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1. n. [Geophysics] A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs. See: hydrocarbon indicator, push-down, velocity

gas chimney

1. n. [Geophysics] A migration technique for processing seismic data in areas where lateral velocity changes are not too severe, but structures are complex. Time migration has the effect of moving dipping events on a surface seismic line from apparent locations to their true locations in time. The resulting image is shown in terms of traveltime rather than depth, and must then be converted to depth with an accurate velocity model to be compared to well logs. See: depth migration, diffraction, migration, processing, velocity

time migration

1. n. [Geophysics] A distinguishing feature of a waveform in a seismic event, such as shape, frequency, phase or continuity. Alternate Form: signature See: frequency, wave

character

1. n. [Geophysics] The magnetic field associated with the Earth’s crust arises from induced and remanent magnetism. The crustal field—also referred to as the anomaly field—varies in direction and strength when measured over the Earth’s surface. It is relatively strong in the vicinity of ferrous and magnetic materials, such as in the oceanic crust and near concentrations of metal ores, and is a focus of geophysical mineral exploration. See: main magnetic field, external disturbance field, local magnetic interference

crustal magnetic field

1. n. [Geophysics] The change in the direction of travel of a wavefront, or the bending of a ray, as it passes from one medium to another, expressed mathematically by Snell's law. Refraction is a consequence of changes in wavelength and velocity of propagation of a wave produced by differences in refractive indices of the media. Refraction surveys where the incident and reflected angles are critical can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities. Synonyms: seismic refraction method See: angle of approach, angle of incidence, blind zone, converted wave, critical angle, deep seismic sounding, displacement, Fermat's principle, head wave, reflection, refractive index, Snell's law, weathering correction

refraction

1. n. [Geophysics] A pair of opposite (and equal) electrical charges. The strength of the dipole is a vector quantity whose direction points from the positive to the negative charge and whose magnitude is the product of the absolute value of the charge times the separation. A point dipole is an idealized mathematical representation of a dipole in which the separation of the charges goes to zero while their charge increases so that the product (dipole strength) remains constant.

dipole

1. vb. [Geophysics] To place seismometers on the ground. The seismometer should be firmly stuck or planted in the ground in the proper location and orientation for optimal seismic acquisition. See: acquisition, coupling, jug hustler, seismometer

plant

1. n. [Geophysics] The ratio of stress to strain, abbreviated as K. The bulk modulus is an elastic constant equal to the change in applied pressure (∂P) divided by the ratio of the change in volume to the original volume of a body (∂V/V). K = −V (∂P / ∂V), where K = bulk modulus V = volume ∂P = partial derivative of pressure ∂V = partial derivative of volume. Alternate Form: modulus of compression See: elastic constants, Lamé constant

bulk modulus

1. n. [Geophysics] The seismic data recorded for one channel. A seismic trace represents the response of the elastic wavefield to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array. Synonyms: trace See: array, channel, density, gather, receiver, velocity

seismic trace

1. n. [Geology, Geophysics] The surface or near-surface, unconsolidated sedimentary layer that has been subject to weathering and whose pores are air-filled instead of liquid-filled. An aerated layer typically has a low seismic velocity. See: pore, static correction, weathering, weathering correction

aerated layer

1. n. [Geophysics] In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. See: anomaly, elevation correction, gravity anomaly, gravity survey, survey

free-air correction

1. n. [Geophysics] An agreed and known value, such as the elevation of a benchmark or sea level, to which other measurements are corrected. In seismic data, the term refers to an arbitrary planar surface to which corrections are made and on which sources and receivers are assumed to lie to minimize the effects of topography and near-surface zones of low velocity. See: benchmark, datum correction, elevation correction, receiver, source

datum

1. n. [Geophysics] A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing. See: event, multiple reflection, noise, peg-leg multiple, primary reflection, short-path multiple, simple multiple

long path multiple

1. n. [Geophysics] A class of borehole seismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. In the more general context, VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals hundreds of meters apart. Also, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver. The check-shot survey uses only the direct path traveltime. In addition to tying well data to seismic data, the vertical seismic profile also enables converting seismic data to zero-phase data and distinguishing primary reflections from multiples. Alternate Form: vertical seismic profile See: air gun, apparent anisotropy, borehole seismic data, check-shot survey, depth conversion, drill-noise vertical seismic profile, first break, multiple reflection, one-way time, primary reflection, salt-proximity vertical seismic profile, seismic-while-drilling vertical seismic profile, Stoneley wave, walk-above vertical seismic profile (VSP), walkaway vertical seismic profile, zero-offset vertical seismic profile

vertical seismic profile (VSP)

1. n. [Geophysics] The daily variation in properties of the Earth, such as the temperature or the local geomagnetic field, or the daily change in sunlight. Such variations depend in part on latitude, proximity to the ocean, the effects of solar radiation and tides and other factors.

diurnal variation

1. n. [Geophysics] In surface seismic acquisition, the horizontal distance from source to geophone. In a vertical seismic profile, geophone offset is the horizontal distance between the source and the wellhead or the surface projection of the geophone in the case of a deviated well. Offset between seismic source and receiver creates a delay, or moveout, in the arrival time of a reflection that can be corrected before stacking and can be used to determine velocity. See: acquisition, arrival time, common-offset, extended spread, fold, lithostratigraphic inversion, moveout, mute, normal moveout, offset, perpendicular offset, shotpoint, spread, tail mute, two-way traveltime, velocity analysis, vertical seismic profile, walkaway vertical seismic profile, zero-offset data

geophone offset

1. adj. [Geophysics] Pertaining to traces that have the same offset, or distance between source and receiver. See: trace

common offset

1. n. [Geophysics] A vibrator truck equipped with wide tires to allow access to rugged or soggy terrain while causing less damage to the environment.

buggy vibro

1. n. [Geophysics] The location of an explosive seismic source below the surface. Before acquisition of surface seismic data onshore using explosive sources such as dynamite, holes are drilled at shotpoints and dynamite is placed in the holes. The shotholes can be more than 50 m [164 ft] deep, although depths of 6 to 30 m [20 to 98 ft] are most common and depth is selected according to local conditions. With other "surface" sources, such as vibrators and shots from air shooting, the shots occur at the Earth's surface. See: acquisition, air shooting, dynamite, shotpoint, source, vibrator

shot depth

1. n. [Geophysics] A concave-upward event in seismic data produced by a buried focus and corrected by proper migration of seismic data. The focusing of the seismic wave produces three reflection points on the event per surface location. The name was coined for the appearance of the event in unmigrated seismic data. Synclines, or sags, commonly generate bow ties. See: sag, seismic wave, syncline

bow tie

1. n. [Geophysics] A type of explosive used as a source for seismic energy during data acquisition. Originally, dynamite referred specifically to a nitroglycerin-based explosive formulated in 1866 by Alfred Bernhard Nobel (1833 to 1896), the Swedish inventor who endowed the Nobel prizes. The term is incorrectly used to mean any explosive rather than the original formulation. See: air shooting, shot depth

dynamite

1. n. [Geology] A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. Anticlines are arch-shaped folds in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. Folds typically occur in anticline-syncline pairs. The hinge is the point of maximum curvature in a fold. The limbs occur on either side of the fold hinge. The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial surface is called the axial plane in cases where the fold is symmetrical and the lines containing the points of maximum curvature of the folded layers, or hinge lines, are coplanar. Concentric folding preserves the thickness of each bed as measured perpendicular to original bedding. Similar folds have the same wave shape, but bed thickness changes throughout each layer, with thicker hinges and thinner limbs. See: anomaly, axial surface, collision, competent, competent, competent, competent, competent, competent, concentric fold, crest, disharmonic, drape, flower structure, harmonic, orogeny, parallel fold, plunge, similar fold, strain, structural trap, trend

fold

1. n. [Geophysics] A wave that is far enough from its source that its wavefront has no effective curvature, or is planar, over a short distance. Seismic and electromagnetic waves are treated as plane waves even though that assumption is not strictly correct. See: wave, wavefront

plane wave

1. n. [Geophysics] An event or appearance of seismic data as a reflection, refraction, diffraction, or other similar feature, or the time at which seismic data appear. An event in a seismic section can represent a geologic interface. Synonyms: event See: arrival time, diffraction, dip moveout, first break, Fresnel zone, hodogram, lag, moveout, ray tracing, reflection, refraction, seismic section, spectrum, time slice

arrival

1. n. [Geophysics] A display of seismic traces that share a source. Alternate Form: CSG See: seismic trace, trace

common source gather

1. n. [Geophysics] A device containing a magnetometer and possibly other instruments that can be towed by an aircraft during aeromagnetic surveying or in a marine seismic streamer to provide dynamic information about the streamer position. See: aeromagnetic survey

bird

1. n. [Geology] The phase that occurs after successful exploration and development and during which hydrocarbons are drained from an oil or gas field. See: appraisal, development, exploration, licensing round, retrograde condensation, scout, scout ticket, subsalt, time-lapse seismic data, underpressure, virgin pressure

production

1. n. [Geophysics] A marine seismic data acquisition method that uses one or more vessels to tow source arrays and streamers to record seismic signals, along with one or more source-only vessels sailing parallel to, but at some specified distance from, the recording vessel(s). The source-only vessels provide offset sources that generate reflections from a wide range of azimuths; these reflections are received by streamers towed by the recording vessel(s).

wide-azimuth towed-streamer acquisition

1. n. [Geophysics] A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data or to achieve deconvolution. See: alias filter, aliasing, aperture, backscatter, band, band-pass, band-reject, coherence filtering, deterministic deconvolution, Fourier transform, noise, processing, random noise, signal, signal-to-noise ratio, Walsh-Hadamard transform

filter

1. n. [Geophysics] A time-variant operation performed on seismic data. Normal moveout (NMO) is a dynamic correction. See: normal moveout, processing, static correction

dynamic correction

1. n. [Geophysics] The number of measurements per unit of time, or the inverse of the sample interval. See: sample frequency, sample interval

sample rate

1. n. [Geophysics] A type of distortion of a wave train in which the velocity of the wave varies with frequency. Surface waves and electromagnetic body waves typically exhibit dispersion, whereas P-waves in most rocks show little change in velocity with frequency. See: body wave, distortion, frequency, P-wave, surface wave

dispersion

1. n. [Geophysics, Geology] Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. Alternate Form: low-velocity layer See: pore, static correction, weathering, weathering correction

low velocity layer

1. n. [Geophysics] A single-vessel technique of acquiring marine seismic data at a complete range of azimuths by towing streamers in a circular path.

full-azimuth towed-streamer acquisition

1. n. [Geophysics] A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion. Synonyms: jug, receiver, seismometer See: accelerometer, array, cable, channel, check-shot survey, coupling, damping, datum correction, detector, displacement, downhole receiver, dropout, fan shooting, first break, four-component seismic data, geophone interval, ground roll, group interval, hodogram, hydrophone, image, jug hustler, multicomponent seismic data, nest, ocean-bottom cable, random noise, seismograph, spread, three-component seismic data, velocity analysis

geophone

1. n. [Geology] The movement of hydrocarbons from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system. See: accumulation, basin, caprock, critical moment, generation, migrate, petroleum system, preservation, primary migration, prospect, reservoir, seal, secondary migration, sedimentary basin, source rock, spill point, trap

migration

1. n. [Geophysics] A sound wave that travels through the air at approximately 330 m/s and can be generated and recorded during seismic surveying. Air waves are a type of coherent noise. See: acquisition, mute, random noise, wave

air wave

1. n. [Geophysics] The magnetization retained by rocks from previous magnetic fields, abbreviated NRM. NRM is a record of the Earth's magnetic field as it existed at the time that the rock formed, such as when magnetic crystals in igneous rocks solidified (also known as chemical remanent magnetism, CRM) or at the time of deposition of sedimentary rocks (known as depositional remanent magnetism, DRM). During deposition of sediments that become sedimentary rock, magnetized particles can settle with their magnetic pole aligned with that of the Earth at that time. Alternate Form: NRM See: geomagnetic polarity reversal

natural remanent magnetism

1. n. [Geophysics] A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles. See: acquisition, bubble effect, cavitation, impulsive seismic data, source, vertical seismic profile, water gun

air gun

1. n. [Geophysics] A type of surface wave in which particles move in an elliptical path within the vertical plane containing the direction of wave propagation. At the top of the elliptical path, particles travel opposite to the direction of propagation, and at the bottom of the path they travel in the direction of propagation. Because Rayleigh waves are dispersive, with different wavelengths traveling at different velocities, they are useful in evaluation of velocity variation with depth. Rayleigh waves make up most of the energy recorded as ground roll. See: ground roll, surface wave, water-bottom roll

Rayleigh wave

1. n. [Geophysics] A type of receiver that can be positioned on the seafloor to acquire seismic data. See: acquisition, receiver

seabed geophone

1. n. [Geophysics] The principle that the path taken by a ray of light from one point to another is that which takes the minimum time (or the maximum time in select cases), named for its discoverer, French mathematician Pierre de Fermat (1601 to 1665). Snell's law and the laws of reflection and refraction follow from Fermat's principle. Fermat's principle also applies to seismic waves. See: least-time path, seismic wave, Snell's law, tomography

Fermat's principle

1. n. [Geophysics] A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area. Synonyms: applied-potential method

equipotential method

1. n. [Geophysics] The rate of repetition of complete wavelengths of electrical signals, light, sound and seismic waves measured in cycles per second, or hertz, and symbolized by f. Typical recorded seismic frequencies are in the range of 5 to 100 hertz. See: acoustic, acoustic emission, aliasing, attribute, band, band-limited function, band-pass, band-reject, character, complex trace analysis, correlation, dispersion, f-k domain, f-k plot, filter, mute, natural frequency, processing, signal, sonic, space frequency domain, spectral, spectrum, vibratory seismic data, wave, wavelength, wavelet

frequency

1. n. [Geophysics] A type of seismic amplitude anomaly, seismic event, or characteristic of seismic data that can occur in a hydrocarbon-bearing reservoir. Although "bright spots," as hydrocarbon indicators are loosely called, can originate in numerous ways, they are not all indicative of the presence of hydrocarbons. Criteria to distinguish true hydrocarbon indicators (sometimes called HCIs) from other types of amplitude anomalies include:amplitude variation with offsetbright or dim spot(s) in amplitude as a result of variations in lithology and pore fluids, sometimes occurring in groups of stacked reservoirschange or reversal in polarity because of velocity changes, also called phasingconformity with local structuresdiffractions that emanate from fluid contactsflat spot that represents a fluid (gas-oil or gas-water) contact, which can also show the downdip limit of the reservoir in some casesgas chimneys above leaking reservoirsshadow zones below the accumulationvelocity push-down because of lower velocities of hydrocarbons than rocksdifference in response between reflected pressure and shear energy.Hydrocarbon indicators are most common in relatively young, unconsolidated siliciclastic sediments with large impedance contrasts across lithologic boundaries, such as those in the Gulf of Mexico and offshore western Africa. An ongoing issue in exploration for hydrocarbon indicators is the difficulty in distinguishing between gas accumulations and water with a low degree of gas saturation ("fizz water"). Alternate Form: bright spot See: amplitude anomaly, amplitude variation with offset, attribute, diffraction, dim spot, gas chimney, phase, push-down, velocity

hydrocarbon indicator

1. n. [Geophysics] A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones. See: array, fan shooting, footprint, geophone, ground roll, hydrophone, radial array, receiver, seismic trace, source, source pattern, spread

geophone pattern

1. n. [Geophysics] Generally, the distance between a receiver and a source in a survey, such as an electromagnetic survey. In seismic surveys, perpendicular or normal offset is the component of the distance between the source and geophones at a right angle to the spread. See: electromagnetic method, offset, receiver, source, spread, survey

perpendicular offset

1. n. [Geophysics] The minimum thickness necessary for a layer of rock to be visible or distinct in reflection seismic data. Generally, the detectable limit is at least 1/30 of the wavelength. Acquisition of higher frequency seismic data generally results in better detection or vertical resolution of thinner layers. See: vertical resolution, wavelength

detectable limit

1. n. [Geophysics] A wave entering a relatively high-velocity medium whose incident and refracted angle is the critical angle. Synonyms: refraction See: angle of incidence, critical angle, refractor

head wave

1. n. [Geophysics] An adjustment of the relative positive and negative excursions of reflections during seismic processing by bulk shifting the null point, or baseline, of the data to emphasize peaks at the expense of troughs or vice versa. Some authors describe bias as a systematic distortion of seismic data to achieve greater continuity. See: baseline, distortion, processing, trough, zero crossing

bias

1. n. [Geophysics] The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Birefringence occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808. Synonyms: double refraction See: anisotropy, S-wave, seismology, velocity

birefringence

1. n. [Geophysics] An elastic constant named after British physicist Thomas Young (1773 to 1829) that is the ratio of longitudinal stress to longitudinal strain and is symbolized by E. It can be expressed mathematically as follows: E = (F/A) / (ΔL/L), where E = Young's modulus F = longitudinal force A = area F/A = longitudinal stress ΔL = change in length L = original length ΔL/L = longitudinal strain. See: elastic constants

Young's modulus

1. n. [Geophysics] The difference between the actual value of gravity measured at a location and the value predicted by a particular Earth model. Gravity anomalies are usually determined by adjusting the known value of (absolute) gravity at a reference station by Bouguer, free-air or other corrections and subtracting the final predicted value from the measurement. (A different description is that the various corrections are subtracted from the data to reduce it to the reference level. Both interpretations are valid provided it is remembered that the resulting gravity anomaly can be caused by density anomalies—i.e., differences in density between Earth and the theoretical model—that can lie anywhere either above or below the reference level.) See: anomaly, Bouguer correction, free-air correction, gravity

gravity anomaly

1. n. [Geophysics] A type of static correction that compensates for delays in seismic reflection or refraction times from one point to another, such as among geophone groups in a survey. These delays can be induced by low-velocity layers such as the weathered layer near the Earth's surface. See: static correction, weathered layer

differential weathering correction

1. n. [Geophysics] An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. See: FFT, Fourier transform, waveform

fast Fourier transform

1. n. [Geophysics] The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO. Alternate Form: dip moveout

DMO

1. n. [Geophysics] A system to automatically control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay. Alternate Form: AGC, AGC time constant See: event, processing, Q

automatic gain control

1. n. [Geophysics] A line joining points of equal time or age, such as a reflection in a seismic profile or contours in an isochron map. See: contour, isochron map, isochron map, reflection

isochron

1. n. [Geophysics] An acoustic velocity value used during processing to produce static, vertical shifts in seismic and other time domain data in order to bring a specific point into alignment with some common elevation feature. Most often, the point in question is the 0.0 s time point, while the elevation feature is ground level. In other cases, the elevation feature may be arbitrary, such as 300 m above mean sea level. See: average velocity, base of weathering, check-shot survey, interval velocity, static correction, velocity, velocity layering, vertical seismic profile, weathering correction

replacement velocity

1. n. [Geophysics] The study of the Earth's magnetic field, a branch of geophysics that began with the observation by British scientist William Gilbert (1544 to 1603) that the Earth is a magnet. Variations in the magnetic field can be used to determine the extent of sedimentary basins and the depth to basement rocks, as well as to differentiate between igneous rocks and certain sedimentary rocks such as salt. High-resolution magnetic surveys can also be used to determine the locations of oil pipelines and production equipment. See: deep tow, downward continuation, electromagnetic method, geophysics, inclinometer, permeability, potential field, remote sensing, spherical divergence, survey, upward continuation

magnetics

Faye correction

1. n. [Geophysics] Frequencies within the acceptable limits of a filter. The term is commonly used as an adjective, as in "band-pass filter," to denote a filter that passes a range of frequencies unaltered while rejecting frequencies outside the range. Alternate Form: band-pass

band pass

1. n. [Geophysics] An instrument used to measure the strength or direction of the Earth's magnetic field. See: aeromagnetic survey, bird, detector

magnetometer

1. n. [Geophysics] Another term for aperture, a portion of a dataset, such as seismic data, to which functions or filters are applied. Aperture time, for example, can be specified, such as a window from 1.2 to 2.8 seconds. Synonyms: aperture See: filter, sinc x

window

1. n. [Geophysics] A quantitative measure of the coherence of seismic data from multiple channels that is equal to the energy of a stacked trace divided by the energy of all the traces that make up the stack. If data from all channels are perfectly coherent, or show continuity from trace to trace, the semblance has a value of unity. See: channel, coherence, coherence, stack

semblance

1. n. [Geology] The movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin. See: lithosphere, plate tectonics, transpression, turbidity current

convergence

1. n. [Geophysics] Generally, the return or rebound of particles or energy from the interface between two media. There are two laws of reflection, which state (1) that incident rays, reflected rays and the normal to the reflecting interface at the point of incidence are coplanar, and (2) that the angle of incidence is equal to the angle of reflection. In geophysics, reflection refers to the seismic energy or signal that returns from an interface of contrasting acoustic impedance, known as a reflector, according to Snell's law. Reflection seismic surveys are useful for mapping geologic structures in the subsurface, interpreting sedimentary environments and evaluating hydrocarbon accumulations that might occur as amplitude anomalies. Reflection surveys are complicated by the variation of velocity as well as the various types of wave energy that are propagated within the Earth. In electromagnetics, variation in electrical properties produces reflections. See: acoustic transparency, amplitude anomaly, arrival, attenuation, autotrack, complex trace analysis, converted wave, critical reflection, deep seismic sounding, diffraction, drill-noise vertical seismic profile, electromagnetic method, Fermat's principle, footprint, primary reflection, raypath, reflection coefficient, reflector, refraction, seismic-while-drilling vertical seismic profile, Snell's law

reflection

1. n. [Geophysics] A measure of the similarity of two seismic traces. See: correlation, phantom, seismic trace, semblance

coherence

1. n. [Geophysics] The ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of henries per meter (H/m). See: magnetics, skin depth

magnetic permeability

1. n. [Geophysics] The azimuthal variation of the AVO response. Alternate Form: AVOAZ See: amplitude variation with offset, AVO

amplitude variation with offset and azimuth

1. n. [Geophysics, Formation Evaluation, Enhanced Oil Recovery] A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. See: tomography

interwell tomography

1. n. [Geophysics] A seismic section that has been redisplayed such that a reflection of interest not horizontal in the original display appears horizontal and flat. Such displays can shed light on geological conditions at the time a given sedimentary layer accumulated. See: seismic section

flattened section

1. n. [Geophysics] Anything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy, such as shot generation ground roll, surface waves, multiples, effects of weather and human activity, or random occurrences in the Earth. Noise can be minimized by using source and receiver arrays, generating minimal noise during acquisition and by filtering and stacking data during processing. See: aliasing, bubble effect, coherence filtering, coherent noise, common midpoint method, cultural noise, deep tow, embedded wavelet, filter, filter, ground roll, inversion, long-path multiple, multiple reflection, mute, peg-leg multiple, random noise, record, seismic-while-drilling vertical seismic profile, short-path multiple, signal-to-noise ratio, smile, stack, suppression, surface wave, tail mute

noise

1. n. [Geophysics] The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N or S/(S+N), although S/N is more commonly used. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined. See: filter, inversion, noise, signal, stack, stacking velocity

signal-to-noise ratio

1. n. [Geophysics] A check-shot survey of a well, which can be used to correct the sonic log and generate a synthetic seismogram that displays changes in amplitude versus traveltime. See: check-shot survey, sonic log, synthetic seismogram

one-dimensional (1D) seismic data

1. n. [Geophysics] The velocity that wave energy—comprised of a wave group, train or packet of individual wave phases or components—travels through a medium. The wave energy may be grouped into an envelope that is shrink-wrapped around it. The shape of the envelope around the wave-energy group changes with distance because the individual wave phases move apart from one another. vg = vp − λ (∂vp/∂λ) = vp + f (∂vp/∂f) Relation of group velocity to phase velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (λ), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency. See: phase velocity

group velocity

1. n. [Geophysics] A large set of seismometers whose output is sent to a common data channel to record a seismic trace. See: fold, geophone interval, group, group interval, seismic trace, seismometer, spread

patch

1. n. [Geophysics] A graphical technique to distinguish subsets of data according to their direction and velocity by plotting and contouring frequency and wavenumber. See: contour, f-k domain

f-k plot

1. n. [Geophysics] A type of positive gravity anomaly that results from the presence of a dense cap rock overlying a relatively low-density salt dome. See: caprock

caprock effect

1. n. [Geophysics] A display of seismic data along a line, such a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done. Synonyms: seismic line See: arrival, depth conversion, event, header, synthetic seismogram, trace, two-dimensional survey

seismic section

1. n. [Geophysics] A subset of a 3D seismic survey comprising low fold or simplified processing (such as omitting dip moveout processing) that can be evaluated soon after acquisition. See: acquisition, borehole seismic data, dip moveout, fold, processing, three-dimensional survey

quicklook

1. n. [Geology] An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems. See: depositional environment, petroleum system, sediment

delta

1. n. [Geophysics] A representation of the direction of travel of a seismic wave. See: least-time path, raypath

ray

1. n. [Geophysics] The process of changing volume as stress is applied to a body.

dilatation

1. vb. [Geophysics] To perform a convolution, which is a mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering. See: embedded wavelet, synthetic seismogram, wavelet

convolve

free air correction

1. n. [Geophysics] The fastest route that a seismic ray can travel between two points, generally dictated by Fermat's principle. See: Fermat's principle, Snell's law

minimum-time path

1. n. [Geophysics] An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. Synonyms: acoustic wave, compressional wave, P-wave See: amplitude variation with offset, dilatation, four-component seismic data, Poisson's ratio, rarefaction, S-wave, shadow zone

dilatational wave

1. n. [Geophysics] Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them. Alternate Form: true-amplitude recovery See: amplitude, attenuation, gain, spherical divergence

TAR

1. n. [Geophysics] Another term for shear modulus, an elastic constant for the ratio of shear stress to shear strain. The shear modulus is one of the Lamé constants. It can be expressed mathematically as follows: μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulus τ = Shear stress = ΔF/A ΔF = Increment of shear force A = Area acted on by the shear force γ = Shear strain = ΔL/L ΔL = Increment of transverse displacement parallel to A L = Original length. Alternate Form: shear modulus See: elastic constants, Lamé constant

modulus of rigidity

1. n. [Geophysics] The inability of a system to exactly match input and output, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier. Synonyms: amplitude distortion See: bias, dispersion, dynamic range, harmonic distortion, zero-phase

distortion

1. n. [Geophysics] A display of seismic traces that share a receiver. Alternate Form: CRG See: seismic trace, trace

common-receiver gather

1. n. [Geophysics] The distance between geophones or groups of geophones. Synonyms: geophone interval See: geophone, group, source, spacing

group interval

1. vb. [Geophysics] To annotate a map or other display with data at the appropriate location. For example, geologists post formation tops on well logs, isopach maps and seismic profiles. Geophysicists post velocity values and traveltimes on maps before contouring. Engineers contour maps posted with pressure or production data. Posting can become an iterative process as new data become available and interpretations are updated. See: contour

post

1. n. [Geophysics] The result of one of many forms of forward modeling to predict the seismic response of the Earth. A more narrow definition used by seismic interpreters is that a synthetic seismogram, commonly called a synthetic, is a direct one-dimensional model of acoustic energy traveling through the layers of the Earth. The synthetic seismogram is generated by convolving the reflectivity derived from digitized acoustic and density logs with the wavelet derived from seismic data. By comparing marker beds or other correlation points picked on well logs with major reflections on the seismic section, interpretations of the data can be improved. The quality of the match between a synthetic seismogram depends on well log quality, seismic data processing quality, and the ability to extract a representative wavelet from seismic data, among other factors. The acoustic log is generally calibrated with check-shot or vertical seismic profile (VSP) first-arrival information before combining with the density log to produce acoustic impedance. See: check-shot survey, convolution, forward modeling, marker bed, multiple reflection, one-dimensional seismic data, phase, reflection, reflection coefficient, Ricker wavelet, seismic modeling, seismic record, seismic section, shotpoint, trace, velocity, vertical seismic profile

synthetic seismogram

1. n. [Geophysics] The process of calculating seismic velocity, typically by using common midpoint data, in order to better process seismic data. Successful stacking, time migration and depth migration all require proper velocity inputs. Velocity or stacking velocity can be calculated from normal moveout, or the change in arrival time produced by source-receiver offset. See: common midpoint, geophone, migration, normal moveout, stack, stacking velocity, time migration, velocity

velocity analysis

1. n. [Geophysics] A method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth's surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth. Synonyms: diffraction stack See: diffraction, Kirchhoff equation, migration, ray tracing

Kirchhoff migration

1. n. [Geophysics] A dilatation, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismic trace. See: amplitude, dilatation, seismic trace

rarefaction

1. n. [Geophysics] A geometrical arrangement of seismic sources (a source array), with each individual source being activated in some fixed sequence in time. See: array, fan shooting, footprint, geophone, ground roll, hydrophone, radial array, receiver, seismic trace, source, spread

source pattern

1. n. [Geophysics] A small, electrically activated explosive charge that detonates a larger charge. Caps, also called seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. Alternate Form: blasting cap See: detonator

cap

1. n. [Geophysics] One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. See: acquisition, amplitude variation with offset, common midpoint method, depth migration, dropout, fold, migration, offset, receiver, seismic record, seismogram, shot depth, spacing, spread, synthetic seismogram

shot point

1. n. [Geology, Geophysics] A near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. See: pore, static correction, weathering, weathering correction

weathered layer

1. n. [Geophysics] The velocity at which a single frequency of a wave group or a phase—or part such as the crest or trough—of a wave group travels through a medium. The phase velocity (vp) is defined by a wavelength (λ) and frequency (f) and given by vp = λ × f. vp = vg + λ (∂vp/∂λ) = vg − f (∂vp/∂f) Relation of phase velocity to group velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (λ), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency. Slowness is the reciprocal of phase velocity. See: group velocity, slowness

phase velocity

1. n. [Geology, Geophysics, Formation Evaluation] A measure of a material’s strength. The unconfined compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions—the confining stress is zero. It is also known as the uniaxial compressive strength of a material because the application of compressive stress is only along one axis—the longitudinal axis—of the sample. Alternate Form: uniaxial compressive strength

unconfined compressive strength (UCS)

1. n. [Geophysics] The primary contribution to Earth’s main magnetic field. See: main magnetic field, nondipole field

dipole field

1. n. [Geophysics] Abbreviation for air gun or water gun. An air gun is a source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles. A water gun is a source of energy for acquisition of marine seismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source. See: acquisition, air gun, bubble effect, cavitation, impulsive seismic data, resolution, source, vertical seismic profile, water gun

gun

1. n. [Geophysics] Seismic data acquired with no horizontal distance between the source and receiver. Stacking seismic data acquired with separated sources and receivers gives the data the appearance of zero-offset data. See: offset, receiver, source, stack, two-way traveltime

zero-offset data

1. n. [Geophysics] In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency. Alternate Form: FD See: Fourier analysis, Fourier synthesis, Fourier transform, seismic survey, time domain

frequency domain

1. n. [Geophysics] In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms. Alternate Form: common midpoint

CMP

1. n. [Geophysics] Generally, an area of the Earth from which waves do not emerge or cannot be recorded. In seismology, the term is used to more specifically describe regions of the subsurface where P-waves and S-waves are difficult to detect, such as regions of the core at certain distances from the epicenter of an earthquake, or the point on the Earth's surface directly above an earthquake. Such zones were first observed in 1914 by Beno Gutenberg (1889 to 1960), an American geologist born in Germany. Because of the molten nature of the outer core, S-waves are especially difficult to detect at 103 to 142 degrees from the epicenter of an earthquake and not observable from 142 to 180 degrees from the epicenter. Areas below salt features are also called shadow zones because the high velocity of salt bends and traps energy, so seismic data quality beneath salt is generally poor unless special seismic processing is performed. See: blind zone, P-wave, S-wave, seismology

shadow zone

1. n. [Geophysics] A technique for predicting or determining arrival times of waves at detectors using raypaths. Ray tracing requires a velocity model and the assumption that rays behave according to Snell's law. Ray tracing provides the traveltimes that are required for Kirchhoff migration. See: arrival, raypath, Snell's law, velocity

ray tracing

1. n. [Geophysics] A wave generated from a point source, such as that generated by an underground explosion. Typical seismic sources such as vibrators and air-gun arrays emit elastic waves that are assumed to be spherical waves. See: air gun, vibrator, wave

spherical wave

1. n. [Geology] A linear, commonly concave-based depression through which water and sediment flow and into which sediment can be deposited in distinctive, often elongated bodies. Channels can occur in a variety of morphologies, e.g., straight, meandering or braided. In some areas, coarse sediments can fill channels of streams or rivers that cut through finer grained sediments or rocks. The close proximity of coarse-grained and fine-grained sediments can ultimately lead to the formation of stratigraphic hydrocarbon traps. See: depositional environment, point bar, sediment, stratigraphic trap, trap, turbidity current

channel

1. n. [Geophysics] In seismic acquisition and processing, the attenuation of amplitudes to reduce the effects of noise or to prevent overload from the high energy of first breaks. See: acquisition, amplitude, attenuation, first break, noise

suppression

1. n. [Geophysics] A technique using a seismic source on the surface and receivers in the borehole to acquire a vertical seismic profile (VSP) during pipe connections. Waveforms are transmitted to the surface during drilling operations and can be processed in time to yield reliable time-depth information and sometimes reflection information. Because the survey and analysis can be performed while a well is being drilled, the data can be considered in decisions during drilling operations. See: acquisition, noise, receiver, reflection, source, vertical seismic profile

seismic-while-drilling vertical seismic profile

1. n. [Geophysics] An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. Alternate Form: fast Fourier transform

FFT

1. n. [Geophysics] Seismic data measured with receivers, sources or both in a well, such as a check-shot survey, vertical seismic profile (VSP), crosswell seismic data, or single-well imaging. By directly measuring the acoustic velocity of each formation encountered in a well, the well logs and borehole seismic data can be correlated to surface seismic data more easily. Borehole seismic data, including both S-waves and P-waves, can be gathered in a cased or openhole. This term is commonly used to distinguish between borehole sonic data (with frequencies typically greater than 1,000 Hz) and borehole seismic data (with frequencies typically less than 1,000 Hz). See: hodogram, log, P-wave, quicklook, radial refraction, S-wave

borehole seismic data

1. n. [Geophysics] A Stoneley wave that occurs at the low frequencies of seismic data. See: Stoneley wave, surface wave

tube wave

1. n. [Geophysics] An event in which one deeper and one near-surface reflector, such as the base of weathering or the ocean floor, are involved. The seismic energy bounces twice from the deep reflector and only once from the shallow reflector, causing the multiple to appear at roughly twice the traveltime of the primary reflection. See: long-path multiple

simple multiple

1. n. [Geophysics] A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break, named for French physicist Augustin-Jean Fresnel (1788 to 1827). Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves. See: attenuation, first break, resolution, wave

Fresnel zone

1. n. [Geophysics] Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data (Vibroseis is a mark of Conoco). The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear "sweep" of at least seven seconds, beginning with high frequencies and decreasing with time ("downsweeping") or going from low to high frequency ("upsweeping"). The frequency can also be changed in a nonlinear manner, such that certain frequencies are emitted longer than others. The resulting source wavelet is not impulsive. Vibrators are employed in land acquisition in areas where explosive sources cannot be used, and more than one vibrator can be used simultaneously to improve data quality. See: acquisition, explosive seismic data, frequency, impulsive seismic data, source, vibrator, wave

vibratory seismic data

1. adj. [Geophysics] Pertaining to sound. Generally, acoustic describes sound or vibrational events, regardless of frequency. The term sonic is limited to frequencies and tools operated in the frequency range of 1 to 25 kilohertz. See: acoustic coupler, acoustic log, bel, decibel, elastic wave, hertz, side-scan sonar, synthetic seismogram, transit time, velocity

acoustic

1. n. [Geophysics] A statistical technique used with variograms, or two-point statistical functions that describe the increasing difference or decreasing correlation between sample values as separation between them increases, to determine the value of a point in a heterogeneous grid from known values nearby. See: variogram

kriging

1. n. [Geophysics] Generally, a geometrical configuration of transducers (sources or receivers) used to generate or record a physical field, such as an acoustic or electromagnetic wavefield or the Earth's gravity field. Synonyms: nest Alternate Form: geophone array, geophone pattern, source pattern See: fan shooting, footprint, ground roll, radial array, receiver, seismic trace, source, spread

array

1. n. [Geophysics] A bin is a subdivision of a seismic survey. The area of a three-dimensional survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the source and the receiver, reflection point, or conversion point. Bins are commonly assigned according to common midpoint (CMP), but more sophisticated seismic processing allows for other types of binning. Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold. See: fold, stack, three-dimensional seismic data, trace

bins

1. n. [Geophysics] A seismic source located in a wellbore rather than at the Earth's surface. See: source

downhole source

1. n. [Geophysics] A arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or an indication of seismic energy on a trace. See: check-shot survey, first arrival, first break, suppression, vertical seismic profile

break

1. n. [Geophysics, Formation Evaluation] Also called interval transit time, the amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. Synonyms: interval transit time, slowness See: acoustic, drift, P-wave

delta t

1. n. [Geophysics] The movement of ions or molecules from regions of high concentration to low concentration within a solution. See: spontaneous potential

diffusion

1. n. [Geophysics] The distance-time relationship determined from analysis of normal moveout (NMO) measurements from common depth point gathers of seismic data. The stacking velocity is used to correct the arrival times of events in the traces for their varying offsets prior to summing, or stacking, the traces to improve the signal-to-noise ratio of the data. See: common depth point, gather, normal moveout, root-mean-square velocity, signal-to-noise ratio, stack, velocity, velocity analysis

stacking velocity

1. n. [Geophysics] The velocity, typically P-wave velocity, of a specific layer or layers of rock, symbolized by vint and commonly calculated from acoustic logs or from the change in stacking velocity between seismic events on a common midpoint gather. See: acoustic log, Dix formula, P-wave, velocity

interval velocity

1. n. [Geophysics] An algorithm used in numerical simulation along the boundary of a computational domain to absorb all energy incident upon that boundary and to suppress reflection artifacts. See: domain

absorbing boundary conditions

1. n. [Geophysics] Any one of a set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. Synonyms: elastic constants, elastic moduli See: bulk modulus, elastic, elastic constants, Poisson's ratio, shear modulus, Young's modulus

modulus of elasticity

1. n. [Geophysics] An interpretation of the presumed continuation of an event. In areas of discontinuous, divergent reflectors or incoherent data, drawing phantoms allows the interpreter to generate a map on a discontinuous event. See: coherence, event, interpretation

phantom

1. n. [Geophysics] The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation. Alternate Form: basic wavelet See: reflection coefficient, seismic processing

embedded wavelet

1. n. [Geophysics] Also known as S-wave, an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson's ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. Synonyms: S-wave, tangential wave See: four-component seismic data

shear wave

1. n. [Geophysics] The reciprocal of wavelength, so the number of wave cycles per unit of distance, abbreviated as k. See: f-k domain, f-k plot, propagation constant, wavelength

wavenumber

1. n. [Geophysics] A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures. Alternate Form: extensive dilatancy anisotropy

EDA

1. n. [Geophysics] The variation of seismic velocity in different directions. Synonyms: seismic velocity

angular dispersion

1. adj. [Geophysics, Geology, Shale Gas] Having directionally dependent properties. For a crystal of a mineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales. Antonyms: isotropic See: aeolotropy, anisotropic formation, anisotropy, birefringence, extensive dilatancy anisotropy, heterogeneous formation, raypath, seismic velocity, velocity

anisotropic

1. n. [Geophysics] The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. Synonyms: delta t, slowness, transit time See: drift, wave

interval transit time

1. n. [Geophysics] A set of mathematical formulas used to convert a time function, such as a seismic trace, to a function in the frequency domain (Fourier analysis) and back (Fourier synthesis). The function is expressed as a convergent trigonometric series, similar to that first formulated by French mathematician Jean-Baptiste-Joseph, Baron Fourier (1768 to 1830). The Fourier transform is used extensively in signal processing to design filters and remove coherent noise. Many filtering operations are performed in the frequency domain. The Fourier transform has applications in image analysis and in pattern recognition in geological systems. See: band-limited function, fast Fourier transform, Fourier analysis, Fourier synthesis, frequency domain, FT, seismic trace, sinc x, Walsh-Hadamard transform

Fourier transform

1. n. [Geophysics] The seismic data recorded for one channel. A trace is a recording of the Earth's response to seismic energy passing from the source, through subsurface layers, and back to the receiver. Synonyms: seismic trace See: bins, brute stack, channel, coherent, coherent noise, common-offset, common-receiver, dilatation, fold, Fourier transform, gather, group, mute, one-dimensional seismic data, random noise, rarefaction, Ricker wavelet, seismic record, seismic section, seismogram, signature deconvolution, spectrum, stack, static correction, synthetic seismogram, two-dimensional seismic data, two-dimensional seismic data, zero crossing

trace

1. adj. [Geophysics] Pertaining to traces that have a different receiver but share a source. See: trace

common-source

1. n. [Geophysics] The process of measuring, observing or analyzing features of the Earth from a distance. Satellite photography and radar are techniques commonly used for remote sensing. Many geophysicists do not consider seismic methods to be remote sensing because although seismic methods sense the subsurface remotely, the sources and receivers are in contact with the Earth. See: gravity, image, magnetics, seismic

remote sensing

1. n. [Geophysics] The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation. Synonyms: embedded wavelet See: reflection coefficient, seismic processing

basic wavelet

1. n. [Geophysics] Contribution to Earth’s main magnetic field that is not represented by the dipole field. See: dipole field, main magnetic field

nondipole field

1. n. [Geophysics] The product of the density of a medium and its shear wave velocity. See: acoustic impedance

elastic impedance

1. n. [Geophysics] The use of a function of time rather than frequency to express an independent variable or measurement. In contrast, in the frequency domain, variables are expressed as a function of frequency instead of time. See: domain, Fourier analysis, Fourier synthesis, frequency domain

time domain

source point

1. n. [Geophysics] Disturbances in seismic data that are not coherent (they lack a phase relationship between adjacent traces, unlike air waves and ground roll) and cannot be correlated to the seismic energy source. Random noise can be reduced or removed from data by stacking traces, filtering during processing or using arrays of geophones during acquisition. See: air wave, coherent, coherent noise, filter, geophone, ground roll, noise, processing, stack, trace

random noise

1. n. [Geophysics] A display of traveltime of acoustic waves versus depth in a well. The term is commonly used as a synonym for a sonic log. Some acoustic logs display velocity. Synonyms: acoustic velocity log See: acoustic wave, depth conversion, interval transit time, interval velocity, velocity survey

acoustic log

1. n. [Geophysics] The remaining value of gravitational attraction after accounting for the theoretical gravitational attraction at the point of measurement, latitude, elevation, the Bouguer correction, and the free-air correction (which compensates for height above sea level assuming there is only air between the measurement station and sea level). This anomaly is named for Pierre Bouguer, a French mathematician (1698–1758) who demonstrated that gravitational attraction decreases with altitude. See: gravity, gravity survey

Bouguer anomaly

1. n. [Geophysics] An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. Alternate Form: fixed source method See: detector, electromagnetic method, moving-source method, receiver

fixed-source method

1. n. [Geophysics] The process of decomposing a function of time or space into a sum (or integral) of sinusoidal functions (sines or cosines) with specific amplitudes and phases. See: Fourier transform, frequency domain, Gibbs' phenomenon, sinusoid, time domain

Fourier analysis

1. n. [Geology] An informal term used to denote a surface in or of rock, or a distinctive layer of rock that might be represented by a reflection in seismic data. The term is often used incorrectly to describe a zone from which hydrocarbons are produced. See: hardground

horizon

1. n. [Geophysics] A step in seismic processing in which reflections in seismic data are moved to their correct locations in space, including position relative to shotpoints, in areas where there are significant and rapid lateral or vertical changes in velocity that distort the time image. This requires an accurate knowledge of vertical and horizontal seismic velocity variations. See: depth section, migration, processing, shotpoint, time migration, traveltime, velocity

depth migration

1. n. [Geophysics] An instrument used to measure the dip of the Earth's magnetic field. See: magnetics

inclinometer

1. n. [Geophysics] The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. See: base of weathering, check-shot survey, static correction, suppression, vertical seismic profile

first break

1. n. [Geophysics] The ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (ρ2V2 − ρ1V1) / (ρ2V2 + ρ1V1), where R = reflection coefficient, whose values range from −1 to +1 ρ1 = density of medium 1 ρ2 = density of medium 2 V1 = velocity of medium 1 V2 = velocity of medium 2. Typical values of R are approximately −1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~0.5 from water to rock; and ~0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations. Synonyms: reflectivity See: acoustic impedance, acoustic impedance section, amplitude, density, density contrast, embedded wavelet, impedance, normal incidence, reflection, reflection tomography, Ricker wavelet, Snell's law, synthetic seismogram, velocity, wave, Zoeppritz equations

reflection coefficient

1. n. [Geophysics] An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson's ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. Synonyms: shear wave, tangential wave See: acoustic, acoustic wave, body wave, converted wave, dilatancy theory, elastic, four-component seismic data, fracture, hodogram, P-wave, Poisson's ratio, seismic, SH-wave, shadow zone, shear, Snell's law, SV-wave, wave

S-wave

1. n. [Geophysics] The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth's surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset. See: migration, normal incidence, offset, receiver, reflector, reflector, source, traveltime, zero-offset data

TWT

1. n. [Geophysics] Measurements used to determine average velocity versus depth, such as from an acoustic log or check-shot survey. Acquiring a velocity survey is also known as "shooting a well." Synonyms: check-shot survey See: acoustic log, shoot a well, velocity

velocity survey

1. n. [Geophysics] A technique for inversion, or generating a model that is consistent with the data, of electromagnetic data, including resistivity and magnetotelluric data. The algorithm is named for William of Occam (1300 to 1349), who asserted that scientific hypotheses and reasoning should be as simple as possible. The use of Occam's inversion produces a smooth model that fits a dataset within certain tolerances, although a smooth model might not be the best fit to the data. See: electromagnetic method, inversion, magnetotelluric method

Occam's inversion

1. n. [Geophysics] An S-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Gamma (γ) is the S-wave anisotropy parameter and is equal to half the ratio of the difference between the horizontally and vertically traveling SH-wave velocities squared divided by the vertically traveling SH-wave velocity squared; an SH-wave is a shear wave that is horizontally polarized. γ ≡ ½ [(C66 − C44) ∕ C44] = ½ [(VSH⊥2 − VSH∥2) / VSH∥2] S-wave parameter (γ) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C66 is the modulus for a horizontally polarized and horizontally traveling S-wave (perpendicular to the symmetry axis), C44 is the modulus for a horizontally polarized and vertically traveling S-wave (parallel to the symmetry axis), VSH⊥ is the velocity for a horizontally polarized and horizontally traveling S-wave and VSH∥ is the velocity for a horizontally polarized and vertically traveling S-wave. Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966. See: epsilon (ε), delta, eta (η)

gamma (γ)

1. n. [Geophysics] A subsurface boundary or interface at which a physical quantity, such as the velocity of transmission of seismic waves, changes abruptly. The velocity of P-waves increases dramatically (from about 6.5 to 8.0 km/s) at the Mohorovicic discontinuity between the Earth's crust and mantle. See: Mohorovicic discontinuity, P-wave, seismic wave

discontinuity

1. n. [Geophysics] Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by seismic processing. Synonyms: secondary reflection See: abnormal events, acquisition, attenuate, autocorrelation, coherent noise, event, ghost, long-path multiple, noise, peg-leg multiple, primary reflection, short-path multiple, synthetic seismogram, vertical seismic profile

multiple reflection

1. n. [Geophysics] A representation of a physical property or entity that can be used to make predictions or compare observations with assumptions. Mathematical velocity models are commonly used to predict the depth to a formation of interest. Physical models, such as layers of clay or putty, can be used to simulate rock layers. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given dataset. Alternate Form: modeling See: convergence, seismic modeling

model

1. n. [Geophysics] A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. Synonyms: Love wave See: wave

Q-wave

1. n. [Geophysics] Traces recorded from a single shotpoint. Numerous seismic records are displayed together in a single seismic section. Synonyms: seismogram See: shotpoint, stack, synthetic seismogram, trace

seismic record

1. n. [Geophysics] A type of reflection survey to help define a salt-sediment interface near a wellbore. Alternate Form: salt-proximity vertical seismic profile See: salt proximity survey, vertical seismic profile

salt-proximity vertical seismic profile (VSP)

1. n. [Geophysics] A concave-upward, semicircular event in seismic data that has the appearance of a smile and can be caused by poor data migration or migration of noise. See: event, migration, noise

smile

1. n. [Geology, Geophysics, Formation Evaluation] A measure of a material’s strength. The uniaxial compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand before failing. It is also known as the unconfined compressive strength of a material because confining stress is set to zero. Alternate Form: unconfined compressive strength, unconfined compressive strength

uniaxial compressive strength

1. n. [Geophysics] Also known as Kirchhoff migration, a method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth's surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth. Synonyms: Kirchhoff migration See: diffraction, Kirchhoff equation, migration, ray tracing

diffraction stack

1. n. [Geophysics] A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections. See: four-dimensional seismic data, isochron map, seismic survey

difference map

1. n. [Geophysics] The comparison, simulation or representation of seismic data to define the limits of seismic resolution, assess the ambiguity of interpretation or make predictions. Generation of a synthetic seismogram from a well log and comparing the synthetic, or modeled trace, with seismic data is a common direct modeling procedure. Generating a set of pseudologs from seismic data is the process known as seismic inversion, a type of indirect modeling. Models can be developed to address problems of structure and stratigraphy prior to acquisition of seismic data and during the interpretation of the data. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set. Synonyms: modeling See: acquisition, amplitude variation with offset, interpretation, inversion, model, resolution, synthetic seismogram, work station

seismic modeling

1. n. [Geophysics] A magnetic tape containing data recorded in the field, abbreviated FT. Alternate Form: field tape

1. n. [Geophysics] In geophysics, the velocity of a wavefront in a certain direction, typically measured along a line of receivers and symbolized by va. Apparent velocity and velocity are related by the cosine of the angle at which the wavefront approaches the receivers: va = v cos θ, where va = apparent velocity v = velocity of wavefront θ = angle at which a wavefront approaches the geophone array. See: receiver, velocity, wave

apparent velocity

1. n. [Geophysics] A set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. Synonyms: elastic moduli, modulus of elasticity See: bulk modulus, elastic, Lamé constant, Poisson's ratio, shear modulus, Young's modulus

elastic constants

1. n. [Geophysics] A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. Alternate Form: blasting cap See: cap, detonate, seismic acquisition

detonator

1. n. [Geophysics] The minimum (negative) deflection of the seismic wavelet. Seismic interpreters commonly pick or track seismic data on paper sections along the trough of a wavelet rather than the solid-colored peak. With the advent of workstations, this is no longer necessary because of automatic picking techniques and the ability to reverse the polarity of the data in real time. Antonyms: peak See: amplitude, amplitude, peak, polarity standard, wave, wavelet

trough

1. n. [Geophysics, Formation Evaluation] Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible. Alternate Form: nuclear magnetic resonance See: magnetic resonance

NMR

1. n. [Geophysics] A feature in seismic data that results from changes in velocity, both laterally and vertically. Pull-up and push-down are examples of velocity anomalies. See: anomaly, pull-up, push-down, velocity

velocity anomaly

1. adj. [Geophysics] Pertaining to variation of the frequency while maintaining the geometry of electromagnetic surveying. In contrast, geometric pertains to keeping the same geometry while varying the frequency. See: electromagnetic method, geometric

parametric

1. n. [Geophysics] A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (ε) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared. ε ≡ ½ [(C11 − C33) / C33] = ½ [(VP⊥2 − VP∥2) / VP∥2] P-wave parameter (ε) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C11 is the horizontal P-wave modulus (perpendicular to the symmetry axis), C33 is the vertical P-wave modulus (parallel to the symmetry axis), VP⊥ is the horizontal P-wave velocity and VP∥ is the vertical P-wave velocity. Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966. See: delta, gamma (γ), eta (η)

epsilon (ε)

extensive dilatancy anisotropy

1. adj. [Geophysics] Pertaining to variation of the survey geometry while maintaining the frequency of electromagnetic surveying. In contrast, parametric pertains to keeping frequency the same while varying the geometry. See: electromagnetic method, parametric

geometric

1. n. [Formation Evaluation, Geophysics] A survey technique that measures the seismic signal transmitted from a source, located in one well, to a receiver array in a neighboring well. The resulting data are processed to create a reflection image or to map the acoustic velocity or other properties (velocities of P- and S-waves, for example) of the area between wells. Placement of the source and receiver array in adjacent wells not only enables the formation between wells to be surveyed, it also avoids seismic signal propagation through attenuative near-surface formations. Another advantage is that it places the source and receiver near the reservoir zone of interest, thereby obtaining better resolution than is possible with conventional surface seismic surveys. This technique is often used for high-resolution reservoir characterization when surface seismic or vertical seismic profile (VSP) data lack resolution, or for time-lapse monitoring of fluid movements in the reservoir. See: tomography

crosswell seismic tomography

1. n. [Geophysics] A situation in interpretation of seismic data in which predicted and actual values differ, or when an interpreted reflection does not close, or tie, when interpreting intersecting lines; or when interpreted seismic data do not match results of drilling a well. Mis-ties commonly occur when data of different phases, rather than uniformly zero-phase data, are interpreted together, or data that have different datum corrections are tied. Mis-ties are described as static if they involve a bulk shift of data (as in the case of tying seismic sections with different datum corrections) or dynamic if the magnitude of the mis-tie varies with time (as in the case of data that have been migrated differently). See: interpretation, tie

mis tie

1. n. [Geophysics] An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data. See: acquisition, buggy vibro, shot depth, source, vibratory seismic data

vibrator

1. n. [Geophysics] An array of sources or receivers radiating outward from a central point, usually a borehole. See: array

radial array

1. n. [Geophysics] An anomaly that occurs as a ring around a feature, such as electrical or geochemical rings around hydrocarbon accumulations. See: anomaly

halo effect

1. n. [Geophysics] A marine seismic data acquisition method using one or more seismic vessels to obtain a combination of multiazimuth and wide-azimuth geometries. A rich-azimuth seismic dataset can be formed by combining the data where multiple wide-azimuth surveys intersect.

Rich-azimuth towed-streamer acquisition

1. n. [Geophysics] A horizontal display or map view of 3D seismic data having a certain arrival time, as opposed to a horizon slice that shows a particular reflection. A time slice is a quick, convenient way to evaluate changes in amplitude of seismic data. See: amplitude, arrival, horizon slice, three-dimensional seismic data

time slice

1. n. [Geophysics] An axis of rotational invariance. A material whose properties exhibit cylindrical, or invariant rotational, symmetry may be rotated about this axis by any amount and its properties will be indistinguishable from what they were before the rotation.

axis of rotational symmetry

1. n. [Geophysics] In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data. Alternate Form: CDP See: channel, depth point, dip, fold, stacking velocity

common depth point

1. n. [Geophysics] The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Double refraction, or birefringence, occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808. Synonyms: birefringence See: anisotropy, S-wave, seismology, velocity, petrography

double refraction

1. n. [Geophysics] The state of being attached to another entity: A well-planted geophone has a coupling to the Earth's surface or to a borehole wall that allows it to record ground motion during acquisition of seismic data. See: geophone, plant, transition zone, tube wave

coupling

1. n. [Geophysics] The convention adopted by the Society of Exploration Geophysicists (SEG) for the display of zero-phase seismic data. If the signal arises from a reflection that indicates an increase in acoustic impedance, the polarity is, by convention, positive and is displayed as a peak. If the signal arises from a reflection that indicates a decrease in acoustic impedance, the polarity is negative and is displayed as a trough. There is another standard for minimum-phase data. In order to interpret seismic data acquired at different times within a region, to model data, or to assess bright or dim spots, some knowledge of the polarity of the data is essential to correlate or tie data properly. See: acoustic impedance, bright spot, dim spot, receiver, source, wavelet

polarity standard

1. n. [Geophysics] A magnetic tape containing data recorded in the field, abbreviated FT. Alternate Form: FT

field tape

1. n. [Geophysics] A processed seismic record that contains traces that have been added together from different records to reduce noise and improve overall data quality. The number of traces that have been added together during stacking is called the fold. See: bins, brute stack, common midpoint, common midpoint method, fold, ground roll, noise, processing, random noise, seismic record, semblance, slant stack, stacking velocity, velocity analysis, zero-offset data

stack

1. n. [Geophysics] The practice of taking a model and calculating what the observed values should be, such as predicting the gravity anomaly around a salt dome using a gravity model or predicting the traveltime of a seismic wave from a source to a receiver using a velocity model. See: gravity anomaly, inverse problem, salt dome

forward problem

1. n. [Geophysics] In digital signal processing, a nonsinusoidal transform by addition and subtraction. The Walsh-Hadamard transform is similar to Fourier series analysis, but uses square waves instead of sinusoidal waves. It is used predominantly in communication theory and, to a lesser extent, in filtering logs with a blocky character. See: filter, Fourier analysis, Fourier transform

Walsh-Hadamard transform

1. n. [Geophysics] A device or system that records the ground oscillations that make up exploration seismic data or earthquakes, sometimes used incorrectly as a synonym for geophone. A seismograph can include amplifiers, receivers and a recording device (such as a computer disk or magnetic tape) to record seismograms. A crude seismograph was built in 1855 by Italian physicist Luigi Palmieri (1807 to 1896). The modern seismograph, which used a pendulum, was invented in 1880 by James Ewing, Thomas Gray and Sir John Milne. See: geophone, seismogram, seismology

seismograph

1. n. [Geophysics] The ability of a material to store a charge from an applied electrical field without conducting electricity. Synonyms: electrical permittivity See: dielectric, electromagnetic method

permittivity

1. n. [Geophysics] The opposition, slowing or prevention of oscillation, or decreasing vibration amplitude, as kinetic energy dissipates. Frictional damping can be important in the use of geophones for seismic surveys, since a vibrating instrument is difficult to read. Eddy currents can produce electromagnetic damping. The classic example of damping from physics is the slowing of a swinging pendulum unless it has a steady supply of energy. See: critical damping, eddy current, geophone, seismic survey

damping

1. n. [Geophysics, Shale Gas, Geology] Predictable variation of a property of a material with the direction in which it is measured, which can occur at all scales. For a crystal of a mineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales. Synonyms: aeolotropy Antonyms: isotropy See: anisotropic, anisotropic formation, birefringence, extensive dilatancy anisotropy, heterogeneous formation, raypath, seismic velocity, velocity

anisotropy

1. n. [Geophysics] A device that detects seismic energy in the form of ground motion or a pressure wave in fluid and transforms it to an electrical impulse. See: apparent velocity, common reflection point, detector, fixed-source method, geophone, moving-source method, offset, one-way time, radial refraction, seismic trace, seismic-while-drilling vertical seismic profile, static correction, two-way traveltime, undershooting, walk-above vertical seismic profile (VSP), walkaway vertical seismic profile, zero-offset data

receiver

1. n. [Geophysics] The conversion of one form of energy into another as the energy passes through a medium. For example, seismic waves are partially converted to heat as they pass through rock. See: absorption band, attenuation, Q, wave

absorption

1. n. [Geophysics] The marine equivalent of ground roll. Water-bottom roll consists of a pseudo-Rayleigh wave traveling along the interface of the water and the seafloor. As the use of seabed receiver systems increases, noise from water-bottom roll has become more of a concern. See: ground roll

water bottom roll

1. n. [Geophysics] Another term for magnetic permeability, the ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of H/m. See: magnetics, skin depth

magnetic constant

1. n. [Geophysics] The 0 to 12 scale established by Admiral Francis Beaufort (1774 to 1857) for measurement of wind strength according to its effect on objects such as trees, flags, and water. According to the Beaufort scale, at wind speeds below 1 knot or 1 km/h, seas are calm. Whitecaps on water and blowing dust and leaves correspond to a Beaufort number of 4, with winds of 11 to 16 knots [20 to 28 km/h]. Hurricane-force winds, greater than 64 knots [> 118 km/h], have a Beaufort number of 12.

Beaufort scale

1. n. [Geophysics] A mathematical representation of the principle that a wavefield at a given point in space and time can be considered as the superposition of waves propagating from adjacent points and earlier times. It is an integral form of the wave equation in which the wave function at a point is represented as the sum (integral) of contributions from a surface enclosing the given point. The Kirchhoff equation (also called the Kirchhoff integral) is the basis for Kirchhoff migration. See: Kirchhoff migration, wave equation

Kirchhoff equation

1. n. [Well Completions, Geophysics] A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps, or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. See: cap, detonator

blasting cap

1. n. [Geophysics] A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes. See: depth conversion, structure map, traveltime

depth map

1. n. [Geology] The reversal of features, particularly structural features such as faults, by reactivation. For example, a normal fault might move in a direction opposite to its initial movement. See: fault, normal fault, structure

inversion

1. n. [Geophysics] A type of elastic wave produced by deformation or brittle failure of material and characterized by relatively high frequency. See: wave

acoustic emission

1. n. [Geophysics] A set of equations that describes the partitioning of energy in a wavefield relative to its angle of incidence at a boundary across which the properties of the rock and fluid content changes. See: reflection coefficient

Zoeppritz equations

1. n. [Geophysics] Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures. Alternate Form: TIH, HTI

horizontal transverse isotropy

1. n. [Geophysics] Frequencies beyond the limits of a filter. Alternate Form: band-reject

band reject

1. n. [Geophysics] The loss of energy or amplitude of waves as they pass through media. Seismic waves lose energy through absorption, reflection, and refraction at interfaces; mode conversion and spherical divergence; or spreading of the wave. Alternate Form: attenuate See: amplitude, converted wave, Fresnel zone, Q, suppression, true-amplitude recovery, wave

attenuation

1. vb. [Geophysics] The removal of undesirable features, such as multiple events, from seismic data. Alternate Form: attenuation See: event, multiple reflection

attenuate

1. n. [Geophysics] A type of vertical seismic profile in which the source is a shear-wave source rather than a compressional-wave source. Shear waves travel through the Earth at about half the speed of compressional waves and respond differently to fluid-filled rock, and so can provide different additional information about lithology and fluid content of hydrocarbon-bearing reservoirs. See: P-wave, S-wave, source, vertical seismic profile, wave

shear

1. n. [Geophysics] The angle of incidence according to Snell's law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows: sin θc = V1 / V2, where θc = the critical angle V1 = velocity of the first medium V2 = velocity of the first medium, which is greater than V1. See: angle of incidence, critical reflection, head wave, refraction, refractor, Snell's law

critical angle

acoustic wave

1. n. [Geophysics] The effect of the separation between receiver and source on the arrival time of a reflection that does not dip, abbreviated NMO. A reflection typically arrives first at the receiver nearest the source. The offset between the source and other receivers induces a delay in the arrival time of a reflection from a horizontal surface at depth. A plot of arrival times versus offset has a hyperbolic shape. Alternate Form: normal moveout

NMO

1. n. [Geophysics] A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle. See: angle of incidence, critical angle, reflection

critical reflection

1. n. [Geophysics] Slang term for a member of a seismic acquisition crew or party who lays out cables and plants geophones for seismic acquisition and collects them after surveying. See: acquisition, cable, geophone, party, plant

jug hustler

1. n. [Geology] A connection of points from well to well in which the data suggest that the points were deposited at the same time (chronostratigraphic) or have similar and related characteristics. Alternate Form: correlate See: chronostratigraphy, geomagnetic polarity reversal, paleontology, sequence, sequence stratigraphy

correlation

1. n. [Geophysics] A magnetic disturbance field generated by electric currents flowing in the ionosphere and magnetosphere and “mirror-currents” induced in the Earth and oceans by the external magnetic field time variations. The disturbance field, which is associated with diurnal field variations and magnetic storms, is affected by solar activity (solar wind), the interplanetary magnetic field and the Earth’s magnetic field. The external magnetic field exhibits variations on several time scales, which may affect the applicability of magnetic reference models. Very long-period variations are related to the solar cycle of about 11 years. Short-term variations result from daily changes in solar radiation, atmospheric tides and conductivity. Irregular time variations are influenced by the solar wind. Perturbed magnetic states, called magnetic storms, occur and show impulsive and unpredictable rapid time variations. See: main magnetic field, crustal magnetic field, local magnetic interference

external disturbance field

1. vb. [Geophysics] To remove the contribution of selected seismic traces in a stack to minimize air waves, ground roll and other early-arriving noise. Low-frequency traces and long-offset traces are typical targets for muting. See: air wave, frequency, ground roll, noise, offset, stack, trace

mute

1. n. [Geophysics] A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers. There are several types of tomography used by geophysicists, including transmission tomography (which uses measurements between boreholes, surface-to-surface, or between a borehole and the surface), reflection or seismic tomography (based on standard reflection seismology), and diffraction tomography (using Fermat's principle for computations instead of Snell's law). Variations in velocity can be attributed to changes in density and elastic properties of rocks, which in turn are affected by the increasing temperature with depth in the Earth. Tomographic techniques have been used to construct maps of the Earth's interior, deep in the mantle, as well as for mapping the shallow subsurface by borehole tomography. See: crosswell tomography, Fermat's principle, receiver, reflection tomography, Snell's law, source, velocity

tomography

1. n. [Geophysics] A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. Alternate Form: BM See: datum, monument, survey

benchmark

1. n. [Geophysics] A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. Synonyms: Q-wave See: wave

Love wave

1. n. [Geophysics] The distance between analogous points in a wave train, measured perpendicular to the wavefront. In seismic data, the wavelength is the seismic velocity divided by frequency. Wavelength is defined as: λ = v / f, where λ = wavelength v = velocity of propagation f = frequency. See: absorption band, apparent wavelength, band, band-limited function, detectable limit, refraction, wavenumber

wavelength

1. n. [Geophysics] Those thicknesses of rock or sediment that have a common velocity, as opposed to the sedimentary layering or bedding of the rock or sediments. See: velocity

velocity layering

1. n. [Geophysics] A local geophysical anomaly generated by a man-made feature, such as electrical and communications wires, steel beams and tanks and railroad tracks. See: cultural noise

cultural anomaly

1. n. [Geophysics] The modulus of the magnetic field vector. The magnetic total field is the magnitude, or absolute value, of the magnetic field vector. The magnetic total field describes the strength, or intensity, of the magnetic field, which is measured in units of nanoTesla (nT). The symbol for the magnetic total field is often F or Btotal. See: magnetic field, main magnetic field

magnetic total field

1. n. [Reservoir Characterization, Geophysics] The act of constructing a model. Alternate Form: model

modeling

1. n. [Geophysics] The loss of energy from a wavefront as a consequence of geometrical spreading, observable as a decrease in wave amplitude. Spherical divergence decreases energy with the square of the distance. Cylindrical divergence decreases energy with the distance. See: spherical divergence

divergence

1. n. [Geophysics] An electromagnetic method in which naturally occurring, low-frequency electric currents (telluric currents), are measured at a base station and compared with values measured at other stations. The normalized measurements of telluric current provide information about the direction of current flow and the conductance (conductivity times thickness) of sediments in the surveyed area. Extremely low-frequency telluric currents (with periods of days or months) provide information about conductivity in the deep interior of the Earth. See: base station, survey, telluric current

telluric-current method

1. n. [Geophysics] Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them. Alternate Form: TAR See: amplitude, attenuation, gain, processing, spherical divergence

true-amplitude recovery

1. n. [Geophysics] A bundle of electrical wires that connects geophones, or the entire carrier system for marine hydrophones, which includes the hydrophones, the electrical wires, the stress member, spacers, the outer skin of the cable, and the streamer filler, which is typically kerosene or a buoyant plastic. The cable relays data to the seismic recording truck or seismic vessel. See: channel, eel, geophone, geophone cable, hydrophone, jug hustler, ocean-bottom cable, spacer, streamer

cable

1. n. [Geology] The ability, or measurement of a rock's ability, to transmit fluids, typically measured in darcies or millidarcies. The term was basically defined by Henry Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock. Effective permeability is the ability to preferentially flow or transmit a particular fluid through a rock when other immiscible fluids are present in the reservoir (for example, effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows for comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow. For more details, see The Defining Series: Defining and Determining Permeability. See: absolute permeability, caprock, effective permeability, fracture, fracture porosity, impermeable, pore, pore pressure, porosity, relative permeability, reservoir, saturation, seal

permeability

1. n. [Geophysics] A method of marine seismic acquisition in which a boat tows a receiver well below the surface of the water to get closer to features of interest or to reduce noise due to conditions of the sea. Deep tow devices are used for some side-scan sonar, gravity and magnetic surveys. See: acquisition, gravity, gravity survey, magnetics, noise, seismic acquisition, side-scan sonar

deep tow

1. n. [Geophysics] Earth’s main magnetic field generated in the Earth’s fluid outer core by a self-exciting dynamo process. Approximately 95% of the total magnetic field measured at Earth’s surface comes from this main field, a significant portion of which may be described as the field of a dipole placed at the Earth’s center and tilted approximately 11° from the Earth’s rotational axis. The magnitude of the main magnetic field is nearly 60,000 nT near the Earth’s poles and about 30,000 nT near the equator. However, there are significant nondipole contributions to the main magnetic field that complicate its mathematical and graphical representation, including that the relative strengths of nondipole components change. As additional complications, the main field varies slowly because of changes within the Earth’s core and the magnetic dipole axis pole position itself wanders over time. See: dipole field, nondipole field, crustal magnetic field, external disturbance field, local magnetic interference

main magnetic field

1. n. [Geophysics] The location, acquisition and processing parameters, and other pertinent information attached to a well log, seismic record and traces. See: parameter, seismic record, seismic section, well log

header

1. n. [Geophysics] A function or time series whose Fourier transform is restricted to a finite range of frequencies or wavelengths. See: band, frequency, wavelength

band-limited function

1. n. [Geophysics] An acquisition technique most commonly used in electromagnetic methods whereby the energy source or transmitter and detectors or receivers are kept in the same relative position and moved together to different spots to compile a profile or map. See: electromagnetic method, fixed-source method, receiver, source

moving-source method

1. n. [Geophysics] A display of seismic data along a line, such as a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done. Synonyms: seismic section See: trace, two-dimensional survey

seismic line

1. n. [Geophysics] A filter, or a set of limits used to eliminate unwanted portions of the spectra of the seismic data, to remove frequencies that might cause aliasing during the process of sampling an analog signal during acquisition or when the sample rate of digital data is being decreased during seismic processing. See: aliasing, filter

alias filter

1. n. [Geophysics] A method of compensating for delays in seismic reflection or refraction times induced by low-velocity layers such as the weathered layer near the Earth's surface. It is a type of static correction. See: static correction, velocity, weathered layer, weathering

weathering correction

1. n. [Geophysics] A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially. See: trace

two-dimensional seismic data

1. n. [Geophysics] A type of short-path multiple, or multiply-reflected seismic energy, having an asymmetric path. Short-path multiples are added to primary reflections, tend to come from shallow subsurface phenomena and highly cyclical deposition, and can be suppressed by seismic processing. In some cases, the period of the peg-leg multiple is so brief that it interferes with primary reflections, and its interference causes a loss of high frequencies in the wavelet. See: event, long-path multiple, multiple reflection, noise, primary reflection, processing, short-path multiple

peg-leg multiple

water-bottom roll

1. n. [Geophysics] A display, also known as the f-k domain, of seismic data by wavenumber versus frequency rather than the intuitive display of location versus time for convenience during seismic processing. Working in the space-frequency domain provides the seismic processor with an alternative measure of the content of seismic data in which operations such as filtering of certain unwanted events can be accomplished more effectively. See: event, f-k domain, frequency, processing

space frequency domain

1. n. [Geophysics] The case in which a wavefront is parallel to an interface and its raypath is perpendicular, or normal, to the interface as the wave impinges upon the interface. See: angle of incidence, raypath, reflection coefficient, Snell's law, two-way traveltime, wave

normal incidence

1. n. [Geophysics] The increase in the volume of rocks as a result of deformation, such as when fractures develop. See: fracture

dilatancy

1. n. [Geophysics] A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. Alternate Form: benchmark

1. n. [Geophysics] The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. A 2D survey commonly contains numerous widely spaced lines acquired orthogonally to the strike of geological structures and a minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the seismic data and interpretation and mapping of structures. See: acquisition, processing

three-dimensional (3D) survey

1. n. [Geophysics] An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator. Synonyms: bright spot See: amplitude, anomaly, processing, reflection

amplitude anomaly

1. n. [Geophysics] The dimensionless quality factor. It is the ratio of the peak energy of a wave to the dissipated energy. As waves travel, they lose energy with distance and time due to spherical divergence and absorption. Such energy loss must be accounted for when restoring seismic amplitudes to perform fluid and lithologic interpretations, such as amplitude versus offset (AVO) analysis. Q is also described as the reciprocal of attenuation, but that is not strictly correct because the attenuation coefficient has units of inverse length. See: absorption, attenuation, automatic gain control, crosswell tomography, spherical divergence

seismic wave

1. n. [Geophysics] In seismic data, the ratio of the velocity determined from normal moveout (i.e., primarily a horizontal measurement) to velocity measured vertically in a vertical seismic profile or similar survey. Apparent anisotropy is of particular importance when migrating long-offset seismic data and analyzing AVO data accurately. The normal moveout velocity involves the horizontal component of the velocity field, which affects sources and receivers that are offset, but the horizontal velocity field is not involved in velocity calculations from vertically measured time-depth pairs. See: amplitude variation with offset

apparent anisotropy

1. n. [Geophysics] The minimum damping that will prevent or stop oscillation in the shortest amount of time, typically associated with oscillatory systems like geophones. Critical damping is symbolized by μc. See: damping, geophone

critical damping

1. n. [Geophysics] A type of event in 2D seismic data in which a feature out of the plane of a seismic section is apparent, such as an anticline, fault or other geologic structure. A properly migrated 3D survey will not contain sideswipes. See: event, migration, two-dimensional seismic data

sideswipe

1. n. [Geophysics] A mode of the electromagnetic field that involves only one component of the magnetic field and the two components of the electric field perpendicular to it; e.g., the x-component of the magnetic field and y- and z-components of the electric field. The TM mode is useful in describing 2D models in which the magnetic field is perpendicular to the 2D plane of the model. For this case, Maxwell's equations can be reduced to a single scalar equation for the magnetic field component, which simplifies calculations tremendously. Alternate Form: transverse magnetic mode

1. n. [Geophysics] A type of acoustic log that displays traveltime of P-waves versus depth. Sonic logs are typically recorded by pulling a tool on a wireline up the wellbore. The tool emits a sound wave that travels from the source to the formation and back to a receiver. See: acoustic log, P-wave, sonic, traveltime, velocity

sonic log

1. n. [Geophysics] The mathematical description of refraction, or the physical change in the direction of a wavefront as it travels from one medium to another with a change in velocity and partial conversion and reflection of a P-wave to an S-wave at the interface of the two media. Snell's law, one of two laws describing refraction, was formulated in the context of light waves, but is applicable to seismic waves. It is named for Willebrord Snel (1580 to 1626), a Dutch mathematician. Snell's law can be written as: n1 sin i = n2 sin r, where n1 = refractive index of first medium n2 = refractive index of second medium sin i = sine of the angle of incidence sin r = sine of the angle of refraction. See: angle of approach, angle of incidence, critical angle, Fermat's principle, least-time path, normal incidence, ray tracing, reflection coefficient, refractive index, tomography, wave

Snell's law

1. n. [Geophysics] The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. See: autocorrelation, correlation, lag, waveform

crosscorrelation

1. n. [Geophysics] The measurement of gravitational acceleration over an area, usually presented as a map or profile of Bouguer or free-air anomalies. See: Bouguer anomaly, free-air correction, survey

gravity survey

1. n. [Geophysics] Slang term to describe a seismologist performing seismic field work.

doodlebugger

1. n. [Geophysics] A change made in seismic data to present reflectors realistically. Velocity corrections typically require that assumptions be made about the seismic velocities of the rocks or sediments through which seismic waves pass. See: velocity, wave

velocity correction

1. n. [Geophysics] A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a high seismic velocity (e.g., a salt layer or salt dome, or a carbonate reef) surrounded by rock with a lower seismic velocity causes what appears to be a structural high beneath it. After such features are correctly converted from time to depth, the apparent structural high is generally reduced in magnitude. Antonyms: push-down See: depth conversion, salt dome, velocity anomaly

pull-up

1. n. [Geophysics] A seismic line within a 3D survey parallel to the direction in which the data were acquired. In marine seismic data, the in-line direction is that in which the recording vessel tows the streamers. See: acquisition, crossline, extended spread, spread, streamer, three-dimensional seismic data

in line

1. n. [Geophysics] The solution to the Laplace equation expressed as spherical coordinates. The normal modes of the Earth, or the reverberations that follow earthquakes, have the form of spherical harmonics. Love waves and Rayleigh waves can also be expressed as spherical harmonics. See: Laplace equation

spherical harmonic

two-way traveltime

1. n. [Geophysics] A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling decide where to set casing in a well evaluate drilling hazards, such as anomalous pore pressure. Alternate Form: drill noise vertical seismic profile See: acquisition, noise, receiver, reflection, source, vertical seismic profile

drill-noise vertical seismic profile

1. n. [Geophysics] Abbreviation for automatic gain control. A system to automatically control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay. Alternate Form: automatic gain control

AGC

1. n. [Geophysics] Often called statics, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. Alternate Form: near-surface correction, near-surface correction, statics See: base of weathering, brute stack, differential weathering correction, dynamic correction, elevation correction, first break, moveout, processing, receiver, source, velocity, weathered layer, weathering correction

static correction

1. n. [Geophysics] A type of multicomponent seismic data acquired in a land, marine, or borehole environment by using three orthogonally oriented geophones or accelerometers. 3C is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, as for example, on land. This technique allows determination of both the type of wave and its direction of propagation. Alternate Form: three-component seismic data See: accelerometer, geophone, hydrophone, multicomponent seismic data

three-component (3C) seismic data

1. n. [Geophysics] The fastest route that a seismic ray can travel between two points, generally dictated by Fermat's principle. See: Fermat's principle, Snell's law

least-time path

1. n. [Geophysics] The edge of an advancing wave, which includes adjacent points that have the same phase. See: phase, plane wave, wave

wavefront

1. vb. [Geophysics] To detect and measure energy.

record

1. n. [Geophysics] A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane. See: converted wave, S-wave, SV-wave

SH-wave

1. n. [Geophysics] A device used to measure the acceleration due to gravity, or, more specifically, variations in the gravitational field between two or more points. See: detector, gravimetry

gravimeter

1. n. [Geophysics] The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell's law describes the relationship between the angle of incidence and the angle of refraction of a wave. See: angle of approach, critical reflection, head wave, raypath, refraction, refractive index, refractor, Zoeppritz equations

angle of incidence

1. n. [Geophysics] A partial differential equation that governs potential fields (in regions where there are no sources) and is equivalent, in three dimensions, to the inverse square law of gravitational or electrical attraction. In Cartesian coordinates, the Laplace equation equates the sum of the second partial (spatial) derivatives of the field to zero. (When a source is present, this sum is equal to the strength of the source and the resulting equation is called Poisson's equation). The differential equation is named for French mathematician Pierre-Simon de Laplace (1749 to 1827), and applies to electrical, gravity and magnetic fields. ∇2u = ∂2u/∂x2 + ∂2u/∂y2 + ∂2u/∂z2 = 0, where u(x,y,z) is a potential function. See: potential field, spherical harmonic

Laplace equation

1. n. [Geophysics] Abbreviation for common receiver gather. A display of seismic traces that share a receiver. Alternate Form: common receiver gather See: seismic trace, trace

CRG

1. n. [Geophysics] How Earth’s magnetic field varies with time. These time variations, called secular variations, necessitate periodic updating of magnetic field maps and models. Two types of processes in the Earth’s core produce these variations. One process is related to variations in Earth’s main dipole field, which operate on time scales of hundreds or thousands of years. The other process is related to variations in Earth’s nondipole field, which operate on time scales on the order of tens of years. See: main magnetic field, dipole field, nondipole field

geomagnetic secular variation

1. n. [Geophysics] A display of seismic traces that share a receiver. Alternate Form: CRG See: seismic trace, trace

common receiver gather

1. n. [Geophysics] The frequency of the normal, free oscillation or vibration of an entity or a system, such as the vibration of a tuning fork when struck or the open string of a musical instrument when plucked. A system oscillating at its natural frequency is said to resonate. See: frequency

natural frequency

1. n. [Geophysics] A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a low seismic velocity (e.g., a shale diapir or a gas chimney) surrounded by rock with a higher seismic velocity causes what appears to be a structural low beneath it. After such features are converted from time to depth, the apparent structural low is generally reduced in magnitude. Hydrocarbon indicators can display velocity pushdowns because the velocity of hydrocarbon is slower than that of rock. Antonyms: pull-up See: depth conversion, hydrocarbon indicator, velocity, velocity anomaly

pushdown

1. n. [Geophysics] Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow.

single-azimuth towed-streamer acquisition

1. n. [Geophysics] A function commonly used in seismic processing. Sinc x is the Fourier transform of a boxcar function, which is a function with a rectangular-shaped aperture. See: aperture, Fourier transform, processing

sinc x

1. n. [Geophysics] The set of values an independent variable can take. For example, the independent variable of the time domain is time; and for the frequency domain, it is frequency. See: absorbing boundary conditions, f-k domain, frequency domain, time domain

domain

1. n. [Geophysics] The ratio of absorbed incident energy to the total energy to which a body is exposed.

absorptance

1. n. [Geophysics] A dataset measured and recorded with reference to a particular area of the Earth's surface, such as a seismic survey. See: accelerometer, base station, baseline, baseline, benchmark, benchmark, cultural noise, depth point, drift, electromagnetic method, free-air correction, gravity, gravity survey, magnetics, monument, perpendicular offset, salt proximity survey, seismic, side-scan sonar, telluric-current method

survey

1. n. [Geophysics] In geophysics, the depth divided by the traveltime of a wave to that depth. Average velocity is commonly calculated by assuming a vertical path, parallel layers, and straight raypaths, conditions that are quite idealized compared to those actually found in the Earth. See: raypath, velocity

average velocity

1. n. [Geophysics] A line joining base stations whose transmissions are synchronized during surveying. See: base station, survey

baseline

1. n. [Geophysics] A step in seismic processing by which the signature of the seismic source in the seismic trace is changed to a known, shorter waveform by using knowledge of the source waveform. If the source waveform is known for each shot, then the process also minimizes variations between seismic records that result from changes in the source output. See: deconvolution, processing, trace, waveform

signature deconvolution

1. n. [Geophysics] A conveyance, such as a sled with runners or pontoons, used to transport geophysical gear to a location. Skids are commonly deployed in acquisition of seismic data in marshes or other areas of soft, soggy terrain. See: acquisition, marsh

skid

1. n. [Geophysics] Method of seismic reflection surveying and processing that exploits the redundancy of multiple fold to enhance data quality by reducing noise. During acquisition, an energy source is supplied to a number of shotpoints simultaneously. Once data have been recorded, the energy source is moved farther down the line of acquisition, but enough overlap is left that some of the reflection points are re-recorded with a different source-to-receiver offset. Multiple shotpoints that share a source-receiver midpoint are stacked. The number of times that a common midpoint is recorded is the fold of the data. See: processing, shotpoint, stack

common midpoint method

1. n. [Geophysics] The null point of a seismic trace. At zero deflection, the phase of a periodic signal is zero or pi. See: bias, signal

zero crossing

1. n. [Formation Evaluation, Geophysics] A crosswell seismic technique that incorporates reflection traveltimes and direct traveltimes into a tomographic inversion algorithm to produce images of seismic velocity between wells. See: crosswell seismic tomography, tomography

crosswell reflection tomography

1. n. [Geophysics] A mathematical method to determine seismic attributes, including reflection strength and instantaneous frequency, by using the Hilbert transform, a special form of the Fourier transform, and the quadrature trace, or the component of the signal that is 90 degrees out of phase. The Fourier transform a(t) = h(t) + jx(t), where h(t) = seismic trace x(t) = quadrature trace can be used to determine reflection strength by combining h and x: r(t) = [h(t)2 + x(t)2]1/2, and to determine instantaneous phase: θ(t) = tan−1[x(t)/h(t)]. See: attribute

complex trace analysis

1. n. [Geophysics] The geometrical pattern of groups of geophones relative to the seismic source. The output from a single shot is recorded simultaneously by the spread during seismic acquisition. Common spread geometries include in-line offset, L-spread, split-spread and T-spread. See: acquisition, aperture, array, extended spread, geophone, group, in-line, offset, perpendicular offset, shotpoint

spread

1. n. [Geophysics] Seismic data acquired in a land, marine, or borehole environment by using more than one geophone or accelerometer. 3C seismic data, a type of multicomponent seismic data, uses three orthogonally oriented geophones or accelerometers. 4C seismic data, another type of multicomponent seismic data, involves the addition of a hydrophone to three orthogonally oriented geophones or accelerometers. 3C multicomponent seismic data is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, for example, on land. This technique allows determination of both the type of wave and its direction of propagation. See: accelerometer, four-component seismic data, geophone, three-component seismic data

multicomponent seismic data

1. n. [Geophysics] The unit of measurement to describe or compare the intensity of acoustic or electrical signal, named for American inventor Alexander Graham Bell (1847 to 1922). Measurements are typically given in tenths of a bel, or decibels. The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. Sounds on the order of one decibel are barely audible to humans but can cause pain when on the order of 1012 decibels. The symbol for the unit is B, but dB is the standard unit. See: acoustic, decibel, signal

bel

1. vb. [Geology] For hydrocarbons to move from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system. See: accumulation, basin, caprock, critical moment, generation, petroleum system, preservation, primary migration, prospect, reservoir, seal, secondary migration, sedimentary basin, source rock, spill point, trap

migrate

1. n. [Geophysics] In acoustics, the product of velocity times density, also called acoustic impedance and symbolized by Z. The reflection coefficient of an interface depends on the contrast in acoustic impedance of the rock on either side of the interface. See: acoustic impedance, reflection coefficient

impedance

1. n. [Formation Evaluation, Geophysics] Also called interval transit time, the amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. Synonyms: delta t, interval transit time See: acoustic, drift, P-wave

slowness

1. n. [Geophysics] The shape of a wave, typically shown as a graph of amplitude (or other quantity of interest) versus time. See: correlation, crosscorrelation, fast Fourier transform, signature, signature deconvolution

waveform

1. n. [Geophysics] Abbreviation for amplitude variation with offset and azimuth, the azimuthal variation of the AVO response. Alternate Form: amplitude variation with offset and azimuth See: amplitude variation with offset, AVO

AVOAZ

1. n. [Geophysics] A display of traveltime of acoustic waves versus depth in a well. The term is commonly used as a synonym for a sonic log. Some acoustic logs display velocity. See: acoustic wave, depth conversion, interval transit time, interval velocity, velocity survey

acoustic velocity log

1. n. [Geophysics] The magnetic field measured near the Earth’s surface is the superposition of magnetic fields arising from various time-varying physical processes that are grouped into four general components: the main magnetic field the crustal field external disturbance field local magnetic interference. The significance of these contributions to direction, strength and stability of the magnetic field varies with geographic region and with magnetic survey direction. See: main magnetic field, crustal magnetic field, external disturbance field, local magnetic interference

magnetic field

1. n. [Geophysics] An elastic constant for the ratio of shear stress to shear strain. The shear modulus is one of the Lamé constants. It can be expressed mathematically as follows: μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulus τ = Shear stress = ΔF/A ΔF = Increment of shear force A = Area acted on by the shear force γ = Shear strain = ΔL/L ΔL = Increment of transverse displacement parallel to A L = Original length. Synonyms: modulus of rigidity See: elastic constants, Lamé constant

shear modulus

1. n. [Geophysics] An inline offset spread.

extended spread

1. n. [Geophysics] A possible explanation for volume changes in rocks due to strain, such as microfracturing or cracking, and the accompanying change in the ratio of P- to S-wave velocity. Support for dilatancy theory comes in the form of porosity increases from 20 to 40% that have been measured in laboratory experiments using rock samples. See: P-wave, S-wave

dilatancy theory

1. n. [Geophysics] A type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters. See: acquisition, array, coherent noise, mute, random noise, Rayleigh wave, stack, surface wave, tail mute, water-bottom roll

ground roll

1. n. [Geophysics] The ratio of the speed of light in a vacuum to the speed of light in a given material, commonly symbolized by n. According to Snell's law, the refractive index is also the ratio of sine of the angle of incidence to the sine of the angle of refraction. See: angle of incidence, refraction

refractive index

1. n. [Geophysics] Explosives designed to affect a certain direction preferentially. Shaped charges are most commonly used to perforate wells, but can be an energy source for seismic acquisition. Alternate Form: perforating charge See: directivity

shaped charge

1. n. [Geophysics] A seismic aquisition method in which the incident and reflected angles are critical. These refraction surveys can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities. See: angle of incidence, critical angle, Fermat's principle, reflection, refraction, Snell's law

seismic refraction method

1. n. [Geophysics] Abbreviation for amplitude variation with angle of incidence. See: angle of incidence, AVO, AVOAZ

AVA

1. n. [Geophysics] Abbreviation for amplitude variation with azimuth. See: anisotropy, AVA, AVO, AVOAZ, azimuth, fault, fracture, stress

AVAZ

1. vb. [Perforating, Geophysics] To set off an explosive material. Explosive sources are used in seismic acquisition and explosive charges are used to perforate wells in preparation for production or injection. See: blasting cap, cap, detonating cord

detonate

1. n. [Geophysics] Another term for acoustic impedance, the product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient. Synonyms: acoustic impedance See: acoustic impedance section, acoustic transparency, amplitude anomaly, bright spot, density contrast, dim spot, polarity standard, reflector

seismic impedance

1. n. [Geophysics] Measurements of the Earth's magnetic field gathered from aircraft. Magnetometers towed by an airplane or helicopter can measure the intensity of the Earth's magnetic field. The differences between actual measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type or in thickness of rock units. See: anomaly, bird, magnetometer, rock, upward continuation

aeromagnetic survey

1. n. [Geophysics] A nonlinear change in waveform in which simple multiples of (1,2, ... n times) the input frequencies, or harmonics, are generated. See: distortion

harmonic distortion

1. n. [Geophysics] A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. Synonyms: velocity survey, well shoot See: correlate, correlation, depth conversion, drift, first break, interpretation, one-dimensional seismic data, one-way time

checkshot survey

1. n. [Geophysics] In marine seismic acquisition, the lateral deviation of a streamer away from the towing direction because of a water current. See: acquisition, streamer

streamer feathering

1. n. [Geophysics] Another term for static correction, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. See: base of weathering, brute stack, differential weathering correction, dynamic correction, elevation correction, first break, moveout, processing, receiver, source, velocity, weathered layer, weathering correction

near-surface correction

1. n. [Geophysics] The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft. Alternate Form: dB See: acoustic, bel, dynamic range, signal

decibel

1. n. [Geophysics] A modeling technique to assess the geologic history of rock layers through the use of geologic cross sections or seismic sections. Removal of the youngest layers of rock at the top of the section allows restoration of the underlying layers to their initial, undisturbed configurations. Successively older layers can be removed sequentially to further assess the effects of compaction, development of geologic structures and other processes on an area. See: cross section

back-stripping

1. n. [Geophysics] A mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering. The mathematical form of the convolution of two functions, a filter f(t) and a time-series x(t), is y(t) = ∫ f(t−τ)x(τ)dτ, where y(t) is the output of the convolution. In the frequency domain, convolution is simply the product of the Fourier transforms (FT) of the two functions: Y(ω) = F(ω)*X(ω), where X(ω) = FT of the time series x(t) F(ω) = FT of the filter f(t) Y(ω) = FT of the output y(t) ω = angular frequency. See: embedded wavelet, synthetic seismogram, wavelet

convolution

1. n. [Geophysics] A group of four partial differential equations that describe all classical phenomena, involving electric and magnetic fields. James Clerk Maxwell (1831 to 1879), a British physicist, first wrote out this complete set of equations: (1.) ∇·D = ρ (2.) ∇×H = J + (∂D/∂t) (3.) ∇·B = 0 (4.) ∇×E = −(∂B/∂t), where D = electric displacement ρ = electric charge density H = magnetic field strength J = electric current density B = magnetic flux density E = electric field strength. Equation (1) is equivalent to Coulomb's law, the inverse square attraction of static electric charges. Equation (2) is Ampere's law relating magnetic fields and currents, which was extended by Maxwell to include induction of a magnetic field by a time-varying electric displacement. Equation (3) is Coulomb's law for magnetic flux, expressing the absence of isolated magnetic charges. Equation (4) is Faraday's law of induction, relating an electric field to a time-varying magnetic flux. Maxwell's equations are the starting point for all calculations involving surface or borehole EM methods. See: electromagnetic method

Maxwell's equations

1. adj. [Geophysics] Pertaining to traces that have a different receiver but share a source. See: trace

common source

1. n. [Geophysics] A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. Synonyms: TEM, transient electromagnetic method Alternate Form: TDEM

time-domain electromagnetic method

1. n. [Geophysics] A display of seismic traces that share a source. Alternate Form: CSG See: seismic trace, trace

common-source gather

1. n. [Geophysics] Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry. Alternate Form: TI See: axis of rotational symmetry, vertical transverse isotropy, horizontal transverse isotropy

transverse isotropy

1. vb. [Geophysics] To interpret data, such as seismic sections, by selecting and tracking marker beds or other events. See: event, marker bed

pick

1. n. [Geophysics] An electromagnetic method used to map the spatial variation of the Earth's resistivity by measuring naturally occurring electric and magnetic fields at the Earth's surface. These natural EM fields are generated (at all frequencies) in the Earth's atmosphere mainly by lightning strokes and by interactions between the solar wind and the ionosphere. In the most general MT method, the horizontal components of the electric field and all three components of the magnetic field are measured at the surface. The measurements are used to determine specific ratios of electric to magnetic field components called tensor impedances. The technique was introduced the French geophysicist Louis Cagniard in the 1950s and has been popular for mineral exploration and regional geophysical mapping. It is used in oil exploration for low-cost reconnaissance of sedimentary basins and for exploration in areas where seismic surveys are difficult because of severe topography or the presence high-impedance volcanic rocks near the surface. The resolution of MT surveys is limited by the diffusive nature of EM propagation in the earth; it is usually on the order of hundreds of meters to kilometers. But the MT method can probe the Earth to depths of several tens of kilometers. Alternate Form: magnetotelluric method

tangential wave

1. n. [Geophysics] A source of energy for acquisition of marine seismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source. See: acquisition, air gun, cavitation, gun, resolution, source

water gun

common midpoint

1. n. [Geophysics] The ability to distinguish between separate points or objects, such as sedimentary sequences in a seismic section. High frequency and short wavelengths provide better vertical and lateral resolution. Seismic processing can greatly affect resolution: deconvolution can improve vertical resolution by producing a broad bandwidth with high frequencies and a relatively compressed wavelet. Migration can improve lateral resolution by reducing the size of the Fresnel zone. See: acquisition, band, deconvolution, migration, phase, processing, seismic modeling, tuning effect, water gun, wavelet

resolution

1. adj. [Geology] Pertaining to the movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin. See: lithosphere, plate tectonics, transpression, turbidity current

convergent

1. n. [Geophysics] A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth's surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data. Synonyms: seismic reflection tomography See: reflection coefficient, seismology, tomography, velocity

reflection tomography

long-path multiple

1. n. [Geology] The offset of segments or points that were once continuous or adjacent. Layers of rock that have been moved by the action of faults show displacement on either side of the fault surface. See: fault, transform fault

displacement

1. n. [Geophysics] A device that records seismic energy in the form of ground motion and transforms it to an electrical impulse. Synonyms: geophone, jug, receiver See: accelerometer, acquisition, fold, geophone, group, hydrophone, plant

seismometer

1. n. [Geophysics] The wavelength measured by receivers when a wave approaches at an angle. The relationship between true and apparent wavelength can be shown mathematically as follows: λ = λa sin θ, where λ = wavelength λa = apparent wavelength θ = angle at which a wavefront approaches the geophone array. See: receiver, wavelength

apparent wavelength

1. n. [Geophysics] A type of local seismic event that, in contrast to a bright spot, shows weak rather than strong amplitude. The weak amplitude might correlate with hydrocarbons that reduce the contrast in acoustic impedance between the reservoir and the overlying rock, or might be related to a stratigraphic change that reduces acoustic impedance. Antonyms: bright spot See: acoustic impedance, anomaly, hydrocarbon indicator, polarity standard

dim spot

1. n. [Geophysics] A portion of a dataset, such as seismic data, to which functions or filters are applied. Aperture time, for example, can be specified, such as a window from 1.2 to 2.8 seconds. Synonyms: window See: filter, sinc x

aperture

1. n. [Geophysics] The lower boundary of the near-surface, low-velocity zone in which rocks are physically, chemically, or biologically broken down, in some cases coincident with a water table. Static corrections to seismic data can compensate for the low velocity of the weathered layer in comparison with the higher-velocity strata below. See: first break, ghost, static correction, velocity, weathering

base of weathering

1. n. [Geophysics] A marine seismic data acquisition method in which a conventional narrow-azimuth towed-streamer configuration is used to acquire data over a survey area in more than one direction. The number of directions is typically three or more. The azimuthal range for a multiazimuth survey is not continuous in azimuth, but is well sampled along the shooting directions.

multiazimuth towed-streamer acquisition

1. n. [Geophysics] A point on the surface for which the depth to a horizon has been calculated in a refraction seismic survey. The term is commonly misused as a synonym for common depth point. See: common depth point, displacement

depth point

1. n. [Geophysics] A mode of the electromagnetic field that involves only one component of the electric field and the two components of the magnetic field perpendicular to it; e.g., the x-component of the electric field and y- and z-components of the magnetic field. The TE mode is useful in describing 2D models in which the electric field is perpendicular to the 2D plane of the model. For this case, Maxwell's equations can be reduced to a single scalar equation for the electric field component, which simplifies calculations tremendously. There is an analogous mode for the magnetic field called the TM mode. A general EM field in a region without sources can be expressed as a sum of TE and TM modes. Alternate Form: TE See: electromagnetic method, transverse magnetic mode, wave

transverse electric mode

1. n. [Geophysics] Another term for bulk modulus, the ratio of stress to strain, abbreviated as k. The bulk modulus is an elastic constant equal to the applied stress divided by the ratio of the change in volume to the original volume of a body. See: elastic constants, Lamé constant

modulus of compression

1. n. [Geophysics] A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. Synonyms: TDEM, time-domain electromagnetic method Alternate Form: transient electromagnetic method

TEM

1. n. [Geophysics] An interactive computer suitable for seismic data processing, interpretation and modeling that is particularly useful for studies of large quantities of seismic data, particularly 3D seismic data. See: seismic modeling, three-dimensional seismic data

work station

1. n. [Geophysics] The time measured from a check-shot survey or vertical seismic profile (VSP), which is the time energy takes to travel from an energy source at the surface of the Earth to a receiver at a depth of interest. See: check-shot survey, receiver, source, vertical seismic profile

one-way time

1. n. [Geophysics] An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix's assumptions. The equation is as follows: Vint = [(t2 VRMS22 − t1 VRMS12) / (t2 − t1)]1/2, where Vint = interval velocity t1 = traveltime to the first reflector t2 = traveltime to the second reflector VRMS1 = root-mean-square velocity to the first reflector VRMS2 = root-mean-square velocity to the second reflector. Reference: Dix CH: "Seismic Velocities from Surface Measurements," Geophysics 20, no. 1 (January 1955): 68–86. See: interval velocity, reflector, root-mean-square velocity, traveltime

Dix formula

1. n. [Geophysics] A correction for variations in the density or thickness of the Earth's crust. Isostatic corrections are commonly applied to gravity data and are made according to a specific model for isostasy. See: gravity, isostasy

isostatic correction

1. n. [Geophysics] A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth's surface or in boreholes in order to map variations in the Earth's electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, "electromagnetic method" tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods). See: array, cultural noise, diffusion equation, dipole, eddy current, electrical conductivity, electrical permittivity, fixed-source method, geometric, induced polarization, magnetics, magnetotelluric method, Maxwell's equations, moving-source method, Occam's inversion, parametric, perpendicular offset, probe, reflection, skin depth, survey, transient electromagnetic method, transverse electric mode, transverse magnetic mode

electromagnetic method

1. n. [Geophysics] A range of frequencies or wavelengths. Audible frequencies of sound and visible wavelengths of light are examples of bands. In seismic data, band-pass frequencies are within the limits of a specific filter, while band-reject frequencies exceed the acceptable range of frequencies. See: absorption band, band-limited function, filter, frequency, resolution, wavelength

band

1. n. [Geophysics] A seismic dataset measured and recorded with reference to a particular area of the Earth's surface, to evaluate the subsurface. See: gravity survey, telluric-current method

seismic survey

band-pass

1. n. [Geophysics] In multichannel seismic acquisition, the common midpoint on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver that is shared by numerous locations if the reflector is flat-lying. Like common depth point, this term is commonly misused, because in the case of dipping layers, common reflection points do not exist. See: channel, dipping bed, fold

common reflection point

mis-tie

1. n. [Geophysics] A display of seismic traces that share an acquisition parameter, such as a common midpoint gather, which contains traces having a common midpoint. See: common midpoint, seismic trace, stacking velocity, trace

gather

1. n. [Geophysics] A process used in seismic processing to stack, or sum, traces by shifting traces in time in proportion to their offset. This technique is useful in areas of dipping reflectors. See: processing, stack, trace

slant stack

1. n. [Geophysics] A type of large-amplitude interface, or surface, wave generated by a sonic tool in a borehole. Stoneley waves can propagate along a solid-fluid interface, such as along the walls of a fluid-filled borehole and are the main low-frequency component of signal generated by sonic sources in boreholes. Analysis of Stoneley waves can allow estimation of the locations of fractures and permeability of the formation. Stoneley waves are a major source of noise in vertical seismic profiles. See: signal, sonic, surface wave, tube wave, vertical seismic profile, wave

Stoneley wave

1. n. [Geophysics] A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer.

detector

1. n. [Geophysics] The variation of elastic properties with direction. For example, elastic anisotropy occurs when seismic, or elastic, waves travel through rock at differing velocities in various directions. Elastic anisotropy occurs if there is a preferred alignment of a material’s fabric elements—crystals, grains, cracks, bedding planes, joints or fractures—on a scale smaller than the length of the wave. This alignment causes waves to propagate fastest in its direction. Elastic anisotropy is sometimes called seismic anisotropy, velocity anisotropy, traveltime anisotropy, acoustic anisotropy or slowness anisotropy.

elastic anisotropy

1. n. [Geology, Geophysics, Formation Evaluation] Test in which the fluid in the sample is not able to flow and equilibrate to imposed pore pressure conditions; the fluid mass remains the same while the fluid volume and pressure will vary. See: drained test

undrained test

1. n. [Geophysics] The distance between geophones or the centers of groups of geophones. Synonyms: group interval See: geophone, group, source

geophone interval

1. n. [Geophysics] Surface seismic data acquired using an explosive energy source, such as dynamite. See: acquisition, impulsive seismic data, vibratory seismic data

explosive seismic data

1. n. [Geophysics] Abbreviation for horizontal transverse isotropy. Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures. Alternate Form: horizontal transverse isotropy, HTI

TIH

1. vb. [Geophysics] To use computer software to pick a particular reflection or attribute in seismic data automatically. Autotracking can speed interpretation of three-dimensional seismic data but must be checked for errors, especially in areas of faulting and stratigraphic changes. See: interpretation

autotrack

1. n. [Geology, Geophysics, Formation Evaluation] Test in which the pore fluid in the sample is able to flow and equilibrate to imposed pore pressure conditions; the fluid mass and volume will vary but its pressure will be constant. A drained test could be on a dry sample. See: undrained test

drained test

antialias filter

1. n. [Geophysics] The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing. See: acoustic positioning, air wave, aperture, bubble effect, circle shooting, common midpoint method, crossline, deep tow, depth controller, eel, explosive seismic data, four-dimensional seismic data, gather, geophone, header, hydrophone, impulsive seismic data, in-line, offset vertical seismic profile, seismic modeling, seismic-while-drilling vertical seismic profile, seismometer, shot depth, shotpoint, skid, spread, streamer feathering, suppression, tail buoy, three-dimensional seismic data, time-lapse seismic data, transition zone, two-dimensional seismic data, undershooting, vibratory seismic data, water gun

acquisition

1. n. [Geophysics] A cutoff in time, offset or both that has the effect of eliminating some types of noise from seismic data. A tail mute can be used to exclude slow surface waves such as ground roll. See: ground roll, noise, offset, surface wave

tail mute

1. n. [Geophysics] The duration of time for a P-wave to travel one foot, typically displayed on an acoustic log. The unit of microseconds per foot (or meter) is called the slowness, which is the inverse of velocity. Transit time is measured in microseconds per foot (μs/ft) or in microseconds per meter (μs/m). Synonyms: interval transit time See: acoustic, cycle skip, delta t, drift, P-wave

transit time

shotpoint

1. n. [Geophysics] A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.

back-propagation

1. n. [Geophysics] An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. Synonyms: acoustic wave, dilatational wave, P-wave See: amplitude variation with offset, body wave, dilatation, four-component seismic data, Poisson's ratio, rarefaction, S-wave, shadow zone

compressional wave

1. n. [Geophysics] The difference in the arrival times or traveltimes of a reflected wave measured by receivers at two different offset locations. Normal moveout (NMO) is moveout caused by the separation between a source and a receiver in the case of a flat reflector. Dip moveout (DMO) occurs as an effect in addition to NMO when reflectors dip. Problems that require static corrections can also produce moveout. See: arrival, common depth point, dip moveout, normal moveout, offset, receiver, reflector, source, static correction

moveout

1. n. [Geophysics] A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones. Alternate Form: geophone array See: array, fan shooting, footprint, geophone, ground roll, hydrophone, radial array, receiver, seismic trace, source, source pattern, spread

nest

1. n. [Geophysics] A term to indicate features in seismic data other than reflections, including events such as diffractions, multiples, refractions, and surface waves. Although the term suggests that such events are not common, they often occur in seismic data. See: diffraction, event, multiple reflection, reflection, refraction, surface wave

abnormal events

1. n. [Geophysics] A scientist trained in the study of the physics of the Earth, particularly its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. In the petroleum industry, geophysicists perform a variety of functions, chiefly the processing and interpretation of seismic data and generation of subsurface maps on the basis of seismic data. Such interpretations enhance understanding of subsurface geology. See: elastic wave, seismic wave

geophysicist

1. n. [Geophysics] The variation in the mass per unit volume of rocks, which affects the local gravitational field of the Earth. A density contrast also contributes to an acoustic impedance contrast, which affects the reflection coefficient. See: acoustic impedance, density, gravity, reflection coefficient

density contrast

1. n. [Geophysics] An area in which water is too shallow for acquisition of marine seismic data with towed streamers, such as near the shoreline, marshes and lagoons. In some cases, source explosives can be rammed into the unconsolidated sediments of transition zone environments rather than drilling more costly shot holes. Likewise, hydrophones can be placed by ramming to couple the receiver to the Earth better and to save time and money during survey acquisition. See: acquisition, coupling

transition zone

1. n. [Geophysics] Multiply-reflected seismic energy with a shorter travel path than long-path multiples. Short-path multiples tend to come from shallow subsurface phenomena or highly cyclical sedimentation and arrive soon after, and sometimes very near, the primary reflections. Short-path multiples are less obvious than most long-path multiples and are less easily removed by seismic processing. See: event, ghost, long-path multiple, multiple reflection, noise, peg-leg multiple, primary reflection, processing

short-path multiple

1. n. [Geophysics] A one-dimensional pulse, usually the basic response from a single reflector. Its key attributes are its amplitude, frequency and phase. The wavelet originates as a packet of energy from the source point, having a specific origin in time, and is returned to the receivers as a series of events distributed in time and energy. The distribution is a function of velocity and density changes in the subsurface and the relative position of the source and receiver. The energy that returns cannot exceed what was input, so the energy in any received wavelet decays with time as more partitioning takes place at interfaces. Wavelets also decay due to the loss of energy as heat during propagation. This is more extensive at high frequency, so wavelets tend to contain less high-frequency energy relative to low frequencies at longer traveltimes. Some wavelets are known by their shape and spectral content, such as the Ricker wavelet. See: convolution, embedded wavelet, peak, polarity, polarity standard, resolution, Ricker wavelet, spectral, synthetic seismogram, trough, wavelet extraction, zero-phase

wavelet

1. n. [Geophysics] The product of conductivity and thickness, typically measured in units of siemens (S). In the inversion of electrical and electromagnetic measurements, the conductance of a layer or zone is usually much better determined than either the conductivity or thickness individually.

conductance

1. n. [Geophysics] A short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as at the base of weathering or between sources and receivers and the sea surface. See: base of weathering, multiple reflection, receiver, short-path multiple, source

ghost

incident angle

1. n. [Geophysics] A hydrophone array in a cable that can be attached to a streamer for acquisition of marine seismic data. The eel can be suspended from the streamer so that the eel is close to the seafloor but the streamer remains high enough to avoid obstacles on the seafloor such as reefs or debris from human activity.

eel

1. n. [Geophysics] A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. Air shooting is also called the Poulter method after American geophysicist Thomas Poulter. See: dynamite, shot depth, source

air shooting

1. vb. [Geophysics] To measure the lateral variation of a property, such as gravity or magnetic fields. Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods. See: gravity, probe

profile

1. n. [Geophysics] A device used in acquisition of marine seismic data that keeps streamers at a certain depth in the water. See: streamer

depth controller

1. n. [Geophysics] The rate at which a sound wave travels through a medium. Unlike the physicist's definition of velocity as a vector, its usage in geophysics is as a property of a medium: distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium. See: acoustic, acoustic impedance, angular dispersion, anisotropy, apparent velocity, attribute, average velocity, base of weathering, birefringence, channel wave, check-shot survey, depth conversion, depth migration, discontinuity, dispersion, extensive dilatancy anisotropy, gas chimney, horizon, hydrocarbon indicator, interval velocity, processing, pull-up, push-down, ray tracing, reflection coefficient, reflection tomography, refraction, refractor, root-mean-square velocity, seismic trace, sonic log, stacking velocity, static correction, synthetic seismogram, time migration, tomography, velocity, velocity analysis, velocity anomaly, velocity correction, velocity layering, velocity survey, vertical seismic profile, wave, wave equation, wavelength, weathering correction

acoustic velocity

1. n. [Geophysics] An implosion produced by locally low pressure, such as the collapse of a gas bubble in liquid (the energy of which is used as the source of seismic energy from air guns). See: air gun, bubble effect, water gun

cavitation

1. n. [Geophysics] The ringing near a discontinuity in a signal that is caused by incomplete Fourier synthesis, or missing frequencies. See: Fourier analysis, Fourier synthesis, frequency

Gibbs' phenomenon

1. n. [Geophysics] A seismic reflectivity section, or a 2D or 3D seismic section, that has been inverted for acoustic impedance. Sonic and density logs can be used to calibrate acoustic impedance sections. See: reflection coefficient, sonic log, three-dimensional seismic data, two-dimensional seismic data

acoustic impedance section

1. n. [Geophysics] The ability of a material to store a charge from an applied electrical field without conducting electricity. Synonyms: permittivity See: dielectric, electromagnetic method

electrical permittivity

TDEM

1. adj. [Geophysics] Pertaining to traces that have the same offset, or distance between source and receiver. See: trace

common-offset

1. n. [Geophysics] A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. Synonyms: TDEM, time-domain electromagnetic method Alternate Form: TEM

transient electromagnetic method

1. n. [Geophysics] The area covered by an array of towed streamers in marine seismic acquisition. See: acquisition, array, streamer

footprint

1. n. [Geophysics] An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. Synonyms: acoustic wave, compressional wave, dilatational wave See: acoustic, amplitude variation with offset, body wave, converted wave, dilatancy theory, dilatation, discontinuity, elastic, elastic wave, interval velocity, Poisson's ratio, rarefaction, S-wave, seismic, shadow zone, shear, Snell's law, sonic, sonic log, transit time, wave

P-wave

1. n. [Geophysics] Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods. See: cultural anomaly, electromagnetic method, noise, survey

cultural noise

1. n. [Geophysics] Traces recorded from a single shotpoint. Numerous seismograms are displayed together in a single seismic section. Synonyms: seismic record See: seismograph, shotpoint, synthetic seismogram, trace

seismogram

1. n. [Geophysics] A reflection phenomenon of energy in which a nonreflective surface, which is a surface that does not reflect energy coherently, randomly scatters energy. No coherent reflected energy can be identified and the energy is scattered in all directions, including back in the direction from which it came. For example, light can be scattered or redistributed by rough, nonreflective surfaces. See: deconvolution, filter, wave

backscatter

1. n. [Geophysics] A crew that acquires a survey or geophysical data. See: jug hustler, observer, party chief, party manager

party

1. n. [Geophysics] A low-frequency electrical current that occurs naturally over large areas at or near the surface of the Earth. Telluric currents are induced by changes in Earth's magnetic field which are usually caused by interactions between the solar wind and the ionosphere (part of the upper atmosphere). See: telluric-current method

telluric current

1. n. [Geophysics] The fastest route that a seismic ray can travel between two points, generally dictated by Fermat's principle. Synonyms: least-time path See: Fermat's principle, minimum-time path, Snell's law

brachistochrone

1. n. [Geophysics] A set of seismometers whose output is sent to a common data channel to record a seismic trace. A large group is known as a patch. See: fold, geophone interval, group interval, patch, seismic trace, seismometer, spread

group

1. n. [Drilling] A term to describe the inclination from vertical of a wellbore. See: deviation survey, inclination

drift

1. n. [Geophysics] Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves. See: borehole seismic data, compressional wave, geophone, ocean-bottom cable, P-wave, shear wave

four-component (4C) seismic data

1. n. [Geophysics] A step in seismic processing to determine the shape of the wavelet, also known as the embedded wavelet, that would be produced by a wave train impinging upon an interface with a positive reflection coefficient. Wavelets may also be extracted by using a model for the reflections in a seismic trace, such as a synthetic seismogram. A wavelet is generated by deconvolving the trace with the set of reflection coefficients of the synthetic seismogram, a process also known as deterministic wavelet extraction. Wavelets may be extracted without a model for the reflections by generating a power spectrum of the data. By making certain assumptions, such as that the power spectrum contains information about the wavelet (and not the geology) and that the wavelet is of a certain phase (minimum, zero), a wavelet may be generated. This is also called statistical wavelet extraction. A particular processing approach to establishing the embedded wavelet is to compare the processed seismic response with the response measured by a vertical seismic profile (VSP) or generated synthetically through a synthetic seismogram in which the embedded wavelet is known. The wavelet can also be extracted through the autocorrelation of the seismic trace, in which case the phase of the wavelet has to be assumed. See: synthetic seismogram, vertical seismic profile

wavelet extraction

1. n. [Geophysics] A step in seismic signal processing to recover high frequencies, attenuate multiples, equalize amplitudes, produce a zero-phase wavelet or for other purposes that generally affect the waveshape. Deconvolution, or inverse filtering, can improve seismic data that were adversely affected by filtering, or convolution that occurs naturally as seismic energy is filtered by the Earth. Deconvolution can also be performed on other types of data, such as gravity, magnetic or well log data. See: autocorrelation, backscatter, deterministic deconvolution, filter, inverse filter, processing, resolution, signature deconvolution, zero-phase

deconvolution

1. n. [Geophysics] A measurable property of seismic data, such as amplitude, dip, frequency, phase, and polarity. Attributes can be measured at one instant in time or over a time window, and may be measured on a single trace, on a set of traces, or on a surface interpreted from seismic data. Attribute analysis includes assessment of various reservoir parameters, including a hydrocarbon indicator, by techniques such as amplitude variation with offset (AVO) analysis. See: complex trace analysis, horizon slice, time-lapse seismic data, velocity

attribute

1. n. [Geophysics] The quality of a medium whose acoustic impedance is constant throughout, such that it contains no seismic reflections. An example of an acoustically transparent medium is water. See: reflection

acoustic transparency

1. n. [Geophysics] A regular spatial arrangement of points, such as x-y coordinates.

grid

1. n. [Geophysics] The product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient. See: acoustic impedance section, acoustic transparency, amplitude anomaly, bright spot, density contrast, dim spot, polarity standard, reflector

acoustic impedance

1. n. [Geophysics] A seismic inversion technique that attempts to describe lithology of individual rock layers and evaluate properties and distribution of pore fluids through analysis of variation of reflected seismic amplitude with offset. See: amplitude variation with offset, inversion, offset

lithostratigraphic inversion

1. n. [Geophysics] A function of time and offset that can be used in seismic processing to compensate for the effects of normal moveout, or the delay in reflection arrival times when geophones and shotpoints are offset from each other. See: geophone, normal moveout, shotpoint

normal-moveout correction

1. n. [Geophysics] A mathematical expression to represent wave displacement and wave velocity (V) as functions of space (x, y, z) and time (t). ∇2ψ = ∂2ψ/∂x2 + ∂2ψ/∂y2 + ∂2ψ/∂z2 = (1/V2) ∂2ψ/∂t2, where ψ = wave displacement V = wave velocity x, y and z = space coordinates t = time. See: back-propagation, diffusion equation, Kirchhoff equation

wave equation

1. n. [Geophysics] The rate at which a wave travels through a medium (a scalar) or the rate at which a body is displaced in a given direction (a vector), commonly symbolized by v. Unlike the physicist's definition of velocity as a vector, its usage in geophysics is as a property of a medium-distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. Velocity can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant velocity (homogeneous) medium. Synonyms: acoustic velocity, seismic velocity See: acoustic, acoustic impedance, angular dispersion, anisotropy, apparent velocity, attribute, average velocity, base of weathering, birefringence, channel wave, check-shot survey, depth conversion, depth migration, discontinuity, dispersion, extensive dilatancy anisotropy, gas chimney, horizon, hydrocarbon indicator, interval velocity, Poisson's ratio, processing, pull-up, push-down, ray tracing, reflection coefficient, reflection tomography, refraction, refractor, root-mean-square velocity, seismic trace, sonic log, stacking velocity, static correction, synthetic seismogram, time migration, tomography, velocity analysis, velocity anomaly, velocity correction, velocity layering, velocity survey, vertical seismic profile, wave, wave equation, wavelength, weathering correction

velocity

1. n. [Geophysics] A graph or curve that displays time versus distance of motion.

hodogram

1. n. [Geophysics] The process of reconstructing a function of time or space from its sinusoidal components determined in Fourier analysis. See: Fourier analysis, Fourier transform, frequency domain, Gibbs' phenomenon, time domain

Fourier synthesis

transverse magnetic mode

1. n. [Geophysics] The duration of the passage of a signal from the source through the Earth and back to the receiver. A time seismic section typically shows the two-way traveltime of the wave. Synonyms: acoustic traveltime See: acoustic log, average velocity, depth map, depth migration, isochron map, receiver, signal, sonic log, source, two-way traveltime, wave

traveltime

1. n. [Geophysics] Naturally occurring (static) electrical potential in the Earth. Spontaneous potentials are usually caused by charge separation in clay or other minerals, by the presence of a semipermeable interface impeding the diffusion of ions through the pore space of rocks, or by natural flow of a conducting fluid (salty water) through the rocks. Variations in SP can be measured in the field and in wellbores to determine variations of ionic concentration in pore fluids of rocks. Alternate Form: SP See: diffusion

spontaneous potential

1. n. [Geophysics] A type of event produced by the radial scattering of a wave into new wavefronts after the wave meets a discontinuity such as a fault surface, an unconformity or an abrupt change in rock type. Diffractions appear as hyperbolic or umbrella-shaped events on a seismic profile. Proper migration of seismic data makes use of diffracted energy to properly position reflections. See: abnormal events, acquisition, arrival, hydrocarbon indicator, Kirchhoff migration, raypath, reflection, time migration, wavefront

diffraction

1. n. [Geophysics] Any compensating factor used to bring measurements to a common datum or reference plane. In gravity surveying, elevation corrections include the Bouguer and free-air corrections. Seismic data undergo a static correction to reduce the effects of topography and low-velocity zones near the Earth's surface. Well log headers include the elevation of the drilling rig's kelly bushing and, for onshore locations, the height of the location above sea level, so that well log depths can be corrected to sea level. See: Bouguer correction, free-air correction, gravity survey, kelly bushing, static correction

elevation correction

seismic processing

1. n. [Geophysics] The comparison of a waveform to itself. Autocorrelation is useful in the identification of multiples or other regularly repeating signals, and in designing deconvolution filters to suppress them. See: correlation, crosscorrelation, deconvolution, multiple reflection, signal

autocorrelation

1. n. [Geophysics] Inductive-source induced polarization. See: induced polarization

IIP

1. n. [Geophysics] In geophysics, analysis of data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists. See: autotrack, check-shot survey, geophysicist, mis-tie, model, phantom, pick, processing, seismic modeling, tie, work station

interpretation

p wave

1. n. [Geophysics] A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal. See: surface wave

guided wave

1. n. [Geophysics] An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters Alternate Form: induced polarization

1. n. [Geophysics] Seismic events whose energy has been reflected once. Multiples, in contrast, are events whose energy has been reflected more than once. A goal of seismic data processing is to enhance primary reflections, which are then interpreted as subsurface interfaces. See: event, long-path multiple, multiple reflection, peg-leg multiple, reflection, short-path multiple, vertical seismic profile

primary reflection

1. n. [Geophysics] A zero-phase wavelet commonly convolved with a reflectivity trace to generate a synthetic seismogram. See: reflection coefficient

Ricker wavelet

1. n. [Geophysics] A series of gravity measurements made along a line or over an area of a locally high topographic feature to remove or compensate for the effect of topography on deeper density readings. See: topographic map

density profile

1. n. [Geophysics] The maximum positive or upward deflection, also known as the crest, of the seismic wavelet. The trough is the maximum negative amplitude or downward deflection of the wave. Seismic interpreters commonly pick or interpret seismic data on paper sections along the trough of a wavelet rather than the normally solid-filled peak for ease of viewing. Synonyms: crest Antonyms: trough See: amplitude, polarity, polarity standard, wave, wavelet

peak

1. n. [Geophysics] Seismic data whose energy source is impulsive and of short duration, as with an air gun, rather than vibratory, as with a vibrator. See: acquisition, air gun, explosive seismic data, vibratory seismic data

impulsive seismic data

1. n. [Geophysics] A method of seismic inversion whereby the effects of rock layers having different seismic characteristics are removed from layers below. See: inversion

layer stripping

induced polarization

1. n. [Geophysics] A type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition. This allows imaging to some distance away from the wellbore. See: acquisition, source, vertical seismic profile

offset vertical seismic profile

1. n. [Geophysics] A seismic line within a 3D survey perpendicular to the direction in which the data were acquired. See: acquisition, in-line, three-dimensional seismic data

crossline

1. n. [Geophysics] The study of the physical characteristics and behavior of rock. Rock mechanics can include analysis of and relationships between properties such as velocity, density, porosity, permeability, shear strength, and bending and crushing behavior, as well as the greater geological context of forces that deform strata and produce geological structures. See: geomechanics

rock mechanics

1. n. [Geophysics] A surveying technique used to identify local, high-velocity features such as salt domes, also called fan shooting. See: salt dome

radial refraction

1. n. [Geophysics] An appearance of seismic data as a diffraction, reflection, refraction or other similar feature produced by an arrival of seismic energy. An event can be a single wiggle within a trace, or a consistent lining up of several wiggles over several traces. An event in a seismic section can represent a geologic interface, such as a fault, unconformity or change in lithology. See: abnormal events, attenuate, bow tie, character, coherence filtering, dim spot, interval time, isochron map, lag, long-path multiple, multiple reflection, peg-leg multiple, phantom, pick, primary reflection, processing, seismic section, short-path multiple, sideswipe, signature, smile, space frequency domain, tuning effect

event

HTI

1. n. [Geology] The deformation that can be recovered when an applied stress has been removed. When the elastic limit of a material has been exceeded, nonrecoverable, permanent deformation occurs. See: elastic limit, plastic deformation, rheology

elastic deformation

1. n. [Geophysics] Abbreviation for amplitude variation with offset. Variation in seismic reflection amplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments. Alternate Form: amplitude variation with offset

AVO

1. n. [Geophysics] An obsolete piece of equipment that converts acoustic signals from analog to electrical form and back. A common use of an acoustic coupler was to provide an interface between a telephone and an early type of computer modem. See: signal

acoustic coupler

1. n. [Geophysics] The study of seismic or elastic waves, such as from earthquakes, explosions or other causes. Interpretation of the structure and composition of the Earth from artificially created seismic waves is a chief concern of seismologists exploring for hydrocarbons and other resources. English physicist John Mitchell (1724 to 1793) is known as the founder of seismology in part because of his observation that one can determine an earthquake's epicenter, or point of origin in the subsurface, by measuring the arrival time of earthquake waves at different locations. The invention of the modern seismograph in 1880 promoted further studies of earthquakes. See: birefringence, earthquake, elastic, reflection tomography, seismograph, shadow zone, wave

seismology

1. n. [Geophysics] A technique for acquiring seismic refraction data around local, high-velocity features such as salt domes by using a fan or arc-shaped geophone array around a central shotpoint. The data from the fan-shaped array are calibrated against a control profile acquired some distance from the anomalous feature. See: anomaly, calibration, geophone array, radial refraction, salt dome

fan shooting

1. n. [Geophysics] A phenomenon of constructive or destructive interference of waves from closely spaced events or reflections. At a spacing of less than one-quarter of the wavelength, reflections undergo constructive interference and produce a single event of high amplitude. At spacing greater than that, the event begins to be resolvable as two separate events. The tuning thickness is the bed thickness at which two events become indistinguishable in time, and knowing this thickness is important to seismic interpreters who wish to study thin reservoirs. The tuning thickness can be expressed by the following formula:Z = VI/2.8 fmax,where Z = tuning thickness of a bed, equal to 1/4 of the wavelengthVI = interval velocity of the targetfmax = maximum frequency in the seismic section.The equation assumes that the interfering wavelets are identical in frequency content and are zero-phase and is useful when planning a survey to determine the maximum frequency needed to resolve a given thickness. Spatial and temporal sampling requirements can then be established for the survey. See: bed thickness, event, resolution, wave

tuning effect

1. n. [Geology] The horizontal displacement between points on either side of a fault, which can range from millimeters to kilometers. Perhaps the most readily visible examples of offset are features such as fences or roads that have been displaced by strike-slip faults, such as the San Andreas fault of California, USA. Synonyms: offset well See: fault, strike-slip fault

offset

1. n. [Geophysics] A type of refraction survey to help define a salt-sediment interface near a wellbore. The source is typically placed directly above the top of a salt dome and the receivers are placed at a number of locations within the borehole. This technique takes advantage of the fact that sound travels faster through the salt than the surrounding soft sediments, such as in the US Gulf Coast. This survey measures the fastest travel path, with part of its path through the salt. The resultant traveltimes are then inverted via a model to obtain a profile of the salt flanks relative to the borehole. See: radial refraction, refraction, salt-proximity vertical seismic profile, source, survey

salt proximity survey

1. n. [Geophysics] The smallest change in a measurement that can be recorded by an instrument.

sensitivity

1. n. [Geophysics] A layer of rock that is sufficiently thick, areally extensive, and has a distinctly higher velocity than the rocks immediately above it such that it can transmit a head wave, or a wave transmitted at its critical incident angle. See: angle of incidence, critical angle, velocity, wave

refractor

1. n. [Geophysics] Seismic data from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4D seismic data), crosswell and VSP data. Synonyms: four-dimensional seismic data See: acquisition, attribute, production, reservoir, secondary recovery, vertical seismic profile

time-lapse seismic data

1. n. [Geophysics] The number of data points or measurements per unit of time or distance. See: sample interval, sample rate

sample frequency

1. n. [Geophysics] Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. See: acquisition, time-lapse seismic data

four-dimensional (4D) seismic data

1. n. [Geophysics] A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth's crust, which is 5 to 75 km [3 to 45 miles] thick. See: DSS, Mohorovicic discontinuity, reflection, refraction

deep seismic sounding

1. n. [Geophysics] The actual leader of a survey crew. The party manager reports to the party chief. See: party, party chief

party manager

1. n. [Geophysics] Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by seismic processing. Synonyms: multiple reflection See: abnormal events, ghost, long-path multiple, short-path multiple

secondary reflection

1. n. [Geophysics] A conventional vertical seismic profile in which the energy source is positioned directly above the receivers, typically very close to the wellbore. Synonyms: vertical seismic profile

zero-offset vertical seismic profile (VSP)

1. vb. [Geophysics] In electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations. Synonyms: sound See: electromagnetic method, magnetotelluric method, profile

probe

dip moveout

1. adj. [Geophysics] Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers. See: distortion, phase, polarity standard, Ricker wavelet, wavelet

zero phase

1. n. [Geophysics] A measure of the anellipticity of the P-wave phase slowness—the inverse of P-wave phase velocity—in rock exhibiting vertical transverse isotropy. η = (ε − δ) / (1 + 2δ) Anellipticity of P-wave phase slowness for a medium in which the elastic properties exhibit vertical transverse isotropy. Eta (η) is the anellipticity and ε and δ are the P-wave anisotropy parameters. When ε and δ are equal, η = 0 and the P-wave phase slowness is an ellipse. When ε = δ = 0, the P-wave phase slowness is isotropic. Reference: Alkhalifah T and Tsvankin I: “Velocity Analysis for Transversely Isotropic Media,” Geophysics 60, no. 5 (September–October 1995): 1550–1566. See: epsilon (ε), delta

eta (η)

1. n. [Geology, Geophysics, Shale Gas] Predictable variation of a property of a material with the direction in which it is measured, which can occur at all scales. For a crystal of a mineral, variation in physical properties observed in different directions is aeolotropy (also known as anisotropy). In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of aeolotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, aeolotropy is common in shales. Synonyms: anisotropy Antonyms: isotropy See: birefringence, extensive dilatancy anisotropy, heterogeneous formation, raypath, seismic velocity, velocity

aeolotropy

DSS

1. n. [Geophysics] The value of the square root of the sum of the squares of the velocity values divided by the number of values, symbolized by vrms. The root-mean-square velocity is that of a wave through subsurface layers of different interval velocity along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity approach equality when source-receiver offset approaches zero and layers are horizontal and isotropic. See: Dix formula, raypath, stacking velocity, velocity

root-mean-square velocity

1. n. [Geophysics] A device designed for use in detecting seismic energy in the form of pressure changes under water during marine seismic acquisition. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. Geophones, unlike hydrophones, detect motion rather than pressure. See: acoustic positioning, acquisition, array, cable, detector, eel, four-component seismic data, geophone, ocean-bottom cable, receiver, seismometer, streamer, three-component seismic data

hydrophone

1. n. [Geophysics] The measurement of gravity or the study of its variations. See: gravimeter, gravity

gravimetry

1. n. [Geophysics] Conventional marine seismic data acquired using a single vessel to tow one or two seismic source arrays in front of a receiver spread. The resulting angle between the source and receivers, is about 20°.

narrow-azimuth seismic data

1. n. [Geophysics] A two-point statistical function that describes the increasing difference or decreasing correlation, or continuity, between sample values as separation between them increases. See: kriging

variogram

1. n. [Geophysics] The path or direction along which wave energy propagates through the Earth. In isotropic media, the raypath is perpendicular to the local wavefront. The raypath can be calculated using ray tracing. Seismic energy travels through media of variable anisotropy and can propagate by diffraction, factors that complicate determination of raypaths. See: angle of approach, angle of incidence, anisotropy, average velocity, diffraction, image, normal incidence, ray, ray tracing, reflection, root-mean-square velocity

raypath

1. n. [Geophysics] A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effect, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator. Antonyms: dim spot See: polarity standard

bright spot

1. n. [Geophysics] The generation of a theoretical equation to define a given dataset. In contrast, curve matching involves the comparison of well-understood data to a dataset of interest. See: curve matching

curve fitting

1. n. [Geophysics] A type of inverse filtering, or deconvolution, in which the effects of the filter are known by observation or assumed, as opposed to statistical deconvolution. See: inverse filter

deterministic deconvolution

1. n. [Geophysics] Generally, a display of entities or properties according to magnitude. In geophysics, spectrum refers to a display of characteristics of a wavetrain or trace as a function of frequency, wavenumber, or arrival time. A common display of spectrum is amplitude as a function of frequency. See: amplitude, arrival, frequency, phase, trace

spectrum

in-line

true amplitude recovery

1. n. [Geophysics] Use of computer software to pick a particular reflection or attribute in seismic data automatically. Autotracking can speed interpretation of three-dimensional seismic data but must be checked for errors, especially in areas of faulting and stratigraphic changes. See: interpretation

autotracking

1. adj. [Geophysics] Pertaining to traces that have a different source but share a receiver. See: trace

common receiver

1. n. [Geophysics] The inability of a system to exactly match input and output amplitude, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier. See: bias, dispersion, distortion, dynamic range, harmonic distortion, zero-phase

amplitude distortion

1. n. [Geophysics] The distance or time between data points or measurements. See: sample frequency, sample rate

sample interval

1. n. [Geophysics] Variation in seismic reflection amplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments. Alternate Form: AVO See: attribute, cementation, converted wave, hydrocarbon indicator, lithostratigraphic inversion, P-wave, processing, seismic modeling

amplitude variation with offset

1. n. [Geophysics] A seismic wave that changes from a P-wave to an S-wave, or vice versa, when it encounters an interface. See: amplitude variation with offset, attenuation, reflection, refraction, SH-wave, SV-wave

converted wave

1. n. [Geophysics] A partial differential equation describing the variation in space and time of a physical quantity that is governed by diffusion. The diffusion equation provides a good mathematical model for the variation of temperature through conduction of heat and the propagation of electromagnetic waves in a highly conducting medium. The diffusion equation is a parabolic partial differential equation whose characteristic form relates the first partial derivative of a field with respect to time to its second partial derivatives with respect to spatial coordinates. It is closely related to the wave equation. ∇2E = j ω μ σ E, where E = electrical field ω = angular frequency μ = magnetic permeability σ = electrical conductivity ∇ = vector differential operator. See: electromagnetic method, wave equation

diffusion equation

1. n. [Geophysics] A comparison, or the location of a comparison, of data. Properly processed and interpreted seismic lines can show good ties, or correlations, at intersection points. See: correlation, interpretation, mis-tie

tie

1. n. [Geophysics] An elastic constant that is a measure of the compressibility of material perpendicular to applied stress, or the ratio of latitudinal to longitudinal strain. This elastic constant is named for Simeon Poisson (1781 to 1840), a French mathematician. Poisson's ratio (σ) can be expressed in terms of properties that can be measured in the field, including velocities of P-waves (VP) and S-waves (VS) as shown below. σ = ½ (VP2 − 2VS2) / (VP2 − VS2) Note that if VS = 0, then Poisson's ratio equals 0.5, indicating either a fluid, because shear waves do not pass through fluids, or a material that maintains constant volume regardless of stress, also known as an ideal incompressible material. Poisson's ratio for carbonate rocks is ~0.3, for sandstones ~0.2, and greater than 0.3 for shale. The Poisson's ratio of coal is ~0.4. See: elastic constants, P-wave, S-wave, velocity

Poisson's ratio

1. n. [Geophysics] A relatively permanent, fixed marker used in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. Synonyms: benchmark See: survey

monument

1. adj. [Geophysics] Pertaining to traces that have a different source but share a receiver. See: trace

common-receiver

1. n. [Geophysics] The distance between sources and receivers, particularly in logging tools. See: normal moveout, receiver, source

spacing

1. n. [Drilling] A particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly. While a dogleg is sometimes created intentionally by directional drillers, the term more commonly refers to a section of the hole that changes direction faster than anticipated or desired, usually with harmful side effects. In surveying wellbore trajectories, a standard calculation of dogleg severity is made, usually expressed in two-dimensional degrees per 100 feet [degrees per 30 m] of wellbore length. There are several difficulties associated with doglegs. First, the wellbore is not located in the planned path. Second is the possibility that a planned casing string may no longer easily fit through the curved section. Third, repeated abrasion by the drillstring in a particular location of the dogleg results in a worn spot called a keyseat, in which the bottomhole assembly components may become stuck as they are pulled through the section. Fourth, casing successfully cemented through the dogleg may wear unusually quickly due to higher contact forces between the drillstring and the inner diameter (ID) of the casing through the dogleg. Fifth, a relatively stiff bottomhole assembly may not easily fit through the dogleg section drilled with a relatively limber BHA. Sixth, excessive doglegs increase the overall friction to the drillstring, increasing the likelihood of getting stuck or not reaching the planned total depth. Usually these problems are manageable. If the dogleg impairs the well, remedial action can be taken, such as reaming or underreaming through the dogleg, or even sidetracking in extreme situations. See: bottomhole assembly, casing string, directional driller, inside diameter, ream, sidetrack, total depth, underream

dogleg

1. n. [Geophysics] A contour map that displays the variation in time between two seismic events or reflections. See: difference map, isochron

isochron map

1. n. [Geophysics] The elapsed time between the release of seismic energy from a source and its arrival at the receiver. See: arrival, event, normal moveout, receiver

arrival time

1. n. [Geophysics] The difference between the maximum displacement of a wave and the point of no displacement, or the null point. The common symbol for amplitude is a. See: amplitude anomaly, attenuation, attribute, bright spot, dilatation, dim spot, filter, footprint, gain, peak, polarity, rarefaction, reflection coefficient, spectral, spectrum, suppression, time slice, trough, true-amplitude recovery, waveform, wavelet, wiggle trace

amplitude

1. n. [Geophysics] A change made to porosity measurements, such as those from sonic logs, to compensate for the lack of compaction, or the predicted loss of pore space as sediments are buried by overburden. Compaction corrections are commonly performed in uncompacted sediments. See: compaction, porosity, sonic log

compaction correction

1. n. [Geophysics] The ratio of or difference between the highest and the lowest reading, or strongest and weakest signal, that can be recorded or reproduced by an instrument without distortion. See: decibel, distortion, signal

dynamic range

1. n. [Geophysics] The ability of a material to support the flow of an electrical current. In linear isotropic materials, the electric current density at any point in space is proportional to the electric field; the constant of proportionality is the electrical conductivity. Conductivity is the inverse of resistivity in isotropic materials, and is measured in siemens per meter or the archaic units of mhos per meter. The electrical conductivity of the Earth can be measured by electromagnetic methods. The electrical conductivity symbol is σ. See: electromagnetic method

electrical conductivity

1. n. [Geophysics] A value added to reflection times of seismic data to compensate for the location of the geophone and source relative to the seismic datum. See: datum, geophone, reflection, source

datum correction

1. n. [Geophysics] A widespread distinctive rock unit that can be correlated readily over a large area. The most useful marker beds tend to form rapidly, such as during volcanic or geologically instantaneous depositional events, and have unusual seismic, magnetic, electrical or other physical properties that aid geological or geophysical interpretation. Coal beds and volcanic ash falls are examples of marker beds. See: correlate, igneous, pick

marker bed

1. n. [Geophysics] Archaic slang for a geophone. Synonyms: geophone, receiver, seismometer See: geophone

jug

1. n. [Geophysics] A field that satisfies the Laplace equation. The Laplace equation is equivalent in three dimensions to the inverse square law of gravitational or electrical attraction (in source-free regions; in regions with sources, it becomes Poisson's equation). Examples of potential fields include the field of the gravity potential and static electric and magnetic fields. See: gravity, Laplace equation, magnetics

potential field

1. n. [Geophysics] The ultimate leader of a survey crew. See: party, party manager

party chief

1. n. [Geophysics] A seismic or acoustic wave, such as a P-wave. See: acoustic emission, acoustic wave, channel wave, P-wave, seismic wave

elastic wave

1. n. [Geophysics] Bubble pulses or bubble noise that affect data quality. In marine seismic acquisition, the gas bubble produced by an air gun oscillates and generates subsequent pulses that cause source-generated noise. Careful use of multiple air guns can cause destructive interference of bubble pulses and alleviate the bubble effect. A cage, or a steel enclosure surrounding a seismic source, can be used to dissipate energy and reduce the bubble effect. See: cavitation

bubble effect

1. n. [Geophysics] A distinguishing feature of a waveform in a seismic event, such as shape, polarity, amplitude, frequency or phase. The signature of the seismic source waveform is of particular interest to geophysicists. Synonyms: character See: event, wave, waveform

signature

1. n. [Geophysics] The exponential rate constant (τ) that determines how quickly the output amplitude of an electrical signal that is under automatic gain control (AGC) responds to a sudden increase or decrease in input signal amplitude. Mathematically, Af(t) = Ai(t) + ΔAi (1 − e−t/τ) where Af is the output signal amplitude, Ai is the input signal amplitude (Ai), ΔAi is the change in input signal amplitude and t is time. When t equals τ, the function (1 − e−t/τ) equals (1 − 1/e) equals 0.63. Therefore, the AGC time constant (τ) is the amount of time that elapses for the output signal of AGC to reflect 63% of the change in the input signal amplitude. See: automatic gain control, AGC

AGC time constant

1. n. [Geophysics] The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. Aliasing can be avoided by sampling at least twice the highest frequency of the waveform or by filtering frequencies above the Nyquist frequency, the highest frequency that can be defined accurately by that sampling interval. See: alias filter, filter, frequency, noise, signal, wave

aliasing

1. n. [Geophysics] Typically an assembly of vertically oriented geophones and hydrophones connected by electrical wires and deployed on the seafloor to record and relay data to a seismic recording vessel. Such systems were originally introduced to enable surveying in areas of obstructions (such as production platforms) or shallow water inaccessible to ships towing seismic streamers (floating cables). Recent developments provide four component (4C) seabed systems to record shear wave (S-wave) as well as P-wave energy. See: acoustic positioning, cable, four-component seismic data, geophone, hydrophone, S-wave, streamer

ocean-bottom cable

1. n. [Geophysics] A set of constants that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. Synonyms: elastic constants, modulus of elasticity See: bulk modulus, elastic, Lamé constant, Poisson's ratio, shear modulus, Young's modulus

elastic moduli

1. n. [Geophysics] A description of the motion of, or means of comparison of, periodic waves such as seismic waves. Waves that have the same shape, symmetry and frequency and that reach maximum and minimum values simultaneously are in phase. Waves that are not in phase are typically described by the angular difference between them, such as, "180 degrees out of phase." Zero-phase wavelets are symmetrical in shape about zero time whereas non-zero-phase wavelets are asymmetrical. Non-zero-phase wavelets are converted to zero-phase wavelets to achieve the best resolution of the seismic data. Known (zero) phase well synthetics and vertical seismic profiles (VSPs) can be compared with local surface seismic data to determine the relative phase of the surface seismic wavelets. Such knowledge allows the surface seismic data to be "corrected" to zero phase. The units of phase are degrees. See: attribute, character, coherence, coherent noise, complex trace analysis, footprint, hydrocarbon indicator, resolution, spectral, spectrum, synthetic seismogram, wavefront, wavelet, zero-phase

phase

1. n. [Geophysics] Analysis of seismic data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists. Synonyms: seismic modeling See: mis-tie, pick

seismic interpretation

1. n. [Geophysics] A type of vertical seismic profile in which the source is moved to progressively farther offset at the surface and receivers are held in a fixed location, effectively providing a mini 2D seismic line that can be of higher resolution than surface seismic data and provides more continuous coverage than an offset VSP. 3D walkaways, using a surface grid of source positions, provide 3D images in areas where the surface seismic data do not provide an adequate image due to near-surface effects or surface obstructions. Walkaway VSPs in which the receivers are placed just above the reservoir are gaining acceptance as a tool to quantify seismic attributes and calibrate surface seismic data. See: offset, receiver, source, vertical seismic profile

walkaway vertical seismic profile (VSP)

CDP

1. adj. [Geophysics] Pertaining to waves of elastic energy, such as that transmitted by P-waves and S-waves, in the frequency range of approximately 1 to 100 Hz. Seismic energy is studied by scientists to interpret the composition, fluid content, extent and geometry of rocks in the subsurface. "Seismic," used as an adjective, is preferable to "seismics," although "seismics" is used commonly as a noun. See: downward continuation, dynamite, elastic, frequency, geophysics, hertz, P-wave, remote sensing, S-wave, survey, wave

seismic

1. n. [Geophysics] The duration of the passage of a signal from the source through the Earth and back to the receiver. A time seismic section typically shows the two-way traveltime of the wave. Synonyms: traveltime See: acoustic log, average velocity, depth map, depth migration, isochron map, receiver, signal, sonic log, source, two-way traveltime, wave

acoustic traveltime

1. n. [Geophysics] A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area. Synonyms: equipotential method

applied-potential method

1. n. [Geophysics] Abbreviation for transverse isotropy. Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry. Alternate Form: transverse isotropy See: axis of rotational symmetry, vertical transverse isotropy, horizontal transverse isotropy

1. n. [Geophysics] A map view of a particular reflection in a 3D seismic survey, as opposed to a horizontal (depth) slice or at a given time (a time slice). Slices are convenient displays for visual inspection of seismic attributes, especially amplitude. See: attribute, three-dimensional survey, time slice

horizon slice

1. n. [Geophysics] A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation. Wavelength is defined as: λ = v / f, where λ = wavelength v = speed of propagation f = frequency. See: absorption, acoustic emission, acoustic log, air wave, aliasing, angle of approach, angle of incidence, apparent velocity, attenuation, average velocity, backscatter, birefringence, body wave, character, coherence, diffraction, displacement, extensive dilatancy anisotropy, Fresnel zone, hertz, image, interval transit time, Love wave, normal incidence, P-wave, peak, plane wave, reflection coefficient, refraction, refractor, S-wave, seismic, seismic wave, seismology, shear, signature, Snell's law, spherical divergence, spherical wave, Stoneley wave, surface wave, transverse electric mode, transverse magnetic mode, traveltime, trough, tuning effect, velocity, velocity correction, vibratory seismic data, wave equation, waveform, wavefront

wave

1. n. [Geophysics] A device that provides energy for acquisition of seismic data, such as an air gun, explosive charge or vibrator. See: air gun, air shooting, downhole source, explosive seismic data, shot depth, shotpoint, spacing, vibratory seismic data, water gun

source

1. n. [Formation Evaluation, Geophysics] A technique used in crosswell seismic and electromagnetic tomography for recording the direct signal from the source or transmitter in one well to the receiver array in another well. This technique is used for mapping the distribution of acoustic velocity and attenuation or electromagnetic resistivity between wells. See: tomography

transmission tomography

1. n. [Geophysics] An alternating or transient electrical current in a conductive medium in the presence of a time-varying magnetic field. The eddy current generates its own electromagnetic field. See: damping, electromagnetic method

eddy current

1. n. [Geophysics] The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing. Synonyms: acquisition See: common midpoint method, explosive seismic data, gather, geophone, hydrophone, shotpoint

seismic acquisition

1. n. [Geophysics] Another term for static correction, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. Alternate Form: near-surface correction, near-surface correction, static correction See: base of weathering, brute stack, differential weathering correction, dynamic correction, elevation correction, first break, moveout, processing, receiver, source, velocity, weathered layer, weathering correction

statics

1. n. [Geophysics] The problem of determining the value or spatial variation of a physical property or feature by comparing measurements to the predictions of a model. For example, seismic traveltimes from a source to a receiver can be used to build a model of seismic velocity in the Earth, or earthquake arrival times can be used to determine the timing and focus (location) of an earthquake. A typical inverse problem in electromagnetics is to determine the variation of electrical conductivity in the Earth from measurements of induced electric and magnetic fields. A forward problem, in contrast, involves taking an assumed model and calculating what the observed values should be, such as the predicting seismic traveltimes between a source and a receiver given a velocity model. See: forward problem

inverse problem

1. n. [Geophysics] An interface between layers of contrasting acoustic, optical or electromagnetic properties. Waves of electromagnetism, heat, light and sound can be reflected at such an interface. In seismic data, a reflector might represent a change in lithology, a fault or an unconformity. A reflector is expressed as a reflection in seismic data. See: acoustic basement, acoustic impedance, common depth point, common midpoint, common reflection point, Fresnel zone, moveout, normal moveout, reflection, tie, two-way traveltime

reflector

1. n. [Geophysics] The apparent source of a received wave. The image is the point in the subsurface that the rays would appear to have come from if they were not reflected, but were shot up from below. A ray that travels from a source and is reflected to a geophone has the same appearance as a ray that travels straight from the image and up to the geophone. See: geophone, raypath, source, wave

image

1. n. [Geophysics] A technique used to estimate the value of a potential field or seismic data at a surface beneath a measured surface. The method is risky because it assumes continuity of the field, so anomalies affect predictions, especially if they occur beneath the measured surface. Noise can be exaggerated and affect calculations adversely. Antonyms: upward continuation See: anomaly, gravity, magnetics, seismic

downward continuation

1. n. [Geophysics] A technique for acquisition of seismic data beneath areas that are difficult to access at the surface of the Earth, such as near rivers, drilling rigs, production platforms, environmentally sensitive areas or around seismically problematic features such as salt domes, which introduce uncertainty because of their high velocity. The sources and receivers are located on opposite sides of the feature. See: acquisition, receiver, source

undershooting

1. n. [Geophysics] Abbreviation for offset vertical seismic profile, a type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition. This allows imaging to some distance away from the wellbore. See: acquisition, source, vertical seismic profile

offset VSP

1. n. [Geophysics] A method of calculating the position of marine seismic equipment. Range measurements are made whereby distance is equal to acoustic signal traveltime from transmitter to hydrophone multiplied by the speed of sound in water. When sufficient acoustic ranges with a proper geometric distribution are collected, location coordinates x, y and z of the marine seismic equipment can be computed by the method of trilateration (measuring the lengths of the sides of overlapping triangles). Acoustic positioning is commonly used in towed streamer and ocean-bottom cable seismic acquisition modes. See: acoustic traveltime, acquisition

acoustic positioning

1. n. [Geophysics] The elapsed time between two seismic events. See: event

interval time

1. n. [Geophysics] A technique for removing noise and emphasizing coherent events from multiple channels of seismic data. Synonyms: coherence filtering See: channel, event, filter

coherent detection

1. n. [Geophysics] A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth's surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data. Synonyms: reflection tomography See: reflection coefficient, tomography, velocity

seismic reflection tomography

1. vb. [Geophysics] To acquire a type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. See: check-shot survey, depth conversion, drift, first break, one-dimensional seismic data, one-way time

shoot a well

1. n. [Geophysics] A layer or body of rock that cannot be detected by seismic refraction, typically because its velocity is lower than that of the overlying rocks; also known as a hidden layer. See: refraction

blind zone

1. n. [Geophysics] A receiver located in a wellbore, as opposed to a location on the Earth's surface. See: geophone, wellbore

downhole receiver

1. n. [Geophysics] A surface marine cable, usually a buoyant assembly of electrical wires that connects hydrophones and relays seismic data to the recording seismic vessel. Multistreamer vessels tow more than one streamer cable to increase the amount of data acquired in one pass. See: acoustic positioning, bird, cable, depth controller, eel, footprint, hydrophone, ocean-bottom cable, streamer feathering, tail buoy

streamer

NRM

1. n. [Geophysics] The range of wavelengths of energy that can be absorbed by a given substance. See: absorption, band, wavelength

absorption band

1. n. [Geophysics] The relationship between voltage (V), electric current (I) and resistance (R), named for German physicist Georg Simon Ohm (1789 to 1854), commonly expressed as the formula below:V/I = R.

Ohm's law

1. n. [Geophysics] A system for acoustic surveying most commonly deployed in marine environments and towed by a ship. The side-scan sonar generates a pulse on the order of 30 to 120 kHz that is reflected from the seafloor. Side-scan sonar records yield an image of the seafloor and shallow sediments. See: acoustic, deep tow, survey

side-scan sonar

1. n. [Geophysics] A processed seismic record that contains traces from a common midpoint that have been added together but has undergone only cursory velocity analysis, so the normal-moveout correction is a first attempt. Typically, no static corrections are made before the brute stack. See: processing, static correction, trace

brute stack

1. n. [Enhanced Oil Recovery, Formation Evaluation, Geophysics] A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. See: tomography

crosswell tomography

1. n. [Geophysics] A technique for removing noise and emphasizing coherent events from multiple channels of seismic data. Alternate Form: coherent detection See: channel, event, filter

coherence filtering

normal moveout

1. adj. [Geophysics] Pertaining to sound waves in the frequency range of 1 to 25 kilohertz. See: acoustic, acoustic log, compaction correction, elastic, frequency, hertz, sonic log, Stoneley wave

sonic

1. n. [Geophysics] The ability of a material to resist or inhibit the flow of an electrical current, measured in ohm-meters. Resistivity is the reciprocal of conductivity. The electrical resistivity symbol is ρ.

electrical resistivity

1. n. [Geophysics] The use of measurements of a field at one elevation, level or surface to determine the values of the field at a higher level. The technique is most often used on potential fields, such as gravity or magnetic fields, to reduce scattered measurements to a common level for a simpler interpretation. Antonyms: downward continuation See: aeromagnetic survey, gravity, magnetics

upward continuation

1. n. [Geophysics] A reference location for a survey, or a survey point whose measured values of a given parameter of interest are understood and can be used to normalize other survey points. Accurate knowledge of base stations is critical in gravity and magnetic surveying. See: baseline, drift, survey, telluric-current method

base station

1. n. [Geophysics] The delay or difference in the arrival time of seismic events that can result from weathering of the rocks or variations in geologic structures in the subsurface. See: arrival, event, weathering

lag

1. n. [Geophysics] A wave that propagates through a medium rather than along an interface. P-waves and S-waves are examples of body waves. See: dispersion, P-wave, S-wave, surface wave

body wave

1. vb. [Geophysics] The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. See: autocorrelation, correlation, lag, waveform

crosscorrelate

1. n. [Geophysics] The loss of information from a magnetic tape that occurs if the tape is damaged or exposed to dirt. See: field tape

dropout

1. n. [Geophysics] The adjustment to a measurement of gravitational acceleration to account for elevation and the density of rock between the measurement station and a reference level. It can be expressed mathematically as the product of the density of the rock, the height relative to sea level or another reference, and a constant, in units of mGal: δgB = 2 π G ρ h = 0.4193 ρ h where δgB = Bouguer correction ρ = rock density in kg/m3 h = height difference between two locations in m G = gravitational constant = 6.67384 × 10−11 m3 kg−1 s−2. Strictly interpreted, the Bouguer correction is added to the known value of gravity at the reference station to predict the value of gravity at the measurement level. The difference between the actual value and the predicted value is the gravity anomaly, which results from differences in density between the actual Earth and reference model anywhere below the measurement station. See: Bouguer anomaly, elevation correction, gravity anomaly

Bouguer correction

1. n. [Geophysics] A technique for acquiring full-azimuth (FAZ) marine seismic data. This technique uses a vessel equipped with source arrays and streamers to shoot and record seismic data; however, unlike conventional surveys acquired in a series of parallel straight lines, circle shooting surveys are acquired as the vessel steams in a series of overlapping, continuously linked circles, or coils. The circular shooting geometry acquires a full range of offset data across every azimuth to sample the subsurface geology in all directions. The resulting FAZ data are used to image complex geology, such as highly faulted strata, basalt, carbonate reefs, and subsalt formations. See: acquisition, salt dome

circle shooting

1. n. [Geophysics] Seismic data or a group of seismic lines acquired individually such that there typically are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line-to-line tying of the seismic data and interpretation and mapping of structures. See: acquisition, correlate, correlation, fault, fold, strike

two-dimensional survey

1. adj. [Geophysics] Pertaining to a spectrum. The spectral content of a wavetrain or wavelet usually refers to its amplitude and phase as a function of frequency. See: amplitude, frequency, phase, wavelet

spectral

1. n. [Geophysics] A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. Synonyms: check-shot survey See: depth conversion, drift, first break, one-dimensional seismic data, one-way time

well shoot

1. n. [Geophysics] The graphical comparison of well-understood datasets, called type curves, to another dataset. If a certain type curve closely corresponds to a dataset, then an interpretation of similarity can be made, although, as Sheriff (1991) points out, there might be other type curves that also match the data of interest. Curve matching differs from curve fitting in that curve fitting involves theoretical models rather than actual examples. See: curve fitting, type curves

curve matching

1. n. [Geophysics] A floating device used in marine seismic acquisition to identify the end of a streamer. Tail buoys allow the seismic acquisition crew to monitor the location and direction of streamers. They are commonly brightly colored, reflect radar signals, and are fitted with Global Positioning System (GPS) receivers. See: acquisition, Global Positioning System, streamer

tail buoy

1. n. [Geophysics] The portion of the seismic wave that contains desirable information. Noise is the undesirable information that typically accompanies the signal and can, to some extent, be filtered out of the data. See: acoustic coupler, acquisition, aliasing, autocorrelation, bel, check-shot survey, coupling, decibel, dynamic range, filter, filter, frequency, inversion, noise, polarity standard, processing, signal-to-noise ratio, Stoneley wave, traveltime, zero crossing

signal

1. n. [Geophysics] A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal. See: surface wave

channel wave

1. n. [Geophysics] The nature of the positive and negative portions of the seismic wavelet, the positive and negative aspects of electrical equipment, or the north and south orientations of magnets and the Earth's magnetic field. See: amplitude, attribute, geomagnetic polarity reversal, peak, polarity standard, wavelet

polarity

1. n. [Geophysics] A method for reconstructing the location and shape of the wave at an earlier time using the wave equation. Alternate Form: back-propagation

back propagation

1. n. [Geophysics] The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value; it can be expressed as: δs = (2/σμω)1/2 = (2/σ)(ε/μ)1/2, where δs = skin depth σ = electrical conductivity ω = 2πf = angular frequency in radians/s f = frequency in Hz μ = μrμ0 = magnetic permeability μr = relative magnetic permeability of the conductor μ0 = relative magnetic permeability of free space = 4π × 10−7 newton per ampere squared (N/A2) ε = εrε0 = dielectric permittivity εr = relative dielectric permittivity of the material ε0 = dielectric permittivity of free space = 8.854 × 10−12 farads per meter (F/m). See: electromagnetic method, permeability

skin depth

1. n. [Geophysics] A modeling technique to assess the geologic history of rock layers through the use of geologic cross sections or seismic sections. Removal of the youngest layers of rock at the top of the section allows restoration of the underlying layers to their initial, undisturbed configurations. Successively older layers can be sequentially removed to further assess the effects of compaction, development of geologic structures, and other processes on an area. Alternate Form: back-stripping

back stripping

1. n. [Geophysics] The Earth's gravitational field, or the attractive force produced by the mass of the Earth. Variations in the gravitational field can be used to map changes in the density of formations in the Earth. Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects. See: anomaly, Bouguer anomaly, Bouguer correction, caprock effect, deep tow, density contrast, density profile, downward continuation, free-air correction, geophysics, gravimeter, gravimetry, gravity anomaly, isostatic correction, potential field, remote sensing, spherical divergence, upward continuation

gravity

1. n. [Geophysics] The portion of the Earth below which strata cannot be imaged with seismic data, or the deepest relatively continuous reflector. Acoustic basement, in some regions, coincides with economic basement and geologic basement, or that portion of the Earth that does not comprise sedimentary rocks. See: basement, reflector, sedimentary, stratum

acoustic basement

1. n. [Geophysics] An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. Alternate Form: fixed-source method See: detector, electromagnetic method, moving-source method, receiver

fixed source method

1. n. [Geophysics] A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity. See: electrical permittivity

dielectric

1. n. [Geophysics] One of two elastic constants named for French mathematician Gabriel Lamé (1795 to 1870). The first Lamé constant is λ, the bulk modulus (K) less two-thirds of the shear modulus (μ): λ = K − (2/3)μ The second Lamé constant is the shear modulus (μ): μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulus τ = Shear stress = ΔF/A ΔF = Increment of shear force A = Area acted on by the shear force γ = Shear strain = ΔL/L ΔL = Increment of transverse displacement parallel to A L = Original length. Lamé constants derived from elastic-wave velocities: λ = ρ(VP2 − 2VS2) μ = ρVS2 λ/μ = (VP/VS)2 − 2, where λ = Lamé's first constant μ = Lamé's second constant, the shear modulus VP = Compressional-wave (P-wave) velocity VS = Shear-wave (S-wave) velocity ρ = Density. See: bulk modulus, elastic constants, shear modulus, P-wave, S-wave

Lamé constant

1. n. [Geophysics] Another term for sequence stratigraphy, a field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time. Sequence stratigraphic study promotes thorough understanding of the evolution of basins, but also allows for interpretations of potential source rocks and reservoir rocks in both frontier areas (having seismic data but little well data) and in more mature hydrocarbon provinces. Prediction of reservoir continuity is currently a key question in mature hydrocarbon provinces where sequence stratigraphy is being applied. The field originated during the 1960s with the study of the stratigraphy of the continental USA, where numerous unconformities could be correlated widely, and led to the proposal that major unconformities might mark synchronous global-scale events. Through sequence stratigraphy, widely-separated sediments that occur between correlatable unconformities could be compared with each other. Studies of outcrops and seismic lines bore out these concepts, which initially were called "Seismic Stratigraphy" and first published widely in 1977. Further study of seismic lines led to the interpretation of the geometry or architecture of seismic events as representing particular styles of sedimentation and depositional environments, and the integration of such interpretations with well log and core data. Because of the simultaneous, competitive nature of the research, numerous oil companies and academic groups use the terminology of sequence stratigraphy differently, and new terms are added continually. See: condensed section, hiatus, onlap, parasequence, sequence boundary, sequence stratigraphy, source rock, systems tract

seismic stratigraphy

1. n. [Geophysics] A bundle of electrical wires that connects geophones and relays data to the seismic recording truck or seismic vessel. See: cable, channel, eel, geophone, hydrophone, jug hustler, ocean-bottom cable, spacer, streamer

geophone cable

1. n. [Geophysics] A shear wave that is polarized so that its particle motion and direction of propagation occur in a vertical plane. See: converted wave, S-wave, SH-wave

SV-wave

1. n. [Geophysics] The acute angle at which a wavefront impinges upon an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell's law describes the relationship between the angle of incidence and the angle of refraction of a wave. See: refraction

angle of approach

1. n. [Geology, Geophysics] Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. See: low velocity layer, pore, static correction, weathering, weathering correction

low-velocity layer

1. n. [Geophysics] A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling, decide where to set casing in a well and evaluate drilling hazards, such as anomalous pore pressure. Alternate Form: drill-noise vertical seismic profile See: acquisition, noise, receiver, reflection, source, vertical seismic profile

drill noise vertical seismic profile

OBC

1. n. [Geophysics] A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be "cut" in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data. See: acquisition, bins, crossline, in-line, migration, two-dimensional seismic data

three-dimensional (3D) seismic data

1. n. [Geophysics] A method of adjusting a dataset against a control that has properties to which the dataset should conform. See: fan shooting

calibration

1. n. [Geophysics] Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer. Alternate Form: TIV, vertical transverse isotropy

vertical transverse isotropy (VTI)

1. n. [Geophysics] The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. See: base of weathering, check-shot survey, first break, static correction, suppression, vertical seismic profile

first arrival

1. n. [Geophysics] A display of seismic data with a scale of units of depth rather than time along the vertical axis. Careful migration and depth conversion are essential for creating depth sections. See: depth conversion, depth migration

depth section

1. n. [Geophysics] The use of frequency (abbreviated as f) and wavenumber (k, the reciprocal of wavelength) as the reference framework, obtained by using the Fourier transform over time and space. See: domain, f-k plot, Fourier transform, space frequency domain, wavelength, wavenumber

f-k domain

1. adj. [Geophysics] Pertaining to seismic events that show continuity from trace to trace. Seismic processing to enhance recognition of coherent events and emphasize discontinuities such as faults and stratigraphic changes has gained popularity since the mid-1990s. See: coherence filtering, discontinuity, event, fault, random noise, stratigraphy

coherent

1. n. [Geophysics] The property of some seismic sources whereby the amplitude, frequency, velocity or other property of the resulting seismic waves varies with direction. A directional charge, such as a length of primer cord or a linear array of charges, can be used when directivity is desirable. Directivity is also a property of geophone arrays, air guns, explosives or vibrators, which can be positioned to reduce horizontal traveling noise such as ground roll. Receivers in the form of groups in which the individual geophones or hydrophones are separated from each other in linear (1D) or areal (2D) arrays are directional, and are designed to suppress signal arriving nearly horizontally and to pass nearly vertical arrivals with minimum attenuation or distortion. Directivity is often present, but the difficulty in accounting for it during seismic processing makes it undesirable in most cases. See: air gun, geophone array, ground roll, hydrophone, primer cord, receiver, seismic processing, shaped charge, source, vibrator, wave

directivity

1. n. [Geophysics] The apparent loss of energy from a wave as it spreads during travel. Spherical divergence decreases energy with the square of the distance. See: divergence, Q, true-amplitude recovery, wave

spherical divergence

1. n. [Geophysics] Abbreviation for vertical transverse isotropy. Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer. Alternate Form: vertical transverse isotropy

TIV

1. n. [Geophysics] A property of a sinusoidal plane wave equal to twice pi divided by the wavelength. Also known as the wavenumber, the propagation constant is fundamental to the mathematical representation of wavefields. It is the spatial equivalent of angular frequency and expresses the increase in the cycle of the wave (measured in radians) per unit of distance. In nondispersive media, the wavespeed is the ratio of the angular frequency to the propagation constant. The propagation vector has magnitude equal to the propagation constant and points in the direction the wave is traveling. See: plane wave, wavenumber

propagation constant

1. n. [Geology] Abbreviation for coal seam gas. Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. Synonyms: coalbed methane, coalbed methane, coalbed methane, CBM Alternate Form: coal seam gas, coal seam gas See: unconventional resource

CSG

1. n. [Geophysics] The unit of measurement of frequency, equivalent to one cycle per second and symbolized by Hz. The unit is named after German physicist Heinrich Hertz (1857 to 1894), who discovered electromagnetic waves. See: acoustic, acoustic, frequency, seismic, sonic, wave

hertz

1. n. [Geophysics] Another term for reflection coefficient, the ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (ρ2V2 − ρ1V1) / (ρ2V2 + ρ1V1), where R = reflection coefficient, whose values range from −1 to +1 ρ1 = density of medium 1 ρ2 = density of medium 2 V1 = velocity of medium 1 V2 = velocity of medium 2. Typical values of R are approximately −1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~0.5 from water to rock; and ~0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations. Synonyms: reflection coefficient See: acoustic impedance, acoustic impedance section, amplitude, density, density contrast, embedded wavelet, impedance, normal incidence, reflection, reflection tomography, Ricker wavelet, synthetic seismogram, velocity, wave, Zoeppritz equations

reflectivity

1. n. [Geophysics] The rate at which a seismic wave travels through a medium, that is, distance divided by traveltime. Seismic velocity can be determined from vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium. See: velocity, velocity survey, wavelength

seismic velocity

1. vb. [Geophysics] Measuring the lateral variation of a property, such as gravity or magnetic fields. Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods. See: gravity, probe

profiling

1. n. [Geophysics] The halfway point between a seismic source and a receiver at the Earth's surface. See: bins, common midpoint, receiver, source

midpoint

1. vb. [Geophysics] Another term for probe, in electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations. Synonyms: probe See: electromagnetic method, magnetotelluric method, profile

sound

1. n. [Geophysics] The director of a seismic acquisition field crew who operates the recording equipment. See: acquisition, party

observer

1. n. [Geophysics] Magnetic interference caused by nearby structures such as metallic rigs and wells. The magnetic permeability of drillstrings and the remanent magnetization in drillstrings contribute to perturbations of the measured magnetic field. Operators may use nonmagnetic drill collars to reduce these effects along with software techniques to compensate for them. See: main magnetic field, crustal magnetic field, external disturbance field

local magnetic interference

1. n. [Geophysics] An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes. See: borehole, drilling mud, fracture, transit time

cycle skip

1. n. [Geophysics] A wave that propagates at the interface between two media as opposed to through a medium. A surface wave can travel at the interface between the Earth and air, or the Earth and water. Love waves and Rayleigh waves are surface waves. See: abnormal events, body wave, dispersion, ground roll, guided wave, Love wave, noise, Rayleigh wave, Stoneley wave, tube wave

surface wave

1. n. [Geophysics] The change in the amplitude of an electrical signal from the original input to the amplified output. See: automatic gain control, true-amplitude recovery

gain

1. n. [Geophysics] A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well. See: deviated hole, vertical seismic profile

walk-above vertical seismic profile (VSP)

1. n. [Geophysics] Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples. See: air wave, multiple reflection, noise, phase, random noise

coherent noise

1. n. [Geophysics] A device used during surveying to measure the acceleration of a ship or aircraft, or to detect ground acceleration in boreholes or on the Earth's surface produced by acoustic vibrations. See: geophone, multicomponent seismic data, receiver, seismometer, survey, three-component seismic data

accelerometer

1. n. [Geophysics] Alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent. Synonyms: seismic processing See: acquisition, amplitude anomaly, amplitude variation with offset, automatic gain control, bias, brute stack, common depth point, common midpoint, common midpoint method, convolution, depth conversion, depth migration, dip moveout, dynamic correction, embedded wavelet, event, filter, frequency, header, interpretation, inversion, lag, migration, normal moveout, peg-leg multiple, quicklook, random noise, resolution, short-path multiple, signature deconvolution, sinc x, slant stack, space frequency domain, stack, static correction, three-dimensional survey, time migration, true-amplitude recovery, velocity, wavelet extraction, work station, zero-phase

processing

geophone array

1. n. [Geophysics] The process of transforming seismic data from a scale of time (the domain in which they are acquired) to a scale of depth to provide a picture of the structure of the subsurface independent of velocity. Depth conversion, ideally, is an iterative process that begins with proper seismic processing, seismic velocity analysis and study of well data to refine the conversion. Acoustic logs, check-shot surveys and vertical seismic profiles can aid depth conversion efforts and improve correlation of well logs and drilling data with surface seismic data. See: acoustic log, check-shot survey, depth map, depth section, pull-up, push-down, seismic processing, seismic section, velocity, vertical seismic profile

depth conversion

1. n. [Geophysics] Frequencies beyond the limits of a filter. Antonyms: band-pass See: band, frequency

band-reject

1. n. [Geophysics] The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. Geophysics plays a critical role in the petroleum industry because geophysical data are used by exploration and development personnel to make predictions about the presence, nature and size of subsurface hydrocarbon accumulations. See: acoustic, gravity, magnetics, seismic, seismic wave

geophysics

magnetotelluric method

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GEOPHYSICS