GEOPHYSICS

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

BM

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. 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] 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] 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 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] 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 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] 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] 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. [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 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 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] 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