GEOPHYSICS

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] 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] 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 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 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] 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] 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] A graph or curve that displays time versus distance of motion.

hodogram

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 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] 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] 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,  TIH

HTI

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 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] Inductive-source induced polarization. See: induced polarization

IIP

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] 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. [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 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 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

incident angle