GEOPHYSICS

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

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] 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] 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] 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] 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 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. 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] 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] 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] 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] 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 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] 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] 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] 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. [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] 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, 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