GEOPHYSICS

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

antialias filter

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] 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] 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] 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 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] 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] 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] 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. 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] 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. 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 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] 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. 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. [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. 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. 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] 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