GEOPHYSICS

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

absorptance

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 range of wavelengths of energy that can be absorbed by a given substance. See: absorption,  band,  wavelength

absorption band

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. 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] 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 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] 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] 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 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] 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] 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 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] 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] 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] 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] 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: dilatational wave See: body wave,  dilatation,  elastic,  rarefaction,  shadow zone,  wave

acoustic wave

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