GEOPHYSICS

1. n. [Geophysics] The area covered by an array of towed streamers in marine seismic acquisition. See: acquisition,  array,  streamer

footprint

1. n. [Geophysics] The practice of taking a model and calculating what the observed values should be, such as predicting the gravity anomaly around a salt dome using a gravity model or predicting the traveltime of a seismic wave from a source to a receiver using a velocity model. See: gravity anomaly,  inverse problem,  salt dome

forward problem

1. n. [Geophysics] Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves. See: borehole seismic data,  compressional wave,  geophone,  ocean-bottom cable,  P-wave,  shear wave

four-component (4C) seismic data

1. n. [Geophysics] Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. See: acquisition,  time-lapse seismic data

four-dimensional (4D) seismic data

1. n. [Geophysics] The process of decomposing a function of time or space into a sum (or integral) of sinusoidal functions (sines or cosines) with specific amplitudes and phases. See: Fourier transform,  frequency domain,  Gibbs' phenomenon,  sinusoid,  time domain

Fourier analysis

1. n. [Geophysics] The process of reconstructing a function of time or space from its sinusoidal components determined in Fourier analysis. See: Fourier analysis,  Fourier transform,  frequency domain,  Gibbs' phenomenon,  time domain

Fourier synthesis

1. n. [Geophysics] A set of mathematical formulas used to convert a time function, such as a seismic trace, to a function in the frequency domain (Fourier analysis) and back (Fourier synthesis). The function is expressed as a convergent trigonometric series, similar to that first formulated by French mathematician Jean-Baptiste-Joseph, Baron Fourier (1768 to 1830). The Fourier transform is used extensively in signal processing to design filters and remove coherent noise. Many filtering operations are performed in the frequency domain. The Fourier transform has applications in image analysis and in pattern recognition in geological systems. See: band-limited function,  fast Fourier transform,  Fourier analysis,  Fourier synthesis,  frequency domain,  FT,  seismic trace,  sinc x,  Walsh-Hadamard transform

Fourier transform

1. n. [Geophysics] In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. See: anomaly,  elevation correction,  gravity anomaly,  survey

free air correction

1. n. [Geophysics] In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. See: anomaly,  elevation correction,  gravity anomaly,  gravity survey,  survey

free-air correction

1. n. [Geophysics] The rate of repetition of complete wavelengths of electrical signals, light, sound and seismic waves measured in cycles per second, or hertz, and symbolized by f. Typical recorded seismic frequencies are in the range of 5 to 100 hertz. See: acoustic,  acoustic emission,  aliasing,  attribute,  band,  band-limited function,  band-pass,  band-reject,  character,  complex trace analysis,  correlation,  dispersion,  f-k domain,  f-k plot,  filter,  mute,  natural frequency,  processing,  signal,  sonic,  space frequency domain,  spectral,  spectrum,  vibratory seismic data,  wave,  wavelength,  wavelet

frequency

1. n. [Geophysics] In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency. Alternate Form: FD See: Fourier analysis,  Fourier synthesis,  Fourier transform,  seismic survey,  time domain

frequency domain

1. n. [Geophysics] A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break, named for French physicist Augustin-Jean Fresnel (1788 to 1827). Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves. See: attenuation,  first break,  resolution,  wave

Fresnel zone

1. n. [Geophysics] A magnetic tape containing data recorded in the field, abbreviated FT. Alternate Form: field tape

FT

1. n. [Geophysics] A single-vessel technique of acquiring marine seismic data at a complete range of azimuths by towing streamers in a circular path.

full-azimuth towed-streamer acquisition

1. n. [Geophysics] The change in the amplitude of an electrical signal from the original input to the amplified output. See: automatic gain control,  true-amplitude recovery

gain

1. n. [Geophysics] An S-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Gamma (γ) is the S-wave anisotropy parameter and is equal to half the ratio of the difference between the horizontally and vertically traveling SH-wave velocities squared divided by the vertically traveling SH-wave velocity squared; an SH-wave is a shear wave that is horizontally polarized. γ ≡ ½ [(C66 − C44) ∕ C44] = ½ [(VSH⊥2 − VSH∥2) / VSH∥2] S-wave parameter (γ) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C66 is the modulus for a horizontally polarized and horizontally traveling S-wave (perpendicular to the symmetry axis), C44 is the modulus for a horizontally polarized and vertically traveling S-wave (parallel to the symmetry axis), VSH⊥ is the velocity for a horizontally polarized and horizontally traveling S-wave and VSH∥ is the velocity for a horizontally polarized and vertically traveling S-wave. Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966. See: epsilon (ε),  delta,  eta (η)

gamma (γ)

1. n. [Geophysics] A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs. See: hydrocarbon indicator,  push-down,  velocity

gas chimney

1. n. [Geophysics] A display of seismic traces that share an acquisition parameter, such as a common midpoint gather, which contains traces having a common midpoint. See: common midpoint,  seismic trace,  stacking velocity,  trace

gather

1. n. [Geophysics] How Earth’s magnetic field varies with time. These time variations, called secular variations, necessitate periodic updating of magnetic field maps and models. Two types of processes in the Earth’s core produce these variations. One process is related to variations in Earth’s main dipole field, which operate on time scales of hundreds or thousands of years. The other process is related to variations in Earth’s nondipole field, which operate on time scales on the order of tens of years. See: main magnetic field,  dipole field,  nondipole field

geomagnetic secular variation

1. adj. [Geophysics] Pertaining to variation of the survey geometry while maintaining the frequency of electromagnetic surveying. In contrast, parametric pertains to keeping frequency the same while varying the geometry. See: electromagnetic method,  parametric

geometric