GEOPHYSICS

1. n. [Geophysics] Surface seismic data acquired using an explosive energy source, such as dynamite. See: acquisition,  impulsive seismic data,  vibratory seismic data

explosive seismic data

1. n. [Geophysics] An inline offset spread.

extended spread

1. n. [Geophysics] A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures. Alternate Form: EDA See: azimuth,  fracture,  wave

extensive dilatancy anisotropy

1. n. [Geophysics] A magnetic disturbance field generated by electric currents flowing in the ionosphere and magnetosphere and “mirror-currents” induced in the Earth and oceans by the external magnetic field time variations. The disturbance field, which is associated with diurnal field variations and magnetic storms, is affected by solar activity (solar wind), the interplanetary magnetic field and the Earth’s magnetic field. The external magnetic field exhibits variations on several time scales, which may affect the applicability of magnetic reference models. Very long-period variations are related to the solar cycle of about 11 years. Short-term variations result from daily changes in solar radiation, atmospheric tides and conductivity. Irregular time variations are influenced by the solar wind. Perturbed magnetic states, called magnetic storms, occur and show impulsive and unpredictable rapid time variations. See: main magnetic field,  crustal magnetic field,  local magnetic interference

external disturbance field

1. n. [Geophysics] A technique for acquiring seismic refraction data around local, high-velocity features such as salt domes by using a fan or arc-shaped geophone array around a central shotpoint. The data from the fan-shaped array are calibrated against a control profile acquired some distance from the anomalous feature. See: anomaly,  calibration,  geophone array,  radial refraction,  salt dome

fan shooting

1. n. [Geophysics] An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. See: FFT,  Fourier transform,  waveform

fast 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,  gravity survey,  survey

Faye correction

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: frequency domain

FD

1. n. [Geophysics] The principle that the path taken by a ray of light from one point to another is that which takes the minimum time (or the maximum time in select cases), named for its discoverer, French mathematician Pierre de Fermat (1601 to 1665). Snell's law and the laws of reflection and refraction follow from Fermat's principle. Fermat's principle also applies to seismic waves. See: least-time path,  seismic wave,  Snell's law,  tomography

Fermat's principle

1. n. [Geophysics] An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. Alternate Form: fast Fourier transform

FFT

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

field tape

1. n. [Geophysics] A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data or to achieve deconvolution. See: alias filter,  aliasing,  aperture,  backscatter,  band,  band-pass,  band-reject,  coherence filtering,  deterministic deconvolution,  Fourier transform,  noise,  processing,  random noise,  signal,  signal-to-noise ratio,  Walsh-Hadamard transform

filter

1. n. [Geophysics] The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. See: base of weathering,  check-shot survey,  first break,  static correction,  suppression,  vertical seismic profile

first arrival

1. n. [Geophysics] The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. See: base of weathering,  check-shot survey,  static correction,  suppression,  vertical seismic profile

first break

1. n. [Geophysics] An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. Alternate Form: fixed-source method See: detector,  electromagnetic method,  moving-source method,  receiver

fixed source method

1. n. [Geophysics] An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. Alternate Form: fixed source method See: detector,  electromagnetic method,  moving-source method,  receiver

fixed-source method

1. n. [Geophysics] The use of frequency (abbreviated as f) and wavenumber (k, the reciprocal of wavelength) as the reference framework, obtained by using the Fourier transform over time and space. See: domain,  f-k plot,  Fourier transform,  space frequency domain,  wavelength,  wavenumber

f-k domain

1. n. [Geophysics] A graphical technique to distinguish subsets of data according to their direction and velocity by plotting and contouring frequency and wavenumber. See: contour,  f-k domain

f-k plot

1. n. [Geophysics] A seismic section that has been redisplayed such that a reflection of interest not horizontal in the original display appears horizontal and flat. Such displays can shed light on geological conditions at the time a given sedimentary layer accumulated. See: seismic section

flattened section

1. n. [Geology] A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. Anticlines are arch-shaped folds in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. Folds typically occur in anticline-syncline pairs. The hinge is the point of maximum curvature in a fold. The limbs occur on either side of the fold hinge. The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial surface is called the axial plane in cases where the fold is symmetrical and the lines containing the points of maximum curvature of the folded layers, or hinge lines, are coplanar. Concentric folding preserves the thickness of each bed as measured perpendicular to original bedding. Similar folds have the same wave shape, but bed thickness changes throughout each layer, with thicker hinges and thinner limbs. See: anomaly,  axial surface,  collision,  competent,  competent,  competent,  competent,  competent,  competent,  concentric fold,  crest,  disharmonic,  drape,  flower structure,  harmonic,  orogeny,  parallel fold,  plunge,  similar fold,  strain,  structural trap,  trend

fold