GEOPHYSICS 3 Flashcards
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.
Faye correction
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.
FD/ Frequency domain
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.
Fermat’s principle
An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly.
FFT/ fast fourier transform
A magnetic tape containing data recorded in the field, abbreviated FT.
field tape
To remove undesirable portions of data during seismic processing to increase the signal-to-noise ratio of seismic data. Filtering can eliminate certain frequencies, amplitudes or other information.
filter
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.
filter
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.
first break/ first arrival
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.
fixed-source method
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.
f-k domain
A graphical technique to distinguish subsets of data according to their direction and velocity by plotting and contouring frequency and wavenumber.
f-k plot
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.
flattened section
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.
fold
A measure of the redundancy of common midpoint seismic data, equal to the number of offset receivers that record a given data point or in a given bin and are added during stacking to produce a single trace. Typical values of fold for modern seismic data range from 60 to 240 for 2D seismic data, and 10 to 120 for 3D seismic data. The fold of 2D seismic data can be calculated by dividing the number of seismometer groups by twice the number of group intervals between shotpoints.
fold
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.
forward problem
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.
four-component seismic data
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.
four-dimensional seismic data
A mathematical algorithm designed by geometrician and physicist Baron J.B.J. Fourier to determine the frequency distribution within a wave pattern as a series of sine waves. Fourier analysis is also used to study any series of repeated signals or patterns. This analysis is sometimes used to study patterns in images such as thin sections, and in geostatistics and log analysis.
Fourier analysis
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.
Fourier analysis
The process of reconstructing a function of time or space from its sinusoidal components determined in Fourier analysis.
Fourier synthesis
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.
free-air correction
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.
frequency
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.
frequency domain / FD
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.
Fresnel zone
A magnetic tape containing data recorded in the field, abbreviated FT
FT (Field tape)
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.
FT (Fourier transform)
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-steamer acquisition
The change in the amplitude of an electrical signal from the original input to the amplified output.
gain
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 − VSHˆ¥2) / VSHˆ¥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 VSHˆ¥ is the velocity for a horizontally polarized and vertically traveling S-wave.
gamma
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.
gas chimney
A display of seismic traces that share an acquisition parameter, such as a common midpoint gather, which contains traces having a common midpoint.
gather
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.
geomagnetic secular variation
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.
geometric
A geometrical arrangement ofseismicreceivers (geophones) with signals recorded by onechannel. The array can contain numerous closely spaced geophones.
geophone array
A bundle of electrical wires that connects geophones and relays data to theseismicrecording truck or seismic vessel.
geophone cable
The distance between geophones or the centers of groups of geophones.
geophone interval/ group interval
A geometrical arrangement ofseismicreceivers (geophones) with signals recorded by onechannel. The array can contain numerous closely spaced geophones.
geophone pattern
A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion.
geophone/ jug/ receiver/ seismometer
A scientist trained in the study of the physics of the Earth, particularly its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. In the petroleum industry, geophysicists perform a variety of functions, chiefly the processing and interpretation of seismic data and generation of subsurface maps on the basis of seismic data. Such interpretations enhance understanding of subsurface geology.
geophysicist
The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. Geophysics plays a critical role in the petroleum industry because geophysical data are used by exploration and development personnel to make predictions about the presence, nature and size of subsurface hydrocarbon accumulations.
geophysics
