GEOPHYSICS Flashcards
A single seismic trace.
1-D seismic data
A check-shot survey of a well, which can be used to correct the sonic log and generate a synthetic seismogram that displays changes in amplitude versus traveltime.
1-D seismic data
A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially.
2D seismic data
A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.
2D seismic data
Seismic data or a group of seismic lines acquired individually such that there typically are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line-to-line tying of the seismic data and interpretation and mapping of structures.
2D survey
A type of multicomponent seismic data acquired in a land, marine, or borehole environment by using three orthogonally oriented geophones or accelerometers. 3C is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, as for example, on land. This technique allows determination of both the type of wave and its direction of propagation.
3C seismic data
A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be “cut” in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data.
3D seismic data
The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. A 2D survey commonly contains numerous widely spaced lines acquired orthogonally to the strike of geological structures and a minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the seismic data and interpretation and mapping of structures.
3D survey
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.
4C 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.
4D seismic data
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.
abnormal events
The range of wavelengths of energy that can be absorbed by a given substance.
absoprtion band
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.
absorbing boundary conditions
The ratio of absorbed incident energy to the total energy to which a body is exposed.
absorptance
The property of some liquids or solids to soak up water or other fluids. The natural gas dehydration process uses glycols (liquids) that absorb the water vapor to finally obtain dehydrated gas. In the same way, light oil, also called absorption oil, is used to remove the heavier liquid hydrocarbons from a wet gas stream to obtain dry gas.
absorption
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.
absorption
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.
accelerometer
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.
acoustic
In geophysics, acoustic refers specifically to P-waves in the absence of S-waves (i.e., in fluids, which do not support S-waves, or in cases in which S-waves in solids are ignored).
acoustic
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.
acoustic coupler
A type of elastic wave produced by deformation or brittle failure of material and characterized by relatively high frequency.
acoustic emission
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.
acoustic impedance
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.
acoustic impedance section
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.
acoustic log
A record of some acoustic property of the formation or borehole. The term is sometimes used to refer specifically to the sonic log, in the sense of the formation compressional slowness. However, it may also refer to any other sonic measurement, for example shear, flexural and Stoneley slownesses or amplitudes, or to ultrasonic measurements such as the borehole televiewer and other pulse-echo devices, and even to noise logs.
acoustic log / acoustic velocity log
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.
acoustic positioning
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.
acoustic transparency
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.
acoustic traveltime
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.
acoustic velocity
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.
acoustic velocity log
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.
acoustic wave / dilatational wave
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.
acquisition
The surface or near-surface, unconsolidated sedimentary layer that has been subject to weathering and whose pores are air-filled instead of liquid-filled. An aerated layer typically has a low seismic velocity.
aerated layer
Measurements of the Earth’s magnetic field gathered from aircraft. Magnetometers towed by an airplane or helicopter can measure the intensity of the Earth’s magnetic field. The differences between actual measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type or in thickness of rock units.
aeromagnetic survey
Abbreviation for automatic gain control. 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.
AGC
The exponential rate constant (τ) that determines how quickly the output amplitude of an electrical signal that is under automatic gain control (AGC) responds to a sudden increase or decrease in input signal amplitude. Mathematically, Af(t) = Ai(t) + ΔAi (1 − e−t/τ) , where Af is the output signal amplitude, Ai is the input signal amplitude (Ai), ΔAi is the change in input signal amplitude and t is time. When t equals τ, the function (1 − e−t/τ) equals (1 − 1/e) equals 0.63. Therefore, the AGC time constant (τ) is the amount of time that elapses for the output signal of AGC to reflect 63% of the change in the input signal amplitude.
AGC time constant
A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles.
air gun
A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. Air shooting is also called the Poulter method after American geophysicist Thomas Poulter.
air shooting
A sound wave that travels through the air at approximately 330 m/s and can be generated and recorded during seismic surveying. Air waves are a type of coherent noise.
air wave
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.
alias filter
The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. Aliasing can be avoided by sampling at least twice the highest frequency of the waveform or by filtering frequencies above the Nyquist frequency, the highest frequency that can be defined accurately by that sampling interval.
aliasing
The difference between the maximum displacement of a wave and the point of no displacement, or the null point. The common symbol for amplitude is a.
amplitude
An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator.
amplitude anomaly / bright spot
A change in the amplitude of awaveformthat is generally undesirable, such as inseismicwaves.
amplitude distortion
The inability of a system to exactly match input and output amplitude, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier.
amplitude distortion
The azimuthal variation of the AVO response.
amplitude variation with offset and azimuth / AVOAZ
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.
amplitude variation with offset/ AVO
The acute angle at which a wavefront impinges upon an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell’s law describes the relationship between the angle of incidence and the angle of refraction of a wave.
angle of approach
The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell’s law describes the relationship between the angle of incidence and the angle of refraction of a wave.
angle of incidence
The variation of seismic velocity in different directions.
angular dispersion / seismic velocity
Having directionally dependent properties. For a crystal of amineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales.
anisotropic
Predictable variation of a property of a material with the direction in which it is measured, which can occur at all scales. For a crystal of a mineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales.
anisotropy/ aeolotropy
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.
antialias filter
A portion of a data set, 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.
aperture/ window
A mechanism to limit the affects of measurements on a device or system. In seismic data acquisition, the length of the spread has the effect of an aperture.
aperture/ window
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.
apparent anisotropy
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, whereva = apparent velocityv = velocity of wavefrontθ = angle at which a wavefront approaches the geophone array.
apparent velocity
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, whereλ = wavelengthλa = apparent wavelengthθ = angle at which a wavefront approaches the geophone array.
apparent wavelength
A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area.
applied-potential method
In computing, code written to access data in more than one dimension according to a name and subscripts that correspond to each dimension.
array
A geometrical arrangement of seismic sources (a source array, with each individual source being activated in some fixed sequence in time) or receivers (a hydrophone or geophone array) that is recorded by one channel.
array
An arrangement or configuration of electrodes or antennas used for resistivity, induced polarization (IP), or other types of electromagnetic surveying. Resistivity arrays typically consist of two current electrodes and two potential electrodes and are distinguished by the relative separations between the electrodes. Examples are the dipole-dipole, Schlumberger and Wenner arrays.
array
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.
array/ nest
The elapsed time between the release of seismic energy from a source and its arrival at the receiver.
arrival time
The removal of undesirable features, such as multiple events, from seismic data.
attenuate/ attenuation
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.
attenuation
The reduction in amplitude of an electromagnetic wave passing through the formation, usually measured in decibels/meter, dB/m. The term is used in particular with reference to the propagation resistivity log and the electromagnetic propagation log.
attenuation/ attenuate
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 ahydrocarbon indicator, by techniques such as amplitude variation with offset (AVO) analysis.
attribute
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.
autocorrelation
A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay.
automatic gain control
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.
autotrack
Use of computer software to pick a particularreflectionorattributeinseismicdata automatically. Autotracking can speed interpretation ofthree-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes.
autotracking
Abbreviation for amplitude variation with angle of incidence.
AVA
Abbreviation for amplitude variation with azimuth.
AVAZ
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.
average velocity
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.
AVO / amplitude variation with offset
Abbreviation for amplitude variation with offset and azimuth. The azimuthal variation of the AVO response.
AVOAZ/ amplitude variation with offset and azimuth
An axis of rotational invariance. A material whose properties exhibit cylindrical, or invariant rotational, symmetry may be rotated about this axis by any amount and its properties will be indistinguishable from what they were before the rotation.
axis of rotational symmetry
A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.
back propagation
A modeling technique to assess the geologic history of rock layers through the use of geologic cross sections or seismic sections. Removal of the youngest layers of rock at the top of the section allows restoration of the underlying layers to their initial, undisturbed configurations. Successively older layers can be removed sequentially to further assess the effects of compaction, development of geologic structures and other processes on an area.
back stripping
A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.
back-propagation
A reflection phenomenon of energy in which a nonreflective surface, which is a surface that does not reflect energy coherently, randomly scatters energy. No coherent reflected energy can be identified and the energy is scattered in all directions, including back in the direction from which it came. For example, light can be scattered or redistributed by rough, nonreflective surfaces.
backscatter
A range of frequencies or wavelengths. Audible frequencies of sound and visible wavelengths of light are examples of bands. In seismic data, band-pass frequencies are within the limits of a specific filter, while band-reject frequencies exceed the acceptable range of frequencies.
band
A function or time series whose Fourier transform is restricted to a finite range of frequencies or wavelengths.
band-limited function
Frequencies within the acceptable limits of a filter. The term is commonly used as an adjective, as in “band-pass filter,” to denote a filter that passes a range of frequencies unaltered while rejecting frequencies outside the range.
band-pass
Frequencies beyond the limits of a filter.
band-reject
The lower boundary of the near-surface, low-velocity zone in which rocks are physically, chemically or biologically broken down, in some cases coincident with a water table. Static corrections to seismic data can compensate for the low velocity of the weathered layer in comparison with the higher-velocity strata below.
base of weathering
A reference location for a survey, or a survey point whose measured values of a given parameter of interest are understood and can be used to normalize other survey points. Accurate knowledge of base stations is critical in gravity and magnetic surveying.
base station
A line joining base stations whose transmissions are synchronized during surveying.
baseline
A reference line, such as a “shale baseline,” a line representing the typical value of a given measurement for a shale on a well log, or the zero-amplitude line of a seismic trace.
baseline
The original survey of a set of surveys covering the same area but acquired over a period of time. In four-dimensional seismic data, it is the first seismic survey, which is then compared to subsequent surveys.
baseline
The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation.
basic wavelet/ embedded wavelet
The 0 to 12 scale for measurement of wind strength according to its effect on objects such as trees, flags and water established by Admiral Francis Beaufort (1774 to 1857). According to the Beaufort scale, at wind speeds below 1 knot or 1 km/hr, seas are calm. Whitecaps on water and blowing dust and leaves correspond to a Beaufort number of 4, with winds of 11 to 16 knots [20 to 28 km/hr]. Hurricane-force winds, greater than 64 knots [> 118 km/hr], have a Beaufort number of 12.
Beaufort scale
The unit of measurement to describe or compare the intensity of acoustic or electrical signal, named for American inventor Alexander Graham Bell (1847 to 1922). Measurements are typically given in tenths of a bel, or decibels. The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. Sounds on the order of one decibel are barely audible to humans but can cause pain when on the order of 1012 decibels. The symbol for the unit is B, but dB is the standard unit.
bel
A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation.
benchmark/ BM
A standard against which the performance of processes are measured.
benchmark/ BM
An adjustment of the relative positive and negative excursions of reflections during seismic processing by bulk shifting the null point, or baseline, of the data to emphasize peaks at the expense of troughs or vice versa. Some authors describe bias as a systematic distortion of seismic data to achieve greater continuity.
bias
To sort seismic data into small areas according to the midpoint between the source and the receiver, reflection point or conversion point prior to stacking.
bin
A subdivision of a seismic survey. The area of a three-dimensional survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the source and the receiver, reflection point or conversion point. Bins are commonly assigned according to common midpoint (CMP), but more sophisticated seismic processing allows for other types of binning. Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold.
bin
A device containing a magnetometer and possibly other instruments that can be towed by an aircraft during aeromagnetic surveying or in a marine seismic streamer to provide dynamic information about the streamer position.
bird
