Seismic Data Types: 1D, 2D, 3C, and 3D

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Questions and Answers

What is 1-D seismic data?

A single seismic trace.

What is 1-D seismic data used for in well surveys?

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.

How is 2D seismic data acquired?

A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.

Describe a 2D survey.

<p>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. 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.</p> Signup and view all the answers

What are abnormal events in seismic data?

<p>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.</p> Signup and view all the answers

What is an absorption band?

<p>The range of wavelengths of energy that can be absorbed by a given substance.</p> Signup and view all the answers

What are absorbing boundary conditions?

<p>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.</p> Signup and view all the answers

What is absorptance?

<p>The ratio of absorbed incident energy to the total energy to which a body is exposed.</p> Signup and view all the answers

What is an accelerometer?

<p>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.</p> Signup and view all the answers

What does acoustic refer to?

<p>Pertaining to sound. Generally describes sound or vibrational events, regardless of frequency. In geophysics, 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).</p> Signup and view all the answers

What is an acoustic coupler?

<p>An obsolete piece of equipment that converts acoustic signals from analog to electrical form and back. A common was to provide an interface between a telephone and an early type of computer modem.</p> Signup and view all the answers

What is acoustic emission?

<p>A type of elastic wave produced by deformation or brittle failure of material and characterized by relatively high frequency.</p> Signup and view all the answers

What is acoustic impedance?

<p>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.</p> Signup and view all the answers

What is an acoustic impedance section?

<p>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.</p> Signup and view all the answers

What is an acoustic log?

<p>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.</p> Signup and view all the answers

What is another name for an acoustic log?

<p>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.</p> Signup and view all the answers

What is acoustic positioning?

<p>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.</p> Signup and view all the answers

What is acoustic transparency?

<p>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.</p> Signup and view all the answers

What is acoustic traveltime?

<p>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.</p> Signup and view all the answers

What is acoustic velocity?

<p>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.</p> Signup and view all the answers

Describe the acquisition of seismic data.

<p>The generation and recording of seismic data. It 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.</p> Signup and view all the answers

What is an aerated layer?

<p>The surface or near-surface, unconsolidated sedimentary layer that has been subject to weathering and whose pores are air-filled instead of liquid-filled. It typically has a low seismic velocity.</p> Signup and view all the answers

What is an aeromagnetic survey?

<p>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.</p> Signup and view all the answers

What is an AGC time constant?

<p>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.</p> Signup and view all the answers

What is an air gun?

<p>A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. It are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles.</p> Signup and view all the answers

What is air shooting?

<p>A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. It is also called the Poulter method after American geophysicist Thomas Poulter.</p> Signup and view all the answers

What is an alias filter?

<p>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.</p> Signup and view all the answers

What is aliasing?

<p>The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. It 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.</p> Signup and view all the answers

What is amplitude?

<p>The difference between the maximum displacement of a wave and the point of no displacement, or the null point.</p> Signup and view all the answers

What is an amplitude anomaly?

<p>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. It 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.</p> Signup and view all the answers

What is amplitude distortion?

<p>A change in the amplitude of a waveform that is generally undesirable, such as in seismic waves.</p> Signup and view all the answers

What is another definition of amplitude distortion?

<p>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.</p> Signup and view all the answers

What is AVOAZ?

<p>The azimuthal variation of the AVO response.</p> Signup and view all the answers

What is the angle of approach?

<p>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.</p> Signup and view all the answers

What is the angle of incidence?

<p>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.</p> Signup and view all the answers

What is angular dispersion?

<p>The variation of seismic velocity in different directions.</p> Signup and view all the answers

What does anisotropic mean?

<p>Having directionally dependent properties. 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.</p> Signup and view all the answers

What is anisotropy?

<p>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.</p> Signup and view all the answers

What is an aperture?

<p>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.</p> Signup and view all the answers

What else can aperture refer to?

<p>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.</p> Signup and view all the answers

What is apparent anisotropy?

<p>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. It 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.</p> Signup and view all the answers

What is apparent velocity?

<p>In geophysics, the velocity of a wavefront in a certain direction, typically measured along a line of receivers and symbolized by va. blalbla 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.</p> Signup and view all the answers

What is the applied-potential method?

<p>A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area.</p> Signup and view all the answers

What is apparent wavelength?

<p>The wavelength measured by receivers when a wave approaches at an angle.</p> Signup and view all the answers

What is an array in computing?

<p>In computing, code written to access data in more than one dimension according to a name and subscripts that correspond to each dimension.</p> Signup and view all the answers

What is another definition of array?

<p>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.</p> Signup and view all the answers

What is another name for an array?

<p>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.</p> Signup and view all the answers

What is arrival time?

<p>The elapsed time between the release of seismic energy from a source and its arrival at the receiver.</p> Signup and view all the answers

What does attenuate mean?

<p>The removal of undesirable features, such as multiple events, from seismic data.</p> Signup and view all the answers

What is attenuation?

<p>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.</p> Signup and view all the answers

What else does attenuation refer to?

<p>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.</p> Signup and view all the answers

What is autocorrelation?

<p>The comparison of a waveform to itself. It is useful in the identification of multiples or other regularly repeating signals, and in designing deconvolution filters to suppress them.</p> Signup and view all the answers

What is automatic gain control?

<p>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. It is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay.</p> Signup and view all the answers

What does autotrack mean?

<p>To use computer software to pick a particular reflection or attribute in seismic data automatically. it can speed interpretation of three-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes.</p> Signup and view all the answers

What is average velocity?

<p>In geophysics, the depth divided by the traveltime of a wave to that depth. It 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.</p> Signup and view all the answers

What is the axis of rotational symmetry?

<p>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.</p> Signup and view all the answers

What is back propagation?

<p>A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.</p> Signup and view all the answers

What is back stripping?

<p>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.</p> Signup and view all the answers

What is backscatter?

<p>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.</p> Signup and view all the answers

What is a band-limited function?

<p>A function or</p> Signup and view all the answers

What can 1-D seismic data be used for?

<p>To conduct 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.</p> Signup and view all the answers

What is another definition of 2D seismic data?

<p>A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.</p> Signup and view all the answers

What is a 2D survey?

<p>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. 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.</p> Signup and view all the answers

What does acoustic mean?

<p>Relating to sound. Generally describes sound or vibrational events, regardless of frequency. In geophysics, 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).</p> Signup and view all the answers

Define Acoustic impedance.

<p>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.</p> Signup and view all the answers

Alternatively, what is an acoustic log/acoustic velocity log?

<p>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.</p> Signup and view all the answers

Define acoustic traveltime.

<p>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.</p> Signup and view all the answers

Define acoustic velocity.

<p>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.</p> Signup and view all the answers

What is an acoustic wave / dilatational wave?

<p>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.</p> Signup and view all the answers

What is acquisition?

<p>The generation and recording of seismic data. It 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.</p> Signup and view all the answers

What is an amplitude anomaly / bright spot?

<p>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. It 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.</p> Signup and view all the answers

Define amplitude distortion.

<p>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.</p> Signup and view all the answers

What is amplitude variation with offset and azimuth / AVOAZ?

<p>The azimuthal variation of the AVO response.</p> Signup and view all the answers

What is amplitude variation with offset/ AVO?

<p>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. This 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.</p> Signup and view all the answers

What is angular dispersion / seismic velocity?

<p>The variation of seismic velocity in different directions.</p> Signup and view all the answers

What is anisotropy/ aeolotropy?

<p>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.</p> Signup and view all the answers

What is aperture/ window?

<p>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.</p> Signup and view all the answers

Alternatively, what is aperture/ window?

<p>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.</p> Signup and view all the answers

Define an array.

<p>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.</p> Signup and view all the answers

Define array/ nest.

<p>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.</p> Signup and view all the answers

What does attenuate/ attenuation mean?

<p>The removal of undesirable features, such as multiple events, from seismic data.</p> Signup and view all the answers

Define attenuation/ attenuate.

<p>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.</p> Signup and view all the answers

What does AVA stand for?

<p>Abbreviation for amplitude variation with angle of incidence.</p> Signup and view all the answers

What is AVO / amplitude variation with offset?

<p>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. this analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks.</p> Signup and view all the answers

What is AVOAZ/ amplitude variation with offset and azimuth?

<p>Abbreviation for amplitude variation with offset and azimuth. The azimuthal variation of the AVO response.</p> Signup and view all the answers

What is another use for 1-D seismic data?

<p>A check-shot survey of a well. It can be used to correct the sonic log and generate a synthetic seismogram that displays changes in amplitude versus traveltime.</p> Signup and view all the answers

What is another way to define 2D seismic data?

<p>A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.</p> Signup and view all the answers

What is a 2D seismic survey?

<p>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. 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.</p> Signup and view all the answers

What does acoustic mean in the context of geophysics?

<p>Pertaining to sound. Generally describes sound or vibrational events, regardless of frequency. In geophysics. 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).</p> Signup and view all the answers

What is another term for an acoustic velocity log?

<p>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.</p> Signup and view all the answers

What is an acoustic wave, also known as a dilatational wave?

<p>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.</p> Signup and view all the answers

What is seismic acquisition?

<p>The generation and recording of seismic data. It 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.</p> Signup and view all the answers

What is air shooting, also known as the Poulter method?

<p>A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. It is also called the Poulter method after American geophysicist Thomas Poulter.</p> Signup and view all the answers

What is an amplitude anomaly, also known as a bright spot?

<p>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. It 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.</p> Signup and view all the answers

Give another definition of amplitude distortion?

<p>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.</p> Signup and view all the answers

What is amplitude variation with offset and azimuth, commonly known as AVOAZ?

<p>The azimuthal variation of the AVO response.</p> Signup and view all the answers

What is amplitude variation with offset, commonly known as AVO?

<p>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. This 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 (&quot;fizz water&quot;). 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.</p> Signup and view all the answers

Define angular dispersion related to seismic velocity.

<p>The variation of seismic velocity in different directions.</p> Signup and view all the answers

What does the term anisotropic mean?

<p>Having directionally dependent properties. 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.</p> Signup and view all the answers

What is anisotropy, also known as aeolotropy?

<p>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.</p> Signup and view all the answers

What is an aperture, also known as a window?

<p>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.</p> Signup and view all the answers

Give another definition of aperture, also known as a window?

<p>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.</p> Signup and view all the answers

What is an array in seismic surveying?

<p>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.</p> Signup and view all the answers

What is an array in resistivity, induced polarization (IP), or other types of electromagnetic surveying?

<p>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.</p> Signup and view all the answers

What does it mean to attenuate or demonstrate attenuation?

<p>The removal of undesirable features, such as multiple events, from seismic data.</p> Signup and view all the answers

What does attenuation/ attenuate mean in the context of electromagnetic waves?

<p>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.</p> Signup and view all the answers

What is an attribute in the context of seismic data?

<p>A measurable property of seismic data, such as amplitude, dip, frequency, phase and polarity. It 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.</p> Signup and view all the answers

What does it mean to autotrack?

<p>To use computer software to pick a particular reflection or attribute in seismic data automatically. it can speed interpretation of three-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes.</p> Signup and view all the answers

Flashcards

1-D seismic data

A single seismic trace.

2D seismic data

A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially.

3D seismic data

Numerous closely-spaced seismic lines providing a high spatially sampled measure of subsurface reflectivity.

4D seismic data

Three-dimensional (3D) seismic data acquired at different times over the same area.

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Abnormal events

Features in seismic data other than reflections, including diffractions, multiples, refractions, and surface waves.

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Acoustic impedance

The product of density and seismic velocity; affects the reflection coefficient.

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Acoustic wave

An elastic body or sound wave in which particles oscillate in the direction the wave propagates.

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Acquisition

The generation and recording of seismic data.

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Aeromagnetic survey

Measurements of the Earth's magnetic field gathered from aircraft.

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Air gun

A source of seismic energy used in marine seismic data acquisition, releasing compressed air into water.

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Aliasing

The distortion of frequency introduced by inadequately sampling a signal.

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Amplitude

The difference between the maximum displacement of a wave and the point of no displacement.

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Amplitude variation with offset/ AVO

Variation in seismic reflection amplitude with change in distance between shotpoint and receiver.

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Anisotropic

Having directionally dependent properties.

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Arrival time

The elapsed time between the release of seismic energy from a source and its arrival at the receiver.

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Attenuation

The loss of energy or amplitude of waves as they pass through media.

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Attribute

A measurable property of seismic data, such as amplitude, dip, frequency, phase and polarity.

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Autocorrelation

The comparison of a waveform to itself.

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amplitude variation with offset and azimuth

The azimuthal variation of the AVO response.

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automatic gain control

A system to control the gain automatically.

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average velocity

The depth divided by the traveltime of a wave to that depth.

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back propagation

A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.

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backscatter

A reflection phenomenon of energy in which a nonreflective surface, which is a surface that does not reflect energy coherently, randomly scatters energy.

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Bin

Seismic data is sorted into small areas prior to stacking.

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Study Notes

  • 1-D Seismic Data: Is a single seismic trace, or a check-shot survey of a well used to correct sonic logs and generate synthetic seismograms displaying amplitude changes versus traveltime.
  • 2D Seismic Data: Is a vertical section of seismic data with adjacent traces acquired sequentially, or a group of 2D seismic lines acquired individually.
  • 2D Survey: Involves seismic data acquired individually with significant gaps (1 km or more) between lines, typically containing lines orthogonal to geological structures and a minimum of lines parallel to allow line-to-line tying and structural interpretation.
  • 3C Seismic Data: Multicomponent seismic data acquired using three orthogonally oriented geophones or accelerometers in land, marine, or borehole environments.
  • 3D Seismic Data: Provides a high spatially sampled measure of subsurface reflectivity from closely-spaced seismic lines, which can be "cut" in any direction while displaying a well-sampled seismic section; original lines are in-lines, and perpendicular lines are crosslines.
  • 3D Survey: Is the acquisition of seismic data with closely spaced receiver and shot lines, often with lines orthogonal to geological structures and a minimum of lines parallel to geological structures.
  • 4C Seismic Data: Borehole or marine seismic data acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor, allowing measurement of shear (S) waves with geophones and compressional (P) waves with the hydrophone.
  • 4D Seismic Data: 3D seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir, such as fluid location, saturation, pressure, and temperature, on the surface or in a borehole.
  • Abnormal Events: Features in seismic data other than reflections, including diffractions, multiples, refractions, and surface waves.
  • Absorption Band: The range of wavelengths of energy that can be absorbed by a given substance.
  • Absorbing Boundary Conditions: Algorithm used in numerical simulation to absorb all energy incident upon a boundary and to suppress reflection artifacts.
  • Absorptance: The ratio of absorbed incident energy to the total energy a body is exposed to.
  • Absorption: The property of liquids or solids to soak up water or other fluids, or the conversion of one form of energy into another as it passes through a medium.
  • Accelerometer: A device used to measure acceleration in ships, aircraft, boreholes, or on the Earth's surface, produced by acoustic vibrations.
  • Acoustic: Pertaining to sound, generally describing vibrational events. In geophysics, specifically refers to P-waves in the absence of S-waves or in cases where S-waves in solids are ignored.
  • Acoustic Coupler: Obsolete equipment converting acoustic signals from analog to electrical form and back.
  • Acoustic Emission: An elastic wave produced by material deformation or brittle failure, characterized by relatively high frequency.
  • Acoustic Impedance: The product of density and seismic velocity (Z), which varies among rock layers and affects the reflection coefficient.
  • Acoustic Impedance Section: A seismic reflectivity section, or a 2D or 3D seismic section, inverted for acoustic impedance, calibrated using sonic and density logs.
  • Acoustic Log: A display of traveltime of acoustic waves versus depth in a well or also referred to as sonic log.
  • Acoustic Positioning: Method to calculate the position of marine seismic equipment using range measurements, where distance equals acoustic signal traveltime multiplied by the speed of sound in water.
  • Acoustic Transparency: Quality of a medium with constant acoustic impedance, containing no seismic reflections, such as water.
  • Acoustic Traveltime: The duration of signal passage from the source through the Earth and back to the receiver, with time seismic sections typically showing two-way traveltime.
  • Acoustic Velocity: The rate at which a sound wave travels through a medium (distance divided by traveltime), determined from laboratory measurements, acoustic logs, vertical seismic profiles, or velocity analysis of seismic data, that varies vertically, laterally, and azimuthally.
  • Acoustic Wave / Dilatational Wave: An elastic body wave in which particles oscillate in the direction of wave propagation (P-waves), which can produce reflected and transmitted S-waves (converted waves) when incident on an interface at other than normal incidence.
  • Acquisition: The generation and recording of seismic data that involves placing geophones or seismometers, towing hydrophones, suspending hydrophones, or placing geophones in a wellbore to record the seismic signal.
  • Aerated Layer: A surface or near-surface, unconsolidated sedimentary layer subject to weathering with air-filled pores, typically having a low seismic velocity.
  • Aeromagnetic Survey: Measurements of the Earth's magnetic field gathered from aircraft, where differences between actual and theoretical measurements indicate anomalies representing changes in rock type or thickness.
  • AGC Time Constant: Indicates 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.
  • Air Gun: A source of seismic energy used in marine seismic data acquisition, and in water-filled pits on land during vertical seismic profiles.
  • Air Shooting: Was a method of seismic acquisition using charges detonated in the air or on poles above the ground as the source.
  • Air Wave: A sound wave traveling through the air at approximately 330 m/s, generated and recorded during seismic surveying.
  • Alias Filter: A filter used to eliminate unwanted portions of seismic data spectra, removing frequencies that might cause aliasing during sampling of an analog signal or when decreasing the sample rate of digital data.
  • Aliasing: Distortion of frequency introduced by inadequately sampling a signal, resulting in ambiguity between signal and noise, which can be avoided by sampling at least twice the highest frequency or filtering frequencies above the Nyquist frequency.
  • Amplitude: The difference between the maximum displacement of a wave and the point of no displacement.
  • Amplitude Anomaly / Bright Spot: An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, but can also result from processing problems, geometric or velocity focusing, or changes in lithology; sudden changes in acoustic impedance causes hydrocarbon indication.
  • Amplitude Distortion: Is a change in the amplitude of a waveform that is generally undesirable.
  • Amplitude Variation with Offset and Azimuth / AVOAZ: The azimuthal variation of the AVO response.
  • Amplitude Variation with Offset/ AVO: The Variation in seismic reflection amplitude with change in distance between shotpoint and receiver which determines thickness, porosity, density, velocity, lithology and fluid content of rocks; requires special processing, modeling, and can be limited by failure to yield a unique solution and is more successful in young, poorly consolidated rocks.
  • Angle of Approach: The acute angle at which a wavefront impinges upon an interface, such as a seismic wave impinging upon strata.
  • Angle of Incidence: The acute angle at which a raypath impinges upon a line normal to an interface.
  • Angular Dispersion / Seismic Velocity: The variation of seismic velocity in different directions.
  • Anisotropic: Having directionally dependent properties, such as variation in seismic velocity measured parallel or perpendicular to bedding surfaces in rocks (common in shales).
  • Anisotropy/ Aeolotropy: Predictable variation of a material's property with the direction in which it is measured, such as seismic velocity measured parallel or perpendicular to bedding surfaces in rocks (common in shales).
  • Antialias Filter: A filter used to eliminate unwanted portions of seismic data spectra, removing frequencies that might cause aliasing during sampling of an analog signal or when decreasing the sample rate of digital data.
  • Aperture/ Window: A portion of a data set, such as seismic data, to which functions or filters are applied. Also can be a mechanism to limit the affects of measurements on a device or system.
  • Apparent Anisotropy: The ratio of velocity determined from normal moveout to velocity measured vertically in a vertical seismic profile, important when migrating long-offset seismic data and analyzing AVO data accurately.
  • Apparent Velocity: The velocity of a wavefront in a certain direction, typically measured along a line of receivers.
  • Applied-Potential Method: Technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area.
  • Apparent Wavelength: The wavelength measured by receivers when a wave approaches at an angle.
  • Array: In computing, code written to access data, in geophysics, a geometrical arrangement of seismic sources or receivers recorded by one channel, or an arrangement of electrodes or antennas used for resistivity, induced polarization (IP), or other types of electromagnetic surveying.
  • Array/ Nest: A geometrical configuration of transducers (sources or receivers) used to generate or record a physical field.
  • Arrival Time: The elapsed time between the release of seismic energy from a source and its arrival at the receiver.
  • Attenuation: The removal of undesirable features from seismic data or the loss of energy or amplitude of waves as they pass through media or the reduction in amplitude of an electromagnetic wave passing through the formation, usually measured in decibels/meter, dB/m.
  • Attribute: A measurable property of seismic data, such as amplitude, dip, frequency, phase, and polarity, measured at one instant or over a time window, or on a single trace, a set of traces, or an interpreted surface.
  • Autocorrelation: The comparison of a waveform to itself, useful in identifying multiples or other regularly repeating signals and in designing deconvolution filters.
  • 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, used in seismic processing to improve visibility of late-arriving events caused amplitude decay.
  • Autotrack: To use computer software to pick a particular reflection or attribute in seismic data automatically.
  • AVA: Abbreviation for amplitude variation with angle of incidence.
  • AVAZ: Abbreviation for amplitude variation with azimuth.
  • Average Velocity: In geophysics, the depth divided by the traveltime of a wave to that depth.
  • AVO / Amplitude Variation With Offset: Abbreviation for amplitude variation with offset, which is Variation in seismic reflection amplitude with change in distance between shotpoint and receiver, indicating differences in lithology and fluid content in rocks above and below the reflector.
  • AVOAZ/ Amplitude Variation With Offset And Azimuth: Abbreviation for amplitude variation with offset and azimuth, or the azimuthal variation of the AVO response.
  • Axis of Rotational Symmetry: An axis of rotational invariance around which a material may be rotated with no change in its properties.
  • Back Propagation: A method for reconstructing the location and shape of the wave at an earlier time using the wave equation.
  • Back Stripping: A modeling technique to assess the geologic history of rock layers through use of cross sections or seismic sections.
  • Backscatter: A reflection phenomenon of energy in which a nonreflective surface randomly scatters energy, lacking coherent reflected energy and scattering energy in all directions.
  • Band: A range of frequencies or wavelengths, such as audible frequencies of sound or visible wavelengths of light.
  • Band-Limited Function: Is a function or time series whose Fourier transform is restricted to a finite range of frequencies or wavelengths.
  • Band-Pass: Frequencies within the acceptable limits of a filter, commonly used to denote a filter that passes a range of frequencies unaltered while rejecting frequencies outside the range.
  • Band-Reject: Frequencies beyond the limits of a filter.
  • Base of Weathering: 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.
  • Base Station: 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.
  • Baseline: A line joining base stations whose transmissions are synchronized during surveying. A reference line, such as a "shale baseline," or a representation of a typical value of a given measurement for a shale on a well log, or the zero-amplitude line of a seismic trace.
  • Basic Wavelet/ Embedded Wavelet: The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient.
  • Beaufort Scale: The 0 to 12 scale for measurement of wind strength according to its effect on objects such as trees, flags and water.
  • Bel: Unit of measurement to describe or compare the intensity of acoustic or electrical signal, typically given in tenths (decibels).
  • Benchmark/ BM: Permanently fixed marker cited in surveying, with an inscription of location and elevation. Standard against which the performance of processes are measured.
  • Bias: An adjustment of the relative positive and negative excursions of reflections during seismic processing.
  • Bin: 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.
  • Bird: 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.

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