GEOPHYSICS 6 Flashcards
Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow.
Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow.
single-azimuth towed-streamer acquistion
A conveyance, such as a sled with runners or pontoons, used to transport geophysical gear to a location. Skids are commonly deployed in acquisition of seismic data in marshes or other areas of soft, soggy terrain.
skid
A steel frame on which portable equipment is mounted to facilitate handling with cranes or flatbed trucks. The skid is robust, is usually designed with attachment points for hooks, chains, or cables, and has at least two lengthwise beams to facilitate sliding the equipment into place on the rigsite.
skid
To slide the rig over, such as to move it from one well slot to another on a fixed offshore platform. This operation is accomplished by disconnecting the rigid attachments from the platform to the rig, and energizing large-capacity hydraulic cylinders that push the rig over greased steel skid beams.
skid
The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value; it can be expressed as: δs = (2/σμω)1/2 = (2/σ)(ε/μ)1/2. Where δs = skin depthσ = electrical conductivityω = 2πf = angular frequency in radians/sf = frequency in Hzμ = μrμ0 = magnetic permeabilityμr = relative magnetic permeability of the conductorμ0 = relative magnetic permeability of free space = 4π × 10−7 newton per ampere squared (N/A2)ε = εrε0 = dielectric permittivityεr = relative dielectric permittivity of the materialε0 = dielectric permittivity of free space = 8.854 × 10−12 farads per meter (F/m).
skin depth
A process used in seismic processing to stack, or sum, traces by shifting traces in time in proportion to their offset. This technique is useful in areas of dipping reflectors.
slant stack
Also called interval transit time, The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot.
slowness
A concave-upward, semicircular event in seismic data that has the appearance of a smile and can be caused by poor data migration or migration of noise.
smile
The mathematical description of refraction, or the physical change in the direction of a wavefront as it travels from one medium to another with a change in velocity and partial conversion and reflection of a P-wave to an S-wave at the interface of the two media. Snell’s law, one of two laws describing refraction, was formulated in the context of light waves, but is applicable to seismic waves. It is named for Willebrord Snel (1580 to 1626), a Dutch mathematician. Snell’s law can be written as: n1 sin 1 = n2 sin r, where n1=refractive index of first medium, n2= refractive index of second medium, sin i = sine of the angle of incidence, sin r= sine of the angle of refraction
snell’s law
Some authors use the term to describe P-waves in fluids, or as a synonym for seismic or elastic.
sonic
Pertaining to sound waves in the frequency range of 1 to 25 kilohertz.
sonic
A type of acoustic log that displays traveltime of P-waves versus depth. Sonic logs are typically recorded by pulling a tool on a wireline up the wellbore. The tool emits a sound wave that travels from the source to the formation and back to a receiver.
sonic log
Another term for probe, in electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations.
sound / probe
A device that provides energy for acquisition of seismic data, such as an air gun, explosive charge or vibrator.
source
A geometrical arrangement ofseismicsources (a source array), with each individual source being activated in some fixed sequence in time.
source pattern
One of a number of locations or stations at the surface of the Earth at which a seismic source is activated.
source point
One of a number of locations or stations at the surface of the Earth at which a seismicsourceis activated.
sourcepoint
A display, also known as the f-k domain, of seismic data by wavenumber versus frequency rather than the intuitive display of location versus time for convenience during seismic processing. Working in the space-frequency domain provides the seismic processor with an alternative measure of the content of seismic data in which operations such as filtering of certain unwanted events can be accomplished more effectively.
space-frequency doman
The distance between successive shotpoints.
spacing
The distance between sources and receivers, particularly in logging tools.
spacing
Pertaining to a spectrum. The spectral content of a wavetrain or wavelet usually refers to its amplitude and phase as a function of frequency.
spectral
The distribution of gamma ray energies, or the number of gamma rays as a function of gamma ray energy.
spectrum
Generally, a display of entities or properties according to magnitude. In geophysics, spectrum refers to a display of characteristics of a wavetrain or trace as a function of frequency, wavenumber, or arrival time. A common display of spectrum is amplitude as a function of frequency.
spectrum
The apparent loss of intensity of a gravitational or magnetic field with distance. Spherical divergence decreases energy with the square of the distance.
spherical divergence
The apparent loss of energy from a wave as it spreads during travel. Spherical divergence decreases energy with the square of the distance.
spherical divergence
The solution to the Laplace equation expressed as spherical coordinates. The normal modes of the Earth, or the reverberations that follow earthquakes, have the form of spherical harmonics. Love waves and Rayleigh waves can also be expressed as spherical harmonics.
spherical harmonic
A wave generated from a point source, such as that generated by an underground explosion. Typical seismic sources such as vibrators and air-gun arrays emit elastic waves that are assumed to be spherical waves.
spherical wave
A log of the natural difference in electrical potential, in millivolts, between an electrode in the borehole and a fixed reference electrode on the surface. The most useful component of this difference is the electrochemical potential since it can cause a significant deflection opposite permeable beds. The magnitude of the deflection depends mainly on the salinity contrast between drilling mud and formation water, and the clay content of the permeable bed. The spontaneous potential (SP) log is therefore used to detect permeable beds and to estimate formation water salinity and formation clay content. The SP log cannot be recorded in nonconductive mud. The SP can be affected by several factors that make interpretation difficult. First, there are other possible sources of electrical potential not related to the electrochemical effect, for example, the electrokinetic potential and bimetallism. Many of these are small and constant throughout the log, and can be lumped together in the shale baseline. Second, the SP can measure only the potential drop in the borehole, and not the full electrochemical potential. The ideal SP opposite a clean bed is known as the static spontaneous potential (SSP), and opposite a shaly bed as the pseudostatic spontaneous potential (PSP). The SP is always less than the SSP or the PSP and more rounded at the boundaries between shales and permeable beds. The SP was first recognized by C. Schlumberger, M. Schlumberger and E.G. Leonardon in 1931, and the first published examples were from Russian oil fields.
Spontaneous potential/ SP
The geometrical pattern of groups of geophones relative to the seismic source. The output from a single shot is recorded simultaneously by the spread during seismic acquisition. Common spread geometries include in-line offset, L-spread, split-spread and T-spread.
spread
To sum traces to improve the signal-to-noise ratio, reduce noise and improve seismic data quality. Traces from different shot records with a common reflection point, such as common midpoint (CMP) data, are stacked to form a single trace during seismic processing. Stacking reduces the amount of data by a factor called the fold.
stack
A processed seismic record that contains traces that have been added together from different records to reduce noise and improve overall data quality. The number of traces that have been added together during stacking is called the fold.
stack
The distance-time relationship determined from analysis of normal moveout (NMO) measurements from common depth point gathers of seismic data. The stacking velocity is used to correct the arrival times of events in the traces for their varying offsets prior to summing, or stacking, the traces to improve the signal-to-noise ratio of the data.
stacking velocity
Often called statics, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers.
statics/ near surface correction/ near-surface correction/ static correction
Another term for static correction, a bulk shift of aseismic tracein time duringseismic processing. A common static correction is theweatheringcorrection, which compensates for a layer of lowseismic velocitymaterial near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers.
statics/ near surface correction/ near-surface correction/ static correction
A type of large-amplitude interface, or surface, wave generated by a sonic tool in a borehole. Stoneley waves can propagate along a solid-fluid interface, such as along the walls of a fluid-filled borehole and are the main low-frequency component of signal generated by sonic sources in boreholes. Analysis of Stoneley waves can allow estimation of the locations of fractures and permeability of the formation. Stoneley waves are a major source of noise in vertical seismic profiles.
Stoneley wave
A surface marine cable, usually a buoyant assembly of electrical wires that connects hydrophones and relays seismic data to the recording seismic vessel. Multistreamer vessels tow more than one streamer cable to increase the amount of data acquired in one pass.
Streamer
In marine seismic acquisition, the lateral deviation of a streamer away from the towing direction because of a water current.
Streamer feathering
In seismic acquisition and processing, the attenuation of amplitudes to reduce the effects of noise or to prevent overload from the high energy of first breaks.
Suprression
A wave that propagates at the interface between two media as opposed to through a medium. A surface wave can travel at the interface between the Earth and air, or the Earth and water. Love waves and Rayleigh waves are surface waves.
surface wave
A data set measured and recorded with reference to a particular area of the Earth’s surface, such as a seismic survey
survey
A completed measurement of the inclination and azimuth of a location in a well (typically the total depth at the time of measurement). In both directional and straight holes, the position of the well must be known with reasonable accuracy to ensure the correct wellbore path and to know its position in the event a relief well must be drilled. The measurements themselves include inclination from vertical, and the azimuth (or compass heading) of the wellbore if the direction of the path is critical. These measurements are made at discrete points in the well, and the approximate path of the wellbore computed from the discrete points. Measurement devices range from simple pendulum-like devices to complex electronic accelerometers and gyroscopes used more often as MWD becomes more popular. In simple pendulum measurements, the position of a freely hanging pendulum relative to a measurement grid (attached to the housing of the tool and assumed to represent the path of the wellbore) is captured on photographic film. The film is developed and examined when the tool is removed from the wellbore, either on wireline or the next time pipe is tripped out of the hole.
survey
A precise and legally binding measurement of surface locations, referenced to known benchmark locations.
survey
The measurement versus depth or time, or both, of one or more physical quantities in or around a well. In early years, the term was used more often than log.
survey
To make the measurements as in definitions 1 or 2.
survey
To measure and record data according to location on the Earth’s surface. In geophysics, the term is used in the context of acquiring seismic, electrical, gravity or magnetic data to evaluate the subsurface.
survey
A shear wave that is polarized so that its particle motion and direction of propagation occur in a vertical plane.
SV-wave
An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques.
S-wave
The result of one of many forms of forward modeling to predict the seismic response of the Earth. A more narrow definition used by seismic interpreters is that a synthetic seismogram, commonly called a synthetic, is a direct one-dimensional model of acoustic energy traveling through the layers of the Earth. The synthetic seismogram is generated by convolving the reflectivity derived from digitized acoustic and density logs with the wavelet derived from seismic data. By comparing marker beds or other correlation points picked on well logs with major reflections on the seismic section, interpretations of the data can be improved. The quality of the match between a synthetic seismogram depends on well log quality, seismic data processing quality, and the ability to extract a representative wavelet from seismic data, among other factors. The acoustic log is generally calibrated with check-shot or vertical seismic profile (VSP) first-arrival information before combining with the density log to produce acoustic impedance.
synthetic seismogram
A floating device used in marine seismic acquisition to identify the end of a streamer. Tail buoys allow the seismic acquisition crew to monitor the location and direction of streamers. They are commonly brightly colored, reflect radar signals, and are fitted with Global Positioning System (GPS) receivers.
Tail buoy
Also known as S-wave, an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques.
Tangential wave/ S-wave/ Shear wave
Steps inseismicprocessingto compensate forattenuation, sphericaldivergenceand other effects by adjusting theamplitudeof the data. The goal is to get the data to a state where thereflection amplitudes relate directly to the change inrock propertiesgiving rise to them.
TAR/ True-amplitude recovery
A variation of theelectromagnetic methodin which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used inexplorationfor minerals andgroundwaterand for environmental mapping.
TDEM/ TEM/ Time-domain electromagnetic methodTransient electromagnetic method
A low-frequency electrical current that occurs naturally over large areas at or near the surface of the Earth. Telluric currents are induced by changes in Earth’s magnetic field which are usually caused by interactions between the solar wind and the ionosphere (part of the upper atmosphere).
Telluric current
An electromagnetic method in which naturally occurring, low-frequency electric currents (telluric currents), are measured at a base station and compared with values measured at other stations. The normalized measurements of telluric current provide information about the direction of current flow and the conductance (conductivity times thickness) of sediments in the surveyed area. Extremely low-frequency telluric currents (with periods of days or months) provide information about conductivity in the deep interior of the Earth.
Telluric current method
An electromagnetic method in which naturally occurring, low-frequency electric currents (telluric currents), are measured at a base station and compared with values measured at other stations. The normalized measurements of telluric current provide information about the direction of current flow and the conductance (conductivity times thickness) of sediments in the surveyed area. Extremely low-frequency telluric currents (with periods of days or months) provide information about conductivity in the deep interior of the Earth.
Telluric-current method
A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping.
TEM / TDEM / Time-domain electromagnetic method/ Transient electromagnetic method
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.
Three component 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.
Three dimensional 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.
Three dimensional 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.
Three-component seismic data / 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.
Three-dimensional 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.
Three-dimensional survey
Abbreviation for transverse isotropy. Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry.
TI
A comparison, or the location of a comparison, of data. Properly processed and interpreted seismic lines can show good ties, or correlations, at intersection points.
Tie
To correlate data in order to formulate or verify an interpretation or to demonstrate the relationship between data sets. Long, regional-scale 2D seismic lines are commonly tied to 3D surveys that cover a limited area, and 3D surveys of different vintages are tied to each other. Well logs are tied into seismic data routinely to determine the relationship between lithologic boundaries in the logs and seismic reflections. Properly tying all available data, including seismic data, well logs, check-shot surveys, synthetic seismograms and vertical seismic profiles, can reduce or, if there are sufficient data, eliminate ambiguity in interpretations.
Tie
Abbreviation for horizontal transverse isotropy. Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures.
TIH / HTI
The use of a function of time rather than frequency to express an independent variable or measurement. In contrast, in the frequency domain, variables are expressed as a function of frequency instead of time.
time domain
A variation of theelectromagnetic methodin which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used inexplorationfor minerals andgroundwaterand for environmental mapping.
time domain electromagnetic method / TEM
Seismicdata from the surface or aboreholeacquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondaryrecovery. The data are examined for changes in attributes related to expressions of fluid content.Time-lapseseismic data can repeat 2D, 3D (which is known as4D seismic data), crosswell and VSPdata.
time lapse seismic data
A migration technique for processing seismic data in areas where lateral velocity changes are not too severe, but structures are complex. Time migration has the effect of moving dipping events on a surface seismic line from apparent locations to their true locations in time. The resulting image is shown in terms of traveltime rather than depth, and must then be converted to depth with an accurate velocity model to be compared to well logs.
time migration
A horizontal display or map view of 3D seismic data having a certain arrival time, as opposed to a horizon slice that shows a particular reflection. A time slice is a quick, convenient way to evaluate changes in amplitude of seismic data.
Time slice
A variation of theelectromagnetic methodin which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used inexplorationfor minerals andgroundwaterand for environmental mapping.
Time-domain electromagnetic method
Seismic data from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4D seismic data), crosswell and VSP data.
Time-lapse seismic data / four-dimensional seismic data
Abbreviation for vertical transverse isotropy. Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer.
TIV/ vertical transverse isotropy
A mode of the electromagnetic field that involves only one component of the magnetic field and the two components of the electric field perpendicular to it; e.g., the x-component of the magnetic field and y- and z-components of the electric field. The TM mode is useful in describing 2D models in which the magnetic field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the magnetic field component, which simplifies calculations tremendously.
TM / transverse magnetic mode
A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers. There are several types of tomography used by geophysicists, including transmission tomography (which uses measurements between boreholes, surface-to-surface, or between a borehole and the surface), reflection or seismic tomography (based on standard reflection seismology), and diffraction tomography (using Fermat’s principle for computations instead of Snell’s law). Variations in velocity can be attributed to changes in density and elastic properties of rocks, which in turn are affected by the increasing temperature with depth in the Earth. Tomographic techniques have been used to construct maps of the Earth’s interior, deep in the mantle, as well as for mapping the shallow subsurface by borehole tomography.
tomography
The presentation on hard copy of log data from a single measurement versus depth. The term originated with the early optical recorders in which log data were recorded on film using an optical trace. Now the term curve is more common.
trace
The seismic data recorded for one channel. A trace is a recording of the Earth’s response to seismic energy passing from the source, through subsurface layers, and back to the receiver.
trace
A variation of theelectromagnetic methodin which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used inexplorationfor minerals andgroundwaterand for environmental mapping.
transient electromagnetic method
The duration of time for a P-wave to travel one foot, typically displayed on an acoustic log. The unit of microseconds per foot (or meter) is called the slowness, which is the inverse of velocity. Transit time is measured in microseconds per foot (μs/ft) or in microseconds per meter (μs/m).
transit time
An area in which water is too shallow for acquisition of marine seismic data with towed streamers, such as near the shoreline, marshes and lagoons. In some cases, source explosives can be rammed into the unconsolidated sediments of transition zone environments rather than drilling more costly shot holes. Likewise, hydrophones can be placed by ramming to couple the receiver to the Earth better and to save time and money during survey acquisition.
transition zone
With reference to invasion, the volume between the flushed zone and the undisturbed zone in which the mud filtrate has only partially displaced the moveable formation fluids. One common model of invasion assumes a smooth transition in resistivity and other formation properties from the flushed to the undisturbed zone. Based on this assumption, the inner and outer diameters of invasion can be determined from array resistivity logs. Another common invasion model, which does not assume a smooth transition, is the annulus.
transition zone
A mode of the electromagnetic field that involves only one component of the electric field and the two components of the magnetic field perpendicular to it; e.g., the x-component of the electric field and y- and z-components of the magnetic field. The TE mode is useful in describing 2D models in which the electric field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the electric field component, which simplifies calculations tremendously. There is an analogous mode for the magnetic field called the TM mode. A general EM field in a region without sources can be expressed as a sum of TE and TM modes.
transverse electric mode / TE
A mode of the electromagnetic field that involves only one component of the electric field and the two components of the magnetic field perpendicular to it; e.g., the x-component of the electric field and y- and z-components of the magnetic field. The TE mode is useful in describing 2D models in which the electric field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the electric field component, which simplifies calculations tremendously. There is an analogous mode for the magnetic field called the TM mode. A general EM field in a region without sources can be expressed as a sum of TE and TM modes.
Transverse electric mode/ TE
Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry.
transverse isotropy
A mode of the electromagnetic field that involves only one component of the magnetic field and the two components of the electric field perpendicular to it; e.g., the x-component of the magnetic field and y- and z-components of the electric field. The TM mode is useful in describing 2D models in which the magnetic field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the magnetic field component, which simplifies calculations tremendously.
transverse magnetic mode /TM
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.
traveltime / acoustic traveltime
The minimum (negative) deflection of the seismic wavelet. Seismic interpreters commonly pick or track seismic data on paper sections along the trough of a wavelet rather than the solid-colored peak. With the advent of workstations, this is no longer necessary because of automatic picking techniques and the ability to reverse the polarity of the data in real time.
trough
Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them.
true-amplitude recovery
Steps inseismicprocessingto compensate forattenuation, sphericaldivergenceand other effects by adjusting theamplitudeof the data. The goal is to get the data to a state where thereflection amplitudes relate directly to the change inrock propertiesgiving rise to them.
true-amplitude recovery / TAR
A stoneley wave that occurs at the low frequencies of seismic data
tube wave
An interface wave that occurs in cased wellbores when a Rayleigh wave encounters a wellbore and perturbs the fluid in the wellbore. The tube wave travels down the wellbore along the interface between the fluid in the wellbore and the wall of the wellbore. A tube wave suffers little energy loss and typically retains a very high amplitude which interferes with reflected arrivals occurring later in time on vertical seismic profile (VSP) data. Because the tube wave is coupled to the formation through which it is traveling, it can perturb the formation across open fractures intersecting the borehole. This squeezing effect can generate secondary tube waves which travel both up and down from the fracture location. Such events can be diagnostic of the presence of open fractures and their amplitude related qualitatively to the length and width, e.g., volume of the fluid-filled fracture space. This effect is generally seen only in shallow formations where the overburden pressure is lower.
tube wave
A phenomenon of constructive or destructive interference of waves from closely spaced events or reflections. At a spacing of less than one-quarter of the wavelength, reflections undergo constructive interference and produce a single event of high amplitude. At spacing greater than that, the event begins to be resolvable as two separate events. The tuning thickness is the bed thickness at which two events become indistinguishable in time, and knowing this thickness is important to seismic interpreters who wish to study thin reservoirs. The tuning thickness can be expressed by the following formula: Z = VI/2.8 fmax,where Z = tuning thickness of a bed, equal to 1/4 of the wavelengthVI = interval velocity of the target, fmax = maximum frequency in the seismic section
tuning effect
A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.
two dimensional seismic data
A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially.
two dimensional 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.
Two dimensional survey
A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially
Two-dimensional 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.
Two-dimensional 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.
Two-way dimensional survey
The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth’s surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset.
Two-way traveltime
