GEOPHYSICS 1 Flashcards
The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Birefringence occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808.
birefringence / double refraction
A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps,seismiccaps or blasting caps, are used for seismicacquisitionwith an explosivesourceto achieve consistent timing of detonation.
blasting cap
A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primaryhigh explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing duringrig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing.
blasting cap
A shadow zone, or a zone through which waves do not pass, or cannot be recorded, or in which reflections do not occur.
blind zone
A layer or body of rock that cannot be detected by seismic refraction, typically because its velocity is lower than that of the overlying rocks; also known as a hidden layer.
blind zone
A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation.
BM / benchmark
A wave that propagates through a medium rather than along an interface. P-waves and S-waves are examples of body waves.
body wave
Seismic data measured with receivers, sources or both in a well, such as a check-shot survey, vertical seismic profile (VSP), crosswell seismic data or single-well imaging. By directly measuring the acoustic velocity of each formation encountered in a well, the well logs and borehole seismic data can be correlated to surface seismic data more easily. Borehole seismic data, including both S- and P-waves, can be gathered in a cased or openhole. This term is commonly used to distinguish between borehole sonic data (with frequencies typically greater than 1000 Hz) and borehole seismic data (with frequencies typically less than 1000 Hz).
borehole seismic data
The remaining value of gravitational attraction after accounting for the theoretical gravitational attraction at the point of measurement, latitude, elevation, the Bouguer correction and the free-air correction (which compensates for height above sea level assuming there is only air between the measurement station and sea level). This anomaly is named for Pierre Bouguer, a French mathematician (1698 to 1758) who demonstrated that gravitational attraction decreases with altitude.
Bouguer anomaly
The adjustment to a measurement of gravitational acceleration to account for elevation and the density of rock between the measurement station and a reference level. It can be expressed mathematically as the product of the density of the rock, the height relative to sea level or another reference, and a constant, in units of mGal: δgB = 2 π G ρ h = 0.4193 ρ h, whereδgB = Bouguer correctionρ = rock density in kg/m3h = height difference between two locations in mG = gravitational constant = 6.67384 × 10−11 m3 kg−1 s−2. Strictly interpreted, the Bouguer correction is added to the known value of gravity at the reference station to predict the value of gravity at the measurement level. The difference between the actual value and the predicted value is the gravity anomaly, which results from differences in density between the actual Earth and reference model anywhere below the measurement station.
Bouguer correction
A concave-upward event in seismic data produced by a buried focus and corrected by proper migration of seismic data. The focusing of the seismic wave produces three reflection points on the event per surface location. The name was coined for the appearance of the event in unmigrated seismic data. Synclines, or sags, commonly generate bow ties.
bow tie
Anarrivalof energy propagated from the energysourceat the surface to thegeophonein the wellbore in verticalseismicprofiles and check-shot surveys, or an indication of seismic energy on a trace.
break
A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effect, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator.
bright spot
A processed seismic record that contains traces from a common midpoint that have been added together but has undergone only cursory velocity analysis, so the normal-moveout correction is a first attempt. Typically, no static corrections are made before the brute stack.
brute stack
Bubble pulses or bubble noise that affect data quality. In marine seismic acquisition, the gas bubble produced by an air gun oscillates and generates subsequent pulses that cause source-generated noise. Careful use of multiple air guns can cause destructive interference of bubble pulses and alleviate the bubble effect. A cage, or a steel enclosure surrounding a seismic source, can be used to dissipate energy and reduce the bubble effect.
bubble effect
A vibrator truck equipped with wide tires to allow access to rugged or soggy terrain while causing less damage to the environment.
buggy vibro
The ratio of stress to strain, abbreviated as K. The bulk modulus is an elastic constant equal to the change in applied pressure (∂P) divided by the ratio of the change in volume to the original volume of a body (∂V/V). K = −V (∂P / ∂V), whereK = bulk modulusV = volume∂P = partial derivative of pressure∂V = partial derivative of volume.
bulk modulus/ modulus of compression
The cable on which wireline logging tools are lowered into the well and through which signals from the measurements are passed. The cable consists of a central section with conductors surrounded by a metal, load-bearing armor.
cable
A bundle of electrical wires that connects geophones, or the entire carrier system for marine hydrophones, which includes the hydrophones, the electrical wires, the stress member, spacers, the outer skin of the cable, and the streamer filler, which is typically kerosene or a buoyant plastic. The cable relays data to the seismic recording truck or seismic vessel.
cable
The process of adjusting a measurement to a standard, so that copies of the same type of logging tool or laboratory instrument will read the same. The tool or instrument is placed in the presence of a calibrator or calibrating environment, for example, a source of gamma rays for a gamma ray tool, or the air, far from the ground, for an induction tool. Calibration coefficients, typically a gain and an offset, are calculated so that the tool or instrument reads correctly in the calibrator. The coefficients are then applied during subsequent measurements. The term master calibration is used for the regular, as for example quarterly, calibration of a logging tool in the workshop. For most wireline tools, a secondary calibrator is adjusted during the master calibration and taken to the wellsite so that a wellsite calibration can be done just prior to the logging job. Some tools, such as the gamma ray, are calibrated only at the wellsite. For most measurements-while-drilling tools, the environment requires that the calibration be performed at the workshop and only a verification made at the wellsite. For some measurements, there is a primary worldwide standard against which calibrators are calibrated, as for example, the radioactive formations at the University of Houston used to define gamma ray API units.
calibration
A method of adjusting a data set against a control that has properties to which the data set should conform.
calibration
A small, electrically activated explosive charge that detonates a larger charge. Caps, also called seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation.
cap/ blasting cap
A type of positive gravity anomaly that results from the presence of a dense cap rock overlying a relatively low-density salt dome.
caprock effect
An implosion produced by locally low pressure, such as the collapse of a gas bubble in liquid (the energy of which is used as the source of seismic energy from air guns).
cavitation
In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data.
CDP/ Common depth point
A device to carry data from a receiver to a recorder, such as from a group of geophones. Simultaneous recording of 500 to 2000 channels is common during 3D seismic acquisition, and 120 to 240 channels during onshore 2D seismic acquisition.
channel
A linear, commonly concave-based depression through which water and sediment flow and into which sediment can be deposited in distinctive, often elongated bodies. Channels can occur in a variety of morphologies, e.g., straight, meandering or braided. In some areas, coarse sediments can fill channels of streams or rivers that cut through finer grained sediments or rocks. The close proximity of coarse-grained and fine-grained sediments can ultimately lead to the formation of stratigraphic hydrocarbon traps.
channel
A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal.
channel wave
A distinguishing feature of a waveform in a seismic event, such as shape, frequency, phase or continuity.
character/ signature
A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.
check shot survey/ velocity survey/ well shoot
A technique for acquiring full-azimuth marine seismic data. This technique uses a vessel equipped with source arrays and streamers to shoot and record seismic data; however, unlike conventional surveys acquired in a series of parallel straight lines, circle shooting surveys are acquired as the vessel steams in a series of overlapping, continuously linked circles, or coils. The circular shooting geometry acquires a full range of offset data across every azimuth to sample the subsurface geology in all directions. The resulting full azimuth (FAZ) data are used to image complex geology, such as highly faulted strata, basalt, carbonate reefs and subsalt formations.
circle shooting
In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms.
CMP/ Common midpoint
A measure of the similarity of two seismic traces.
coherence
The quality of two wave trains, or waves consisting of several cycles, being in phase.
coherence
The similarity of two mathematical functions as evaluated in the frequency domain.
coherence
A quantitative assessment of the similarity of three or more functions, also called semblance.
coherence
Pertaining to seismic events that show continuity from trace to trace. Seismic processing to enhance recognition of coherent events and emphasize discontinuities such as faults and stratigraphic changes has gained popularity since the mid-1990s.
coherent
A technique for removing noise and emphasizing coherent events from multiple channels of seismic data.
coherent detection / coherence filtering
Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples.
coherent noise
In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data.
common depth point
Method of seismic reflection surveying and processing that exploits the redundancy of multiple fold to enhance data quality by reducing noise. During acquisition, an energy source is supplied to a number of shotpoints simultaneously. Once data have been recorded, the energy source is moved farther down the line of acquisition, but enough overlap is left that some of the reflection points are re-recorded with a different source-to-receiver offset. Multiple shotpoints that share a source-receiver midpoint are stacked. The number of times that a common midpoint is recorded is the fold of the data.
common midpoint method
In multichannel seismic acquisition, the common midpoint on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver that is shared by numerous locations if the reflector is flat-lying. Like common depth point, this term is commonly misused, because in the case of dipping layers, common reflection points do not exist.
common reflection point
Pertaining to traces that have a differentreceiver but share a source.
common source
Pertaining to traces that have the same offset, or distance between source and receiver.
common-offset
Pertaining to traces that have a different source but share a receiver.
common-receiver
A display of seismictraces that share a receiver.
common-receiver gather / CRG
Pertaining to traces that have a differentreceiver but share a source.
common-source
A display of seismictraces that share a source.
common-source gather / CSG
A change made to porosity measurements, such as those from sonic logs, to compensate for the lack of compaction, or the predicted loss of pore space as sediments are buried by overburden. Compaction corrections are commonly performed in uncompacted sediments.
compaction correction
A mathematical method to determine seismic attributes, including reflection strength and instantaneous frequency, by using the Hilbert transform, a special form of the Fourier transform, and the quadrature trace, or the component of the signal that is 90 degrees out of phase. The Fourier transform a(t) = h(t) + jx(t), whereh(t) = seismic tracex(t) = quadrature trace, can be used to determine reflection strength by combining h and x: r(t) = [h(t)2 + x(t)2]1/2, and to determine instantaneous phase: θ(t) = tan−1[x(t)/h(t)].
complex trace analysis
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.
compressional wave / acoustic wave/ dilatational wave / p-wave
The reciprocal of resistance in a direct current circuit, measured in siemens (formerly mhos). In an alternating current circuit, conductance is the resistance divided by the square of impedance, also measured in siemens.
conductance
The product of conductivity and thickness, typically measured in units of siemens (S). In the inversion of electrical and electromagnetic measurements, the conductance of a layer or zone is usually much better determined than either the conductivity or thickness individually.
conductance
In mathematics, the process in which a sequence of numbers approaches a fixed value called the “limit” of the sequence. This term is often used in modeling or inversion to describe the situation in which a sequence of calculated values approach, or converge with, measured values.
convergence
The movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin.
convergence
In mathematics, pertaining to the process in which a sequence of numbers approaches a fixed value called the “limit” of the sequence. This term is often used inmodelingorinversionto describe the situation in which a sequence of calculated values approach, or converge with, measured values.
convergent
Pertaining to the movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin.
convergent
A seismic wave that changes from a P-wave to an S-wave, or vice versa, when it encounters an interface.
converted wave
A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Convolution also can use surface rates to transform wellhead pressures to an interpretable form. Convolution assumes a particular model for the pressure-transient response, usually infinite-acting radial flow. This operation is similar to what is done to account for the flow history in rigorous pressure-transient analysis.
convolution
“A mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering. The mathematical form of the convolution of two functions, a filter f(t) and a time-series x(t), is y(t) = ∫ f(t−τ)x(τ)dτ, where y(t) is the output of the convolution. In the frequency domain, convolution is simply the product of the Fourier transforms (FT) of the two functions: Y(ω) = F(ω)*X(ω), whereX(ω) = FT of the time series x(t)F(ω) = FT of the filter f(t)Y(ω) = FT of the output y(t)ω = angular frequency.
“
convolution
To perform a convolution, which is a mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Convolution also can use surface rates to transform wellhead pressures to an interpretable form. Convolution assumes a particular model for the pressure-transient response, usually infinite-acting radial flow. This operation is similar to what is done to account for the flow history in rigorous pressure-transient analysis.
convolve
To perform a convolution, which is a mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering.
convolve
A connection of points from well to well in which the data suggest that the points were deposited at the same time (chronostratigraphic) or have similar and related characteristics.
correlate
The comparison of seismic waveforms in the time domain, similar to coherence in the frequency domain.
correlation
A connection of points from well to well in which the data suggest that the points were deposited at the same time (chronostratigraphic) or have similar and related characteristics.
correlation
A positive relationship between data samples that implies a connection or a relationship between them.
correlation
An electrical or mechanical device that joins parts of systems and can affect the interaction of, or energy transfer between, parts of systems. Electrical couplings promote the passage of certain signals but prevent the passage of others, such as an alternating current coupling that excludes direct current.
coupling
The state of being attached to another entity: A well-planted geophone has a coupling to the Earth’s surface or to a borehole wall that allows it to record ground motion during acquisition of seismic data.
coupling
Abbreviation for common receiver gather. A display of seismictraces that share a receiver.
CRG / common receiver gather
The angle of incidence according to Snell’s law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows: sin θc = V1 / V2, whereθc = the critical angleV1 = velocity of the first mediumV2 = velocity of the first medium, which is greater than V1.
critical angle
The minimum damping that will prevent or stop oscillation in the shortest amount of time, typically associated with oscillatory systems like geophones. Critical damping is symbolized by μc.
critical damping
A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle.
critical reflection
The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero.
crosscorrelate
The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero.
crosscorrelation
A seismic line within a 3D survey perpendicular to the direction in which the data were acquired.
crossline
A survey technique that measures the seismic signal transmitted from a source, located in one well, to a receiver array in a neighboring well. The resulting data are processed to create a reflection image or to map the acoustic velocity or other properties (velocities of P- and S-waves, for example) of the area between wells. Placement of the source and receiver array in adjacent wells not only enables the formation between wells to be surveyed, it also avoids seismic signal propagation through attenuative near-surface formations. Another advantage is that it places the source and receiver near the reservoir zone of interest, thereby obtaining better resolution than is possible with conventional surface seismic surveys. This technique is often used for high-resolution reservoir characterization when surface seismic or vertical seismic profile (VSP) data lack resolution, or for time-lapse monitoring of fluid movements in the reservoir.
crosswell seismic tomography
A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements.
crosswell tomography
The magnetic field associated with the Earth’s crust arises from induced and remanent magnetism. The crustal field—also referred to as the anomaly field—varies in direction and strength when measured over the Earth’s surface. It is relatively strong in the vicinity of ferrous and magnetic materials, such as in the oceanic crust and near concentrations of metal ores, and is a focus of geophysical mineral exploration.
crustal magnetic field
Abbreviation for common source gather. A display of seismictraces that share a source.
CSG/ common source gather
“Abbreviation for coal seam gas. Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates.
Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix.”
CSG/ common source gather/ coalbed methane/ coal bed methane/ coal-bed methane/ CBM/ Coal seam gas
A local geophysical anomaly generated by a man-made feature, such as electrical and communications wires, steel beams and tanks and railroad tracks.
cultural anomaly
Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods.
cultural noise
The generation of a theoretical equation to define a given data set. In contrast, curve matching involves the comparison of well-understood data to a data set of interest.
curve fitting
The graphical comparison of well-understood data sets, called type curves, to another data set. If a certain type curve closely corresponds to a data set, then an interpretation of similarity can be made, although, as Sheriff (1991) points out, there might be other type curves that also match the data of interest. Curve matching differs from curve fitting in that curve fitting involves theoretical models rather than actual examples.
curve matching
An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes.
cycle skip
The opposition, slowing or prevention of oscillation, or decreasing vibration amplitude, as kinetic energy dissipates. Frictional damping can be important in the use of geophones for seismic surveys, since a vibrating instrument is difficult to read. Eddy currents can produce electromagnetic damping. The classic example of damping from physics is the slowing of a swinging pendulum unless it has a steady supply of energy.
damping
A depth reference point, typically established at the time the well is completed, against which subsequent depth measurements should be corrected or correlated.
datum
An agreed and known value, such as the elevation of a benchmark or sea level, to which other measurements are corrected. In seismic data, the term refers to an arbitrary planar surface to which corrections are made and on which sources and receivers are assumed to lie to minimize the effects of topography and near-surface zones of low velocity.
datum
A value added to reflection times of seismic data to compensate for the location of the geophone and source relative to the seismic datum.
datum correction
The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft.
decibel / dB
A step in seismic signal processing to recover high frequencies, attenuate multiples, equalize amplitudes, produce a zero-phase wavelet or for other purposes that generally affect the waveshape. Deconvolution, or inverse filtering, can improve seismic data that were adversely affected by filtering, or convolution that occurs naturally as seismic energy is filtered by the Earth. Deconvolution can also be performed on other types of data, such as gravity, magnetic or well log data.
deconvolution
With reference to induction logging, a method of removing shoulder-bed effects from an induction log. The term refers to early 6FF40 and deep induction logs in which the standard method of deconvolution was based on three measurements separated by 78 in. [198 cm] in depth. The three measurements were weighted by an amount calculated to reduce the effect of shoulder beds on the readings in a high-resistivity bed. Originally, the resistivity of the shoulder beds could be input, but in later usage this resistivity became standardized at 1 ohm-m. The deconvolution was not effective in high-contrast formations. In modern tools, the shoulder effect is corrected by using an inverse filter or an automatic inversion.
deconvolution
A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Deconvolution also can use surface rates to transform wellhead pressures to an interpretable form. Deconvolution has the advantage over convolution that it does not assume a particular model for the pressure-transient response. However, the simplest form of deconvolution often gives a noisy result, and more complex approaches may be computing intensive.
deconvolution
A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth’s crust, which is 5 to 75 km [3 to 45 miles] thick.
deep seismic sounding
A method of marine seismic acquisition in which a boat tows a receiver well below the surface of the water to get closer to features of interest or to reduce noise due to conditions of the sea. Deep tow devices are used for some side-scan sonar, gravity and magnetic surveys.
deep tow
An anisotropy parameter that describes near-vertical P-wave velocity anisotropy and the difference between the vertical and small-offset moveout velocity of P-waves. δ = ½{[(C13 + C44)2 − (C33 − C44)2] / [C33 (C33 − C44)]} . Anisotropy parameter for near-vertical P-waves. Delta (δ) describes near-vertical P-wave velocity anisotropy and the difference between the vertical and small-offset moveout velocity of P-waves. C33 is the vertical P-wave modulus (parallel to the symmetry axis), C44 is the modulus for a vertically traveling and horizontally polarized S-wave (parallel to the symmetry axis) and C13 is the modulus of dilation in the vertical direction induced by compression in the horizontal direction.
delta
An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems.
delta
The variation in the mass per unit volume of rocks, which affects the local gravitational field of the Earth. A density contrast also contributes to an acoustic impedance contrast, which affects the reflection coefficient.
density contrast
A series of gravity measurements made along a line or over an area of a locally high topographic feature to remove or compensate for the effect of topography on deeper density readings.
density profile
A device used in acquisition of marine seismic data that keeps streamers at a certain depth in the water.
depth controller
