GEOPHYSICS 2 Flashcards
The process of transforming seismic data from a scale of time (the domain in which they are acquired) to a scale of depth to provide a picture of the structure of the subsurface independent of velocity. Depth conversion, ideally, is an iterative process that begins with proper seismic processing, seismic velocity analysis and study of well data to refine the conversion. Acoustic logs, check-shot surveys and vertical seismic profiles can aid depth conversion efforts and improve correlation of well logs and drilling data with surface seismic data.
depth conversion
A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes.
depth map
A step in seismic processing in which reflections in seismic data are moved to their correct locations in space, including position relative to shotpoints, in areas where there are significant and rapid lateral or vertical changes in velocity that distort the time image. This requires an accurate knowledge of vertical and horizontal seismic velocity variations.
depth migration
A point on the surface for which the depth to a horizon has been calculated in a refraction seismic survey. The term is commonly misused as a synonym for common depth point.
depth point
A display of seismic data with a scale of units of depth rather than time along the vertical axis. Careful migration and depth conversion are essential for creating depth sections.
depth section
The minimum thickness necessary for a layer of rock to be visible or distinct in reflection seismic data. Generally, the detectable limit is at least 1/30 of the wavelength. Acquisition of higher frequency seismic data generally results in better detection or vertical resolution of thinner layers.
detectable limit
A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer.
detector
A type of inverse filtering, or deconvolution, in which the effects of the filter are known by observation or assumed, as opposed to statistical deconvolution.
deterministic deconvolution
To set off an explosive material. Explosive sources are used in seismic acquisition and explosive charges are used to perforate wells in preparation for production or injection.
detonate
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 primary high 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 during rig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing.
detonator/ blasting cap
A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation.
detonator/ blasting cap
A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity.
dielectric
The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric permittivity.
dielectric constant/ dielectric permittivity
A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections.
difference map
A type of static correction that compensates for delays in seismic reflection or refraction times from one point to another, such as among geophone groups in a survey. These delays can be induced by low-velocity layers such as the weathered layer near the Earth’s surface.
differential weathering correction
A type of event produced by the radial scattering of a wave into new wavefronts after the wave meets a discontinuity such as a fault surface, an unconformity or an abrupt change in rock type. Diffractions appear as hyperbolic or umbrella-shaped events on a seismic profile. Proper migration of seismic data makes use of diffracted energy to properly position reflections.
diffraction
Also known as Kirchhoff migration, a method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth’s surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth.
diffraction stack/ Kirchhoff migration
The process by which particles move over time within a material due to their kinetic motion. The term is most commonly used in pulsed neutron capture logging and in nuclear magnetic resonance (NMR) logging. In a pulsed neutron capture log, the term refers to the spread of neutrons away from the neutron generator. In NMR logging, diffusion refers to the movement of gas, oil or water molecules within the pore space.
diffusion
The movement of ions or molecules from regions of high concentration to low concentration within a solution.
diffusion
The conduction of heat by the movement of molecules.
diffusion
A partial differential equation describing the variation in space and time of a physical quantity that is governed by diffusion. The diffusion equation provides a good mathematical model for the variation of temperature through conduction of heat and the propagation of electromagnetic waves in a highly conducting medium. The diffusion equation is a parabolic partial differential equation whose characteristic form relates the first partial derivative of a field with respect to time to its second partial derivatives with respect to spatial coordinates. It is closely related to the wave equation. ∇2E = j ω μ σ E. whereE = electrical fieldω = angular frequencyμ = magnetic permeabilityσ = electrical conductivity∇ = vector differential operator.
diffusion equation
A fundamental differential equation obtained by combining the continuity equation, flow law and equation of state. Most of the mathematics of well testing were derived from solutions of this equation, which was originally developed for the study of heat transfer. Fluid flow through porous media is directly analogous to flow of heat through solids. Solutions used in well testing usually assume radial flow and homogenous, isotropic formations.
diffusion equation
The increase in the volume of rocks as a result of deformation, such as when fractures develop.
dilatancy
A possible explanation for volume changes in rocks due to strain, such as microfracturing or cracking, and the accompanying change in the ratio of P- to S-wave velocity. Support for dilatancy theory comes in the form of porosity increases from 20 to 40% that have been measured in laboratory experiments using rock samples.
dilatancy theory
The process of changing volume as stress is applied to a body.
dilatation
A rarefaction, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismic trace.
dilatation
The volumetric strain produced by applying stress to a body.
dilatation
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.
dilatational wave/ acoustic wave/ compressional wave/ P-wave
A type of local seismic event that, in contrast to a bright spot, shows weak rather than strong amplitude. The weak amplitude might correlate with hydrocarbons that reduce the contrast in acoustic impedance between the reservoir and the overlying rock, or might be related to a stratigraphic change that reduces acoustic impedance.
dim spot
The procedure in seismic processing that compensates for the effects of a dipping reflector. DMO processing was developed in the early 1980s.
dip moveout/ DMO
The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO.
dip moveout/ DMO
A small antenna used in electromagnetic surveying that can be represented mathematically as a dipole.
dipole
A pair of opposite (and equal) electrical charges. The strength of the dipole is a vector quantity whose direction points from the positive to the negative charge and whose magnitude is the product of the absolute value of the charge times the separation. A point dipole is an idealized mathematical representation of a dipole in which the separation of the charges goes to zero while their charge increases so that the product (dipole strength) remains constant.
dipole
Two poles of opposite polarity that can generate a field, such as an electric or magnetic field or a dipole source and dipole receiver used in sonic logging for excitation and detection of shear waves.
dipole
The primary contribution to Earth’s main magnetic field.
dipole field
The property of some seismic sources whereby the amplitude, frequency, velocity or other property of the resulting seismic waves varies with direction. A directional charge, such as a length of primer cord or a linear array of charges, can be used when directivity is desirable. Directivity is also a property of geophone arrays, air guns, explosives or vibrators, which can be positioned to reduce horizontal traveling noise such as ground roll. Receivers in the form of groups in which the individual geophones or hydrophones are separated from each other in linear (1D) or areal (2D) arrays are directional, and are designed to suppress signal arriving nearly horizontally and to pass nearly vertical arrivals with minimum attenuation or distortion. Directivity is often present, but the difficulty in accounting for it during seismic processing makes it undesirable in most cases.
directivity
A subsurface boundary or interface at which a physical quantity, such as the velocity of transmission of seismic waves, changes abruptly. The velocity of P-waves increases dramatically (from about 6.5 to 8.0 km/s) at the Mohorovicic discontinuity between the Earth’s crust and mantle.
discontinuity
A colloidal system, such as a drilling fluid, that has been dispersed. More generally, the term applies to any two (or more) phase system in which one phase exists as small particles or droplets dispersed in the second phase.
dispersion
The act of breaking up large particles into smaller ones and distributing them throughout a liquid or gaseous medium. For example, in water-base drilling mud, dispersion is the act of degrading clay materials, starches, carboxymethylcellulose, biopolymer, synthetic polymers or oils into submicroscopic particles by applying mechanical energy, heat or chemical means. In oil-mud emulsion terminology, dispersion is the act of forming a fine-grained emulsion of an aqueous phase into an oil. This is usually accomplished with by mechanical shearing or heating in the presence of surfactants. It also includes dispersing solids into an oil mud, such as barite or organophilic clays. Mud viscosity increases as more and more platelets are dispersed. No gel structure can form as long as mechanical shearing is in progress.
dispersion
A type of distortion of a wave train in which the velocity of the wave varies with frequency. Surface waves and electromagnetic body waves typically exhibit dispersion, whereas P-waves in most rocks show little change in velocity with frequency.
dispersion
Spatial separation of components within a fluid. This separation is often driven by diffusion, mixing or differential flow. In an oil field components might be separated because of heterogeneity of permeability, or simply because of different paths taken by the fluid through the pore structure. Hydrodynamic dispersion includes both of these mechanical effects and molecular diffusion. The comp
dispersion
“The phenomenon of a wave separating into its frequency constituents as it passes through a medium. Each frequency component travels at its phase velocity (vp), which is the product of the frequency (f) and wavelength (λ) of that component. Angular dispersion results from anisotropy, which causes velocity to vary with direction. Dispersion affects all types of waves, including light, electromagnetic, sound, elastic, gravity and water waves.
“
dispersion
The act of removing one fluid (usually liquid) from a wellbore and replacing it with another. This is accomplished by pumping a spacer fluid that is benign to both the first and second fluid, followed by the new fluid, down the drillstring and out the bottom of the drillstring or bit. While the spacer and second fluid are pumped into the top of the wellbore, the first fluid is forced out of the annulus between the drillstring and the wellbore or casing. In some cases, this general procedure may be reversed by pumping in the top of the annulus and taking fluid back from the drillstring. Since this is the reverse of the normal circulation path, this is referred to as “reversing out” or “reverse circulation.”
displacement
The act of forcing a cement slurry that has been pumped into a casing string or drillstring to exit the bottom of the casing or drillstring by pumping another fluid behind it. Cement displacement is similar to definition 5 above, with the noted exception that the cement slurry would not normally be pumped out the top of the annulus, but would instead be placed in a particular location in the annulus. This location might be the entire annulus on a short casing string, or filling only a bottom portion of the casing on longer casing strings.
displacement
The offset of segments or points that were once continuous or adjacent. Layers of rock that have been moved by the action of faults show displacement on either side of the fault surface.
displacement
The movement of a particle by wave action, such as movement of rock grains when a seismic wave shakes the ground.
displacement
The horizontal distance between a seismic refraction depth point and the geophone where refracted energy or refraction signal was recorded.
displacement
The shortest distance from the surface location of a well to the vertical projection of the bottom of the well (or other point in the well) to the Earth’s surface. Horizontal wells often have total displacements of 1000 ft [305 m] or more from the surface location, and the world record exceeds 10 km [6.2 miles] of displacement.
displacement
A change in a waveform that is generally undesirable, such as in seismic waves.
distortion
The inability of a system to exactly match input and output, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier.
distortion / amplitude distortion
The daily variation in properties of the Earth, such as the temperature or the local geomagnetic field, or the daily change in sunlight. Such variations depend in part on latitude, proximity to the ocean, the effects of solar radiation and tides and other factors.
diurnal variation
In Cartesian coordinates, divergence is the sum of the partial derivatives of each component of the vector field with respect to the corresponding spatial coordinate: div V = ∇·V = ∂Vx/∂x + ∂Vy/∂y + ∂Vz/∂z
divergence
The loss of energy from a wavefront as a consequence of geometrical spreading, observable as a decrease in wave amplitude. Spherical divergence decreases energy with the square of the distance. Cylindrical divergence decreases energy with the distance.
divergence
In mathematics, a process in which a sequence of numbers does not tend to a fixed limit (the opposite of convergence). Divergence is a mathematical property of a vector field that is a local measure of its rate of spreading.
divergence
An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix’s assumptions. The equation is as follows: Vint = [(t2 VRMS22 − t1 VRMS12) / (t2 − t1)]1/2, whereVint = interval velocityt1 = traveltime to the first reflectort2 = traveltime to the second reflectorVRMS1 = root-mean-square velocity to the first reflectorVRMS2 = root-mean-square velocity to the second reflector.
Dix formula
The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO.
DMO/ dip moveout
A particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly. While a dogleg is sometimes created intentionally by directional drillers, the term more commonly refers to a section of the hole that changes direction faster than anticipated or desired, usually with harmful side effects. In surveying wellbore trajectories, a standard calculation of dogleg severity is made, usually expressed in two-dimensional degrees per 100 feet [degrees per 30 m] of wellbore length. There are several difficulties associated with doglegs. First, the wellbore is not located in the planned path. Second is the possibility that a planned casing string may no longer easily fit through the curved section. Third, repeated abrasion by the drillstring in a particular location of the dogleg results in a worn spot called a keyseat, in which the bottomhole assembly components may become stuck as they are pulled through the section. Fourth, casing successfully cemented through the dogleg may wear unusually quickly due to higher contact forces between the drillstring and the inner diameter (ID) of the casing through the dogleg. Fifth, a relatively stiff bottomhole assembly may not easily fit through the dogleg section drilled with a relatively limber BHA. Sixth, excessive doglegs increase the overall friction to the drillstring, increasing the likelihood of getting stuck or not reaching the planned total depth. Usually these problems are manageable. If the dogleg impairs the well, remedial action can be taken, such as reaming or underreaming through the dogleg, or even sidetracking in extreme situations.
dogleg
An abrupt turn, bend or change of direction in a survey line, a wellbore, or a piece of equipment. Dog-legs can be described in terms of their length and severity and quantified in degrees or degrees per unit of distance.
dogleg
The set of values an independent variable can take. For example, the independent variable of the time domain is time; and for the frequency domain, it is frequency.
domain
A region characterized by a specific feature.
domain
A region of magnetic polarity within a ferromagnetic body. Domains collectively determine the magnetic properties of the body by their arrangement.
domain
The set of values assigned to the independent variables of a function.
domain
The splitting of an incidentwaveinto two waves of different velocities and orthogonal polarizations. Double refraction, or birefringence, occurs in optical mineralogy (seepetrography) when plane-polarized light passes through ananisotropicmineraland 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-shearand 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.
double refraction/ birefringence
A receiver located in a wellbore, as opposed to a location on the Earth’s surface.
downhole receiver
A seismic source located in a wellbore rather than at the Earth’s surface.
downhole source
A technique used to estimate the value of a potential field or seismic data at a surface beneath a measured surface. The method is risky because it assumes continuity of the field, so anomalies affect predictions, especially if they occur beneath the measured surface. Noise can be exaggerated and affect calculations adversely.
downward continuation / upward continuation
A drained test is one in which the pore fluid in the sample is able to flow and equilibrate to imposed pore pressure conditions; the fluid mass and volume will vary but its pressure will be constant. A drained test could be on a dry sample.
drained test
To guarantee the inside diameter of a pipe or other cylindrical tool by pulling a cylinder or pipe (often called a rabbit) of known outside diameter through it. The drift diameter is the inside diameter (ID) that the pipe manufacturer guarantees per specifications. Note that the nominal inside diameter is not the same as the drift diameter but is always slightly larger. The drift diameter is used by the well planner to determine what size tools or casing strings can later be run through the casing, whereas the nominal inside diameter is used for fluid volume calculations such as mud circulating times and cement slurry placement calculations.
Drift
An accurately machined device that is pulled through the casing, tubulars and completion components to ensure minimum-diameter specifications are within tolerance, as described in definition 2. While this tool is usually of a short length, the well planner may specify a special drift that either has a longer length or a nonstandard outside diameter. The large-diameter casing drifts are frequently known as “rabbits.”
Drift
In calibration of a check-shot survey, the difference between geometrically corrected transit time and integrated sonic time.
Drift
A term to describe the inclination from vertical of a wellbore.
Drift
To pass a gauge through casing, tubulars and completion components to ensure minimum-diameter specifications are within tolerance, as described in definition 2. This task is also performed to ensure that there is no junk, dried cement, dirt, rocks or other debris inside the pipe.
Drift
A gradual change in a measurement or recording device during surveying. Reference to or repetition of a measurement at a base station can indicate whether drift is a problem.
Drift
A technique for acquiring a verticalseismicprofile that uses the noise of the drill bitas a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition andprocessingare typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling, decide where to setcasingin a well and evaluate drilling hazards, such asanomalouspore pressure.
drill noise vertical seismic profile
The failure of a channel or geophone to record a shot or shots in a seismic survey, which results in a loss of data.
Dropout
The loss of information from a magnetic tape that occurs if the tape is damaged or exposed to dirt.
Dropout
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.
DSS
A time-variant operation performed on seismic data. Normal moveout (NMO) is a dynamic correction.
dynamic correction
The ratio of or difference between the highest and the lowest reading, or strongest and weakest signal, that can be recorded or reproduced by an instrument without distortion.
dynamic range
A type of explosive used as a source for seismic energy during data acquisition. Originally, dynamite referred specifically to a nitroglycerin-based explosive formulated in 1866 by Alfred Bernhard Nobel (1833 to 1896), the Swedish inventor who endowed the Nobel prizes. The term is incorrectly used to mean any explosive rather than the original formulation.
dynamite
An alternating or transient electrical current in a conductive medium in the presence of a time-varying magnetic field. The eddy current generates its own electromagnetic field.
eddy current
A hydrophone array in a cable that can be attached to a streamer for acquisition of marine seismic data. The eel can be suspended from the streamer so that the eel is close to the seafloor but the streamer remains high enough to avoid obstacles on the seafloor such as reefs or debris from human activity.
eel
The variation of elastic properties with direction. For example, elastic anisotropy occurs when seismic, or elastic, waves travel through rock at differing velocities in various directions. Elastic anisotropy occurs if there is a preferred alignment of a material’s fabric elements—crystals, grains, cracks, bedding planes, joints or fractures—on a scale smaller than the length of the wave. This alignment causes waves to propagate fastest in its direction. Elastic anisotropy is sometimes called seismic anisotropy, velocity anisotropy, traveltime anisotropy, acoustic anisotropy or slowness anisotropy.
elastic anisotropy
A set of constants that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson’s ratio,shearmodulus, and Young’s modulus. Elastic constants are important inseismologybecause thevelocityof waves depends on the elastic constants and density of therock.
elastic moduli / elastic constant/ modulus of elasticity
A seismic or acoustic wave, such as a P-wave.
elastic wave
The ability of a material to support the flow of an electrical current. In linear isotropic materials, the electric current density at any point in space is proportional to the electric field; the constant of proportionality is the electrical conductivity. Conductivity is the inverse of resistivity in isotropic materials, and is measured in siemens per meter or the archaic units of mhos per meter. The electrical conductivity of the Earth can be measured by electromagnetic methods. The electrical conductivity symbol is σ.
electrical conductivity
The ability of a material to store a charge from an applied electrical field without conducting electricity.
electrical permitivity/ permitivity
The ability of a material to resist or inhibit the flow of an electrical current, measured in ohm-meters. Resistivity is the reciprocal of conductivity. The electrical resistivity symbol is ρ.
electrical resistivity
A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth’s surface or in boreholes in order to map variations in the Earth’s electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, “electromagnetic method” tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods).
electromagnetic method
Any compensating factor used to bring measurements to a common datum or reference plane. In gravity surveying, elevation corrections include the Bouguer and free-air corrections. Seismic data undergo a static correction to reduce the effects of topography and low-velocity zones near the Earth’s surface. Well log headers include the elevation of the drilling rig’s kelly bushing and, for onshore locations, the height of the location above sea level, so that well log depths can be corrected to sea level.
elevation correction
The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation.
embedded wavelet / basic wavelet
A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (ε) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared. ε ≡ ½ [(C11 − C33) / C33] = ½ [(VP⊥2 − VPˆ¥2) / VPˆ¥2] . P-wave parameter (ε) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C11 is the horizontal P-wave modulus (perpendicular to the symmetry axis), C33 is the vertical P-wave modulus (parallel to the symmetry axis), VP⊥ is the horizontal P-wave velocity and VPˆ¥ is the vertical P-wave velocity.
Epsilon (ε)
A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area.
Equipotential method
A measure of the anellipticity of the P-wave phase slowness—the inverse of P-wave phase velocity—in rock exhibiting vertical transverse isotropy. η = (ε − δ) / (1 + 2δ) . Anellipticity of P-wave phase slowness for a medium in which the elastic properties exhibit vertical transverse isotropy. Eta (η) is the anellipticity and ε and δ are the P-wave anisotropy parameters. When ε and δ are equal, η = 0 and the P-wave phase slowness is an ellipse. When ε = δ = 0, the P-wave phase slowness is isotropic.
Eta (η)
An appearance of seismic data as a diffraction, reflection, refraction or other similar feature produced by an arrival of seismic energy. An event can be a single wiggle within a trace, or a consistent lining up of several wiggles over several traces. An event in a seismic section can represent a geologic interface, such as a fault, unconformity or change in lithology.
Event
Surface seismic data acquired using an explosive energy source, such as dynamite.
Explosive seismic data
An in-line offset spread.
Extended spread
A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures.
Extensive dilatancy anisotropy / EDA
A magnetic disturbance field generated by electric currents flowing in the ionosphere and magnetosphere and “mirror-currents” induced in the Earth and oceans by the external magnetic field time variations. The disturbance field, which is associated with diurnal field variations and magnetic storms, is affected by solar activity (solar wind), the interplanetary magnetic field and the Earth’s magnetic field. The external magnetic field exhibits variations on several time scales, which may affect the applicability of magnetic reference models. Very long-period variations are related to the solar cycle of about 11 years. Short-term variations result from daily changes in solar radiation, atmospheric tides and conductivity. Irregular time variations are influenced by the solar wind. Perturbed magnetic states, called magnetic storms, occur and show impulsive and unpredictable rapid time variations.
external disturbance field
A technique for acquiring seismic refraction data around local, high-velocity features such as salt domes by using a fan or arc-shaped geophone array around a central shotpoint. The data from the fan-shaped array are calibrated against a control profile acquired some distance from the anomalous feature.
fan shooting
