GEOPHYSICS

1. n. [Geophysics] 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. Alternate Form: blasting cap See: cap,  detonate,  seismic acquisition

detonator

1. n. [Geophysics] A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity. See: electrical permittivity

dielectric

1. n. [Geophysics] 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. See: four-dimensional seismic data,  isochron map,  seismic survey

difference map

1. n. [Geophysics] 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. See: static correction,  weathered layer

differential weathering correction

1. n. [Geophysics] 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. See: abnormal events,  acquisition,  arrival,  hydrocarbon indicator,  Kirchhoff migration,  raypath,  reflection,  time migration,  wavefront

diffraction

1. n. [Geophysics] 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. Synonyms: Kirchhoff migration See: diffraction,  Kirchhoff equation,  migration,  ray tracing

diffraction stack

1. n. [Geophysics] The movement of ions or molecules from regions of high concentration to low concentration within a solution. See: spontaneous potential

diffusion

1. n. [Geophysics] 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, where E = electrical field ω = angular frequency μ = magnetic permeability σ = electrical conductivity ∇ = vector differential operator. See: electromagnetic method,  wave equation

diffusion equation

1. n. [Geophysics] The increase in the volume of rocks as a result of deformation, such as when fractures develop. See: fracture

dilatancy

1. n. [Geophysics] 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. See: P-wave,  S-wave

dilatancy theory

1. n. [Geophysics] The process of changing volume as stress is applied to a body.

dilatation

1. n. [Geophysics] 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. Synonyms: acoustic wave,  compressional wave,  P-wave See: amplitude variation with offset,  dilatation,  four-component seismic data,  Poisson's ratio,  rarefaction,  S-wave,  shadow zone

dilatational wave

1. n. [Geophysics] 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. Antonyms: bright spot See: acoustic impedance,  anomaly,  hydrocarbon indicator,  polarity standard

dim spot

1. n. [Geophysics] 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. Alternate Form: DMO See: moveout,  quicklook,  receiver,  reflector,  traveltime

dip moveout

1. n. [Geophysics] 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

1. n. [Geophysics] The primary contribution to Earth’s main magnetic field. See: main magnetic field,  nondipole field

dipole field

1. n. [Geophysics] 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. See: air gun,  geophone array,  ground roll,  hydrophone,  primer cord,  receiver,  seismic processing,  shaped charge,  source,  vibrator,  wave

directivity

1. n. [Geophysics] 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. See: Mohorovicic discontinuity,  P-wave,  seismic wave

discontinuity

1. n. [Geophysics] 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. See: body wave,  distortion,  frequency,  P-wave,  surface wave

dispersion

1. n. [Geology] 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. See: fault,  transform fault

displacement