Glossary of Geophysical Terms
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D
Decay
In nuclear physics, the process of disintegration of an unstable radioisotope by the spontaneous emission of ged particles or photons.
Decentralize
Forcing a logging probe against one side of the drill hole.
Deconvolution
A data processing technique applied to seismic reflection data to improve the detection and resolution of reflected events. The process reverses the effect of linear filtering processes (convolution) that have been applied to the data by recording instruments or other processes.
Dense-non-aqueous-phase liquids (DNAPLs)
Organic liquids that are more dense than water. They often coalesce in an immiscible layer at the bottom of a saturated geologic unit.
Density log
Also called gamma-gamma log; gamma photons from a radioactive source in the sonde are backscattered to a detector; the backscattering is related to the bulk density of the material around the sonde.
Departure curves
Graphs that show the correction that may be made to logs for some extraneous effects, such as hole diameter, bed thickness, temperature, etc.
Depth of investigation
Limit of signal penetration.
Depth reference or datum
Zero reference point for logs of a borehole.
Depth section
A cross section to which a velocity function has been applied, thus converting arrival times of reflections to depths.
Detector
Any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals.
Deviation
The departure in degrees between the drill hole or probe axis and vertical.
Dielectric constant
A measure of the ability of a material to store charge when an electric field is applied.
Dielectric permittivity
Describes the ge separation or polarization in a medium.
Differential log
A log that records the rate of change of some logged value as a function of depth; the differential log is sensitive to very small changes in absolute value.
Digital log
A log recorded as a series of discrete numerical values (compare analog recording).
Dipmeter
A multi-electrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
Dipole
A pair of equal charges or poles of opposite signs. In resistivity, a pair of closely spaced electrodes approximating a dipole.
Directional survey
A log that provides data on the azimuth and deviation of a borehole from the vertical.
Dispersion
A property of seismic surface waves in which their velocity (as well as their penetration into the subsurface) is frequency dependent. The basis of methods such as MASW in which seismic wave velocity is analyzed as a function of wave frequency.
Displacement currents
The movement of charge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).
Dual laterolog
A focused resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.
A multi-electrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
A pair of equal charges or poles of opposite signs. In resistivity, a pair of closely spaced electrodes approximating a dipole.
A log that provides data on the azimuth and deviation of a borehole from the vertical.
A property of seismic surface waves in which their velocity (as well as their penetration into the subsurface) is frequency dependent. The basis of methods such as MASW in which seismic wave velocity is analyzed as a function of wave frequency.
The movement of charge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).
A focused resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.
