The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.


daughter products

The radioactive natural sources of gamma-rays decay from the original “parent” element (commonly potassium, uranium, and thorium) to one or more lower-energy “daughter” elements. Some of these lower energy elements are also radioactive and decay further. Gamma-ray spectrometry surveys may measure the gamma rays given off by the original element or by the decay of the daughter products.

dB/dt

As the secondary electromagnetic field changes with time, the magnetic field (B) component induces a voltage in the receiving coil, which is proportional to the rate of change of the magnetic field over time.

decay

In time-domain electromagnetic theory, the weakening over time of the eddy currents in the ground, and hence the secondary field after the primary field electromagnetic pulse is turned off. In nuclear physics, including gamma-ray spectrometry, the radioactive breakdown of an element, generally potassium, uranium, thorium, or one of their daughter products.

decay constant

See: time constant

decay series

In gamma-ray spectrometry, a series of progressively lower energy daughter products produced by the radioactive breakdown of uranium or thorium.

decentralise

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

Mass per unit volume, expressed in grams per cubic centimetre. Rock or formation densities are usually measured as either saturated bulk densities or grain densities. For gravity interpretation, the contrasts between rock bulk densities are of primary interest since these contrasts are responsible for the anomalous gravity field. Rock bulk densities have been shown to vary as a function of geologic age, lithology and depth of burial. Rock densities typically range from 1.9 g/cm3 to 3.0 g/cm3.

density contrast

The density of one rock unit relative to another. Density contrasts can be either positive or negative. For example, if Rock A = 2.30 g/cm3 and Rock B = 2.40 g/cm3 then the density contrast of Rock A relative to Rock B is -0.10 g/cm3. Conversely, the relative density contrast of Rock B relative to Rock A is +0.10 g/cm3. gravity anomalies caused by density contrasts within the Earth’s sedimentary section, crust and sub-crust can be analysed and interpreted as lithologic and/or structural anomalies.

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.

density model

A model of the geology in which layers or bodies of given lithologies are replaced by equi-density layers or bodies. The equi-density layers or bodies may or may not correspond to specific geological formations.

density-depth function

The relationship between the change in density with a change in depth. In many areas of the world with thick clastic sections the increase in density with an increase in depth has been shown to be primarily a function of compaction. However, age, lithology and porosity may also influence the relationship. The relationship is important in gravity modelling because a gravity anomaly may be caused by a gradational change in density rather than a relatively abrupt density contrast, such as that which may occur at a fault, contact, or unconformity.

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 exploration or depth of investigation (DOI)

The maximum depth at which the geophysical system can detect the target. The depth of exploration depends very strongly on the type and size of the target, the contrast of the target with the surrounding geology, the homogeneity of the surrounding geology, and the type of geophysical system. One measure of the maximum depth of exploration for an electromagnetic system is the depth at which it can detect the strongest conductive target – generally a highly conductive horizontal layer.

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.

depth slicing

Generically, the use of linear filters to isolate (based on wavelength criteria) anomaly contributions to a map derived from source bodies in a certain depth range. Numerous techniques are used to carry out the isolation.

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 (e)

The capacity of a material to store electrical charge, this is most often measured as the relative permittivity (er), or ratio of the material dielectric to that of free space. The effect of high permittivity may be seen in HEM data at high frequencies over highly resistive geology as a reduced or negative in-phase, and higher quadrature data.

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.

differential resistivity

A process of transforming apparent resistivity to an approximation of layer resistivity at each depth. The method uses multi-frequency HEM data and approximates the effect of shallow layer conductance determined from higher frequencies to estimate the deeper conductivities (Huang and Fraser, 1996)

digital log

A log recorded as a series of discrete numerical values (compare analogue 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.

dipole moment (NIA)

For a transmitter, the product of the area of a coil, the number of turns of wire, and the current flowing in the coil. At a distance significantly larger than the size of the coil, the magnetic field from a coil will be the same if the dipole moment product is the same. For a receiver coil, this is the product of the area and the number of turns. The sensitivity to a magnetic field (assuming the source is far away) will be the same if the dipole moment is the same.

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 analysed as a function of wave frequency.

displacement currents

The movement of charge within a material by polarisation, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).

diurnal

The daily variation in a natural field, normally used to describe the natural fluctuations (over hours and days) of the Earth’s magnetic field.

dose rate

See: exposure rate

drape

To fly a survey following the terrain contours, maintaining a constant altitude above the local ground surface. Also applied to re-processing data collected at varying altitudes above ground to simulate a survey flown at constant altitude.

drift

Long-time variations in the base-level or calibration of an instrument.

dual laterolog

A focussed resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.

dyke model

See: prism. Dyke model descriptions include wide, narrow, thin, vertical, and inclined.