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


Describes geophysical techniques that require direct contact with the ground in order to pass current. The alternative is to induce currents in the earth.


The common unit of magnetic field intensity, equal to one nanoTesla (a Tesla is the SI unit). The Earth’s magnetic field strength is about 50,000 gammas (g) in mid-latitudes.

gamma log

Also called gamma-ray log or natural-gamma log; log of the natural radioactivity of the rocks penetrated by a drill hole; also will detect gamma-emitting artificial radioisotopes (see spectral-gamma log).

gamma ray

A photon that has neither mass nor electrical charge that is emitted by the nucleus of an atom; measured in gamma logging and output from a source used in gamma-gamma logging.

gamma ray spectrometry

Measurement of the number and energy of natural (and sometimes man-made) gamma-rays across a range of photon energies.

Gardner’s equation

An empirically derived equation which describes the relationship between bulk densities and acoustic velocities of rocks: p = 0.23v0.25

geomagnetic field

The Earth’s magnetic field.


Receivers used to record the seismic energy arriving from a source, in seismic geophysical methods.

geophysical mapping

Locating geophysical anomalies in space (as opposed to time, which is geophysical monitoring).

geophysical monitoring

Observing the change in a geophysical measurement with time.


In magnetic surveys, the gradient is the change of the magnetic field over a distance, either vertically or horizontally in either of two directions. Gradient data is often measured, or calculated from the total magnetic field data because it changes more quickly over distance than the total magnetic field, and so may provide a more precise measure of the location of a source. See: analytic signal


A device or set of devices which measures the value of a field in at least two different points in space at the same time. The gradient is the difference in field values per unit of distance between the sensors. By measuring a field’s gradient (that is, its first derivative or rate of change with distance), the total field itself may be computed with varying degrees of accuracy. For potential fields, the direction of the measurement relative to the Earth is critical. Is the gradient being measured horizontally, vertically, and in the case of magnetics, what is the orientation relative to the Earth’s magnetic field? Even with these possible difficulties, measuring just the gradient has the advantage of removing non-geologic field signals, such as when measuring gravity, those introduced by the normal accelerations of the survey aircraft.

grain density

Also called matrix density; the density of a unit volume of rock matrix at zero porosity, in grams per cubic centimetre.

gravity unit

The SI unit of acceleration used with gravity measurements. Abbreviated as gu. 1 gravity unit = 1 μs/s2 = 0.1 milligal.

ground effect

The response from the Earth. A common calibration procedure in many geophysical surveys is to fly to altitude high enough to be beyond any measurable response from the ground, and there establish base levels or backgrounds.

ground electrode

A surface electrode used for SP and resistivity logging.

ground penetrating radar (gPR)

A geophysical method in which bursts of electromagnetic energy are transmitted downwards from the surface, to be reflected and refracted by velocity contrasts within the subsurface. Also known as ground Probing Radar.

guard log

A type of focused resistivity log that derives its name from guard electrodes that are designed to focus the flow of current.