Glossary of Geophysical Terms
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G
Galvanic
Describes geophysical techniques that require direct contact with the ground in order to pass current. The alternative is to induce currents in the earth.
Gamma
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 ge that is emitted by the nucleus of an atom; measured in gamma logging and output from a source used in gamma-gamma logging.
Geomagnetic field
The Earth's magnetic field.
Geophones
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.
Grain density
Also called matrix density; the density of a unit volume of rock matrix at zero porosity, in grams per cubic centimeter.
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.
