Glossary of Geophysical Terms (G,H,I)

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.
Half-life
Radioactively, half-life is the time required for half of a given quantity of material to decay. Chemically, it is the time required for half of a given quantity of material to undergo a chemical reaction.
Imaging work station
Consists of a microcomputer with a high-resolution color monitor and accompanying software which allows the manipulation, enhancement and visual display of digital data.
Induced magnetization
Magnetization caused by an applied magnetic field. Contrast with remanent magnetization.
Induced polarization (IP)
A geophysical effect whereby electrical ge is momentarily polarized within a material, usually a disseminated ore or a clay. This effect is the basis for the IP method, in which a decaying voltage due to this polarization is measured following the turn-off of the activating current in time domain surveying. See also complex resistivity.
Induction (EM), induce
The process, described by Faraday’s Law, whereby a variable magnetic field generates an electric field (voltage) that, in the presence of a conductor, will produce electric currents.
Induction log
A method for measuring resistivity or conductivity that uses an electromagnetic technique to induce a flow of current in the rocks around a borehole; can be used in nonconductive-borehole fluids.
Induction number
A quantitative measure of the quality of a target for EM methods. The formulation varies for different targets but in general it involves the product of target conductivity, magnetic permeability, frequency of the transmitter and a cross-sectional dimension of the target. Dimensionless.
In-phase
That part of a periodic signal that has zero phase shift with a reference signal. See also quadrature.
Interpolation
A method to determine intermediate values from surrounding known values.
Interpretation
Transforming geophysical measurements into subsurface structure. More general term than inversion.
Interval transit time
The time required for a compressional acoustic wave to travel a unit distance (t); transit time usually is measured by acoustic or sonic logs, in microseconds per foot, and is the reciprocal of velocity.
Invaded zone
The annular interval of material around a drill hole where drilling fluid has replaced all or part of the native interstitial fluids.
Inversion, inverting
The process of deriving a model of the subsurface that is consistent with the geophysical data obtained. Generally refers to a more specific methodology than interpretation.
Isotopes
Atoms of the same element that have the same atomic number, but a different mass number; unstable isotopes are radioactive and decay to become stable isotopes.