Federal Highways Administration- Geophysical Imaging Resource Site

Glossary- Geophysical Terminology (G - M)... |A - F | N - T | U - Z | Conversion Table |

G | H | I | J | K | L | M

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.

Karst. Topographic area which has been created by the dissolution of carbonate rock terrain. It is characterized by caverns, sinkholes, and the absence of surface streams.

Lag. The distance a nuclear logging probe moves during one time constant.

Last reading. The depth of the shallowest value recorded on a log.

Lateral log. A multielectrode, resistivity-logging technique that has a much greater radius of investigation than the normal techniques, but requires thick beds and produces an unsymmetrical curve.

Laterolog. A focused-resistivity logging technique; see also guard log.

Light-non-aqueous phase liquids (LNAPLs). Organic fluids that are less dense than water. They are capable of forming an immiscible layer that floats on the water table (e.g. petroleum hydrocarbons or other organic liquids). Also referred to as Floaters.

Long normal log. A resistivity log with AM spacing usually 64 in.; see normal logs.

Lag. The distance a nuclear logging probe moves during one time constant.

Last reading. The depth of the shallowest value recorded on a log.

Laterolog. A focused-resistivity logging technique; see also guard log.

Long normal log. A resistivity log with AM spacing usually 64 in.; see normal logs.

Lag. The distance a nuclear logging probe moves during one time constant.

Last reading. The depth of the shallowest value recorded on a log.

Laterolog. A focused-resistivity logging technique; see also guard log.

Light-non-aqueous phase liquids (LNAPLs). Organic fluids that are less dense than water. They are capable of forming an immiscible layer that floats on the water table (e.g. petroleum hydrocarbons or other organic liquids). Also referred to as Floaters.

Long normal log. A resistivity log with AM spacing usually 64 in.; see normal logs.

M electrode. The potential electrode nearest to the A electrode in a resistivity device.

Magnetics, geomagnetics. Geophysical methodology for studying anomalies in the geomagnetic field due to non-uniform magnetization of the subsurface. Uses magnetometers.

Magnetic permeability. acteristic of a material, it is proportional to the magnetism induced in that material divided by strength of the magnetic field used.

Magnetic susceptibility. A measure of the extent to which a substance may be magnetized; it represents the ratio of magnetization to magnetic field strength.

Magnetization. The magnetic moment per unit volume. It is a vector quantity. See also magnetic susceptibility.

Magnetometer. A device for measuring the earth's magnetic geomagnetic field. Variations in the field strength may indicate changes in magnetic properties of soil and rock or presence of ferrous metals.

Mapping. Locating geological, chemical or geophysical information in space (as opposed to time, which is monitoring). The results are usually summarized as maps.

Matrix. The solid framework of rock or mineral grains that surrounds the pore spaces.

Mho. A unit of electrical conductance that is the reciprocal of ohm.

Micro-gravity survey. A surface geophysical survey method, undertaken on a very small scale (typically station spacings of a few meters), and requiring a high meter sensitivity. Measures the earth's gravitational field at different points over an area of interest. Variations in the field are related to differences in subsurface density distributions, which in turn are associated with changes in soil, rock, and cultural factors. Typically used for cavern or fracture detection.

Microresistivity log. Refers to a group of short-spaced resistivity logs that are used to make measurements of the mud cake and invaded zone.

Migration. The movement of chemicals, bacteria, gases, etc. in flowing water or vapor in the subsurface. Also, a seismic/radar term whose general meaning is the correction of the recorded image for the effects of reflector dip. A very typical result of migration is the removal of hyperbolic events on the record resulting from diffractions from faults and other discontinuities.

Monitoring. Observing the change in a geophysical, hydrogeological or geochemical measurement with time.

Mud cake. Also called filter cake; the layer of mud particles that builds up on the wall of a rotary-drilled hole as mud filtrate is lost to the formation.

Mud filtrate. The liquid effluent of drilling mud that penetrates the wall of the hole.

Muting. Change in the amplitude of all or part of a trace before additional processing. Noisy or clearly erroneous traces are given zero amplitude. Data before the first break and the known refraction arrivals are also often reduced to zero amplitude.