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

imaging work station

Consists of a microcomputer with a high-resolution colour monitor and accompanying software which allows the manipulation, enhancement and visual display of digital data.

in-phase

The component of the measured secondary field that has the same phase as the transmitter and the primary field. The in-phase component is stronger than the quadrature phase over relatively higher conductivity. See also quadrature.

induced magnetisation

Magnetization caused by an applied magnetic field. Contrast with remanent magnetisation.

induced polarisation (IP)

A geophysical effect whereby electrical charge is momentarily polarised 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 polarisation 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. See: eddy 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.

inductive limit

When the frequency of an EM system is very high, or the conductivity of the target is very high, the response measured will be entirely in-phase with no quadrature (phase angle =0). The in-phase response will remain constant with further increase in conductivity or frequency. The system can no longer detect changes in conductivity of the target.

infinite

In geophysical terms, an “infinite’ dimension is one much greater than the footprint of the system, so that the system does not detect changes at the edges of the object.

international geomagnetic Reference Field (IGRF)

An approximation of the smooth magnetic field of the Earth, in the absence of variations due to local geology. Once the IGRF is subtracted from the measured magnetic total field data, any remaining variations are assumed to be due to local geology. The IGRF also predicts the slow changes of the field up to five years in the future.

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.

The annular interval of material around a drill hole where drilling fluid has replaced all or part of the native interstitial fluids.

inversion or inverse modelling

A technique whereby a 2d or 3d density, susceptibility, or geometric (geologic) model is computed to satisfy (invert) a given observed gravity or magnetic field.

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.