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


calibration

Determination of the log values that correspond to environmental units, such as porosity or bulk density; calibration usually is carried out in pits or by comparison with laboratory analyses of core.

calibration coil

A wire coil of known size and dipole moment, which is used to generate a field of known amplitude and phase in the receiver, for system calibration. Calibration coils can be external, or internal to the system. Internal coils may be called Q-coils.

caliper log

A continuous record of borehole diameter, usually made with a mechanical probe having from one to six arms.

casing-collar locator

An electromagnetic device (CCl) that usually is run with other logs to record the location of collars or other changes in casing or pipe.

cement bond log

An acoustic amplitude log that is used to determine the location of cement behind the casing and, under some conditions, the quality of the bonding to casing and rock.

cementation factor

The cementation exponent (m) in Archie’s equation relating formation-resistivity factor and porosity; cementation factor as relates to many aspects of pore and grain geometry that affects permeability.

centralizer

A device designed to maintain a probe in the centre of a borehole.

chargeability

The normalised (using the primary voltage) area under an induced polarisation (IP) decay curve, between two times, after the transmitted current is stopped in a time domain survey. Usually expressed in millivolt-seconds per volt.

coaxial coils (CX)

Coaxial coils in an HEM system are in the vertical plane, with their axes horizontal and collinear in the flight direction. These are most sensitive to vertical conductive objects in the ground, such as thin, steeply dipping conductors perpendicular to the flight direction. Coaxial coils generally give the sharpest anomalies over localised conductors. See: coplanar coils

coherence

A measure of the similarity of two oscillating functions.

coil

A multi-turn wire loop used to transmit or detect electromagnetic fields. Time varying electromagnetic fields through a coil induce a voltage proportional to the strength of the field and the rate of change over time.

compensation

Correction of airborne geophysical data for the changing effect of the aircraft. This process is generally used to correct data in fixed-wing time-domain electromagnetic surveys (where the transmitter is on the aircraft and the receiver is moving), and magnetic surveys (where the sensor is on the aircraft, turning in the Earth’s magnetic field.

complex number

Comprised of a real and imaginary part.

complex resistivity (CR)

A geophysical effect, also the basis of the CR method, in which polarisation within the medium results in the voltage and applied current being out of phase – that is, their ratio is complex. Also known as spectral IP. Induced polarisation (IP) is one form of complex resistivity.

component

In frequency domain electromagnetic surveys this is one of the two phase measurements – in-phase or quadrature. In “multi-component” electromagnetic surveys it is also used to define the measurement in one geometric direction (vertical, horizontal in-line and horizontal transverse – the Z, X and Y components).

compressibility

The relative volume reduction that geological material can undergo when a force is applied or water is removed from the vicinity by pumping.

compressional wave

Compressional acoustic waves (P) are propagated in the same direction as particle displacement; they are faster than shear waves and are used for measuring acoustic velocity or transit time

compton scattering

Gamma ray photons will bounce off electrons as they pass through the Earth and atmosphere, reducing their energy and then being detected by radiometric sensors at lower energy levels. See: stripping.

conductance

The product of conductivity and thickness (Siemens, S). See: conductivity thickness.

conduction currents

Electrical current resulting from the movement of free charges (contrast with displacement current).

conductivity (s)

The ability of a material to conduct electrical current. In isotropic material, it is the reciprocal of resistivity. Units are Siemens/m or usually milli-Siemens per metre (mS/m). It is the inverse of resistivity.

conductivity thickness (st)

The product of the conductivity, and thickness of a large, tabular body. (It is also called the “conductivity-thickness product”). In electromagnetic geophysics, the response of a thin plate-like conductor is proportional to the conductivity multiplied by thickness. For example a 10 metre thickness of 20 Siemens/m mineralisation will be equivalent to 5 metres of 40 S/m; both have 200 S conductivity thickness. Sometimes referred to as conductance.

conductivity-depth imaging

See: conductivity-depth transform

conductivity-depth transform

A process for converting electromagnetic measurements to an approximation of the conductivity distribution vertically in the Earth, assuming a layered Earth.

conductor

Used to describe anything in the ground more conductive than the surrounding geology. Conductors are most often clays or graphite, or hopefully some type of mineralisation, but may also be man-made objects, such as fences or pipelines.

correlation

Determination of the position of stratigraphically equivalent rock units in different wells, often done by matching the character of geophysical logs; also the matching of variables, such as log response and core analyses.

cosmic ray

High energy sub-atomic particles from outer space that collide with the Earth’s atmosphere to produce a shower of gamma rays (and other particles) at high energies.

counts (per second)

The number of gamma-rays detected by a gamma-ray spectrometer. The rate depends on the geology, but also on the size and sensitivity of the detector.

cross-hole

Geophysical methods carried out between boreholes (see also tomography).

crossplot

A term used in log analysis for a plot of one parameter versus another, usually two different types of logs. Useful for the identification of lithology.

cultural environment

The part of the environment which represents man-made features (e.g. roads, buildings, canals, bridges) as opposed to natural features.

culture

A term commonly used to denote any man-made object that creates a geophysical anomaly. Includes, but not limited to, power lines, pipelines, fences, and buildings.

curie

The quantity of any radionuclide that produces 3.70 x 1010 disintegrations per second.

current channelling

Channelling is a restriction of current flow due to an insulating barrier or narrowing of a conductor.

current density

A measure of current flow through a given (oriented) area [Amperes/ m2].

current gathering

The tendency of electrical currents in the ground to channel into a conductive formation. This is particularly noticeable at higher frequencies or early time channels when the formation is long and parallel to the direction of current flow. This tends to enhance anomalies relative to inductive currents (see also induction). The disproportionate influence of lakes and swamps on VLF surveys is a well-known example. Also known as current channelling.