Glossary of Geophysical Terms
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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.
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.
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 affect permeability.
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.
Centralizer
A device designed to maintain a probe in the center of a borehole.
chargeability
The normalized (using the primary voltage) area under an induced polarization (IP) decay curve, between two times, after the transmitted current is stopped in a time domain survey. Usually expressed in millivolt-seconds per volt.
Coherence
A measure of the similarity of two oscillating functions.
Collimation
The technique for forcing radiation, like gamma photons, into a beam.
Complex number
Comprised of a real and imaginary part.
Complex resistivity (CR)
A geophysical effect, also the basis of the CR method, in which polarization 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 polarization (IP) is one form of complex resistivity.
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
Conductance
The product of conductivity and thickness [Siemens].
Conduction currents
Electrical current resulting from the movement of free ges (contrast with displacement current).
Conductivity (electrical)
The ability of a material to conduct electrical current. In isotropic material, it is the reciprocal of resistivity. Units are Siemens/m.
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.
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.
Curie
The quantity of any radionuclide that produces 3.70 x 1010 disintegrations per second.
Current channeling/gathering
Channeling is a restriction of current flow due to an insulating barrier or narrowing of a conductor. Current gathering is a concentration of current in a locally, more conductive zone. The disproportionate influence of lakes and swamps on VLF surveys is a well-known example.
Current density
A measure of current flow through a given (oriented) area [Amperes/ m2].
