Glossary of Geophysical Terms (A,B,C)

A electrode
One of the current-emitting electrodes of a resistivity-imaging system (A); the current return electrode is labeled B.
API unit
The American Petroleum Institute (API) has established test pits for calibrating neutron and gamma logs. The API neutron unit is defined as 1/1,000 of the difference between electrical zero and the logged value opposite the Indiana limestone in the calibration pit with an average porosity of 19 percent. The API gamma unit is defined as 1/200 of the deflection between intervals of high and low radioactivity in the calibration pit.
Accuracy
Closeness of a measurement to the true value.
Acoustic impedance
Seismic velocity multiplied by density.
Acoustic log
Also called sonic log; a record of sound waves as they are transmitted through liquid-filled rock; a record of the transit time (t) is the most common; amplitude and the full acoustic-wave form also are recorded.
Acoustic televiewer log
A record of the amplitude of high-frequency acoustic pulses reflected by the borehole wall; provides location and orientation of bedding, fractures, and cavities.
Acoustic wave
A sound wave transmitted through material by elastic deformation.
Alluvium
A general term for unconsolidated material (e.g. clay, silt, sand, gravel) deposited from running water. Often a sorted or semi-sorted sediment in the bed of a stream or on its floodplain or delta. The deposit may be in the form of an alluvial fan.
Amplitude
The maximum departure of a wave from the average value.
Analog recording
Data are represented as a continuous record of physical variables instead of discrete values, as in digital recording.
Anisotropic
Having a variation in physical properties that are dependent on the orientation of the measurement.
Annulus
The space between the drill pipe or casing and the wall of the drill hole; in rocks saturated with hydrocarbons, the annulus is the transition interval between the invaded zone and the uncontaminated zone.
Anomaly
Refers to a deviation from uniformity in a physical property.
Apparent resistivity/conductivity
The resistivity of a homogeneous isotropic ground that would give the same voltage/current or secondary/primary field ratios as observed in the field with resistivity or EM methods. The apparent conductivity is the reciprocal of the apparent resistivity.
Aquifer
Rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring.
Aquitard
Geologic formation/s of significantly low hydraulic conductivity, typically saturated, but yielding a limited amount of water to wells. Also referred to as a confining unit.
Archie’s Law
An empirical relationship linking formation resistivity ( rt), formation water resistivity (r w) and porosity. The form of the relationship is r t = a rw -m where a and m are experimentally determined constants.
Atomic number (Z)
The number of protons in the nucleus of an atom equal to the number of electrons in a neutral atom.
Attenuation, attenuate
A reduction in energy or amplitude caused by the physical characteristics of a transmitting system.
Automatic Gain Control (AGC)
A process for increasing signal amplitude of a signal through time, thus making all events on the trace appear to be of approximately the same amplitude. Note that this process will expand the amplitudes even if no data are present. Various window lengths are used; the appearance of the data may be greatly affected by the window used in the calculation.
Bedrock
A general term referring to rock that underlies unconsolidated material.
Borehole-compensated
Probes designed to reduce the extraneous effects of the borehole, casing, and of probe position are called borehole-compensated.
Borehole television or video
A downhole television camera; see acoustic-televiewer definition.
Bottom-hole temperature
The bottom-hole temperature (BHT) usually is measured with maximum recording thermometers attached to a logging probe.
Bouguer correction
The process of correcting gravity data for the mass of the rock between a given station and its reference (base) station. Application of the Bouguer correction to the data set, as well as corrections for latitude, topography, meter drift and elevation, yields the Bouguer anomaly.
Brute stack
A common midpoint stack with only preliminary static corrections (often none) and preliminary normal-moveout corrections (often constant velocity). This stack is often done by field computers to verify the existence of actual reflections.
Bulk density
Bulk density is the mass of material per unit volume; in logging, it is the density, in grams per cubic centimeter, of the rock with pore volume filled with fluid.
Bulk modulus
A modulus of elasticity, relating change in volume to the hydrostatic state of stress. It is the reciprocal of compressibility.
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].