The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.
In time-domain electromagnetic surveys, the magnetic field component of the (electromagnetic) field. This can be measured directly, although more commonly it is calculated by integrating the time rate of change of the magnetic field dB/dt, as measured with a receiver coil.
The “normal” response in the geophysical data – that response observed over most of the survey area. Anomalies are usually measured relative to the background. In airborne gamma-ray spectrometric surveys the term defines the cosmic, radon, and aircraft responses in the absence of a signal from the ground.
- base frequency
The frequency of the pulse repetition for a time-domain electromagnetic system. Measured between subsequent positive pulses.
The measured values in a geophysical system in the absence of any outside signal. All geophysical data are measured relative to the system base level.
A general term referring to rock that underlies unconsolidated material.
A common name for the pod towed beneath or behind an aircraft, carrying the geophysical sensor array.
- borehole television or video
A downhole television camera; see acoustic-televiewer definition.
Probes designed to reduce the extraneous effects of the borehole, casing, and of probe position are called borehole-compensated.
- 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.
- Bouguer gravity field
The gravity field obtained after latitude, elevation, Bouguer, and terrain corrections have been applied to the measured (observed or raw) gravity data. The Bouguer (named after Pierre Bouguer, a French geodesist) gravity field is often noted as simple Bouguer for the gravity field before applying terrain corrections or complete Bouguer for the gravity field after applying terrain (and sometimes curvature) corrections. The gravity anomalies observed in the Bouguer field are caused by lateral density contrasts within the sedimentary section, crust and sub-crust of the Earth. A measured above sea level Bouguer gravity field and accurately corrected to sea level datum is not equivalent to gravity measured at sea level. Anomalies caused by mass inhomogeneities between station elevation and datum and which were measured at the original station elevations remain in the data unless special corrections are made.
- 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.
The process of removing the strong signal from the primary field at the receiver from the data, to measure the secondary field. It can be done electronically or mathematically. This is done in frequency-domain EM and to measure on-time in time-domain EM.
- bulk density
Bulk density is the mass of material per unit volume; in logging, it is the density, in grams per cubic centimetre, 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.