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

A electrode

One of the current-emitting electrodes of a resistivity-imaging system (A); the current return electrode is labelled B.


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.


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.

altitude attenuation

The absorption of gamma rays by the atmosphere between the Earth and the detector. The number of gamma rays detected by a system decreases as the altitude increases.


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.

analytic signal

The total amplitude of all the directions of magnetic gradient. Calculated as the sum of the squares.

analytic signal method

The analytic signal method, known also as the total gradient method, as defined here produces a particular type of calculated gravity or magnetic anomaly enhancement map used for defining in a map sense the edges (boundaries) of geologically anomalous density or magnetisation distributions. In exploration potential field applications, the term analytic signal loosely refers to the calculated modulus of the gravity or magnetic anomaly field’s three mutually orthogonal spatial (x, y, z) derivative terms. Mapped maxima (ridges and peaks) in the calculated analytic signal of a gravity or magnetic anomaly map locate the anomalous source body edges and corners (e.g. basement fault block boundaries, basement lithology contacts, fault/shear zones, igneous and salt diapirs, etc.). Analytic signal maxima have the useful property that they occur directly over faults and contacts, regardless of structural dip which may be present, and independent of the direction of the induced and/or remanent body magnetisations. Various extensions to the analytic signal method (as defined here) exist. For example, some extensions to the method include as an additional solved parameter the anomalous source body depth(s).

Having a variation in physical properties that are dependent on the orientation of the measurement.

Having different physical parameters in different directions. This can be caused by layering or fabric in the geology. Note that a unit can be anisotropic, but still homogeneous.


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.


Refers to a deviation from uniformity in a physical property. A localised change in the geophysical data characteristic of a discrete source, such as a conductive or magnetic body.

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.


The physical parameters of the Earth measured by a geophysical system are normally expressed as apparent, as in “apparent resistivity”. This means that the measurement is limited by assumptions made about the geology in calculating the response measured by the geophysical system. Apparent resistivity calculated with HEM, for example, generally assumes that the Earth is a homogeneous half-space – not layered.

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.


Rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring.


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 (rw) and porosity. The form of the relationship is rt = 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.

automated depth estimation

A variety of techniques, which include Werner deconvolution, the Euler method, Naudy’s method, Phillips’ method, and the analytic signal method, which analyse digital analytic signal method magnetic profiles or maps to obtain estimates of source body depth without specific user identification of key portions of anomalies. This contrasts with profile techniques such as Peters’ method (half-slope) or Vacquier’s method (straight slope) which may be implemented as computer programs but require interactive identification of special points on anomalies.

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.