The strength of the total electromagnetic field. In frequency domain it is most often the sum of the squares of in-phase and quadrature components. In multi-component electromagnetic surveys it is generally the sum of the squares of all three directional components.

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.

A localized change in the geophysical data characteristic of a discrete source, such as a conductive or magnetic body.

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.

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.

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.

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 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.

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.

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).

See: conductivity thickness

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 mineralization will be equivalent to 5 metres of 40 S/m; both have 200 S conductivity thickness. Sometimes referred to as conductance.

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

In HEM, the coplanar coils lie in the horizontal plane with their axes vertical, and parallel. These coils are most sensitive to massive conductive bodies, horizontal layers, and the halfspace.

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.

See: current gathering

The radioactive natural sources of gamma-rays decay from the original “parent” element (commonly potassium, uranium, and thorium) to one or more lower-energy “daughter” elements. Some of these lower energy elements are also radioactive and decay further. Gamma-ray spectrometry surveys may measure the gamma rays given off by the original element or by the decay of the daughter products.