The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.
An elastic body wave in which particles move in the direction of propagation. It is the wave assumed in most seismic surveys. Also called primary or push-pull wave.
- percentage frequency effect (PFE)
The percent difference in resistivity measured at two frequencies (one high, one low). It is the basic polarisation parameter measured in frequency domain resistivity surveys. Equivalent to chargeability in time domain surveys.
Perennially frozen ground in areas where the temperature remains at or below 0o C for two or more years in a row.
See: magnetic permeability
The property which enables a three-dimensional material to store electrical charge; i.e. its capacitivity. See: dielectric permittivity
- phase II study
Common nomenclature for the part of an environmental investigation that first involves on-site activities (i.e. geophysics, soil gas surveys and drilling)
- phase shift
A measure of the offset between two periodic signals of the same frequency. Measured in degrees or radians/milliradians.
- phase, phase angle
The angular difference in time between a measured sinusoidal electromagnetic field and a reference, normally the primary field. The phase is calculated from tan-1 (in-phase/quadrature).
- Phillips’ method
An automatic depth estimation method in which the source parameters are estimated from the autocorrelation function of the magnetic anomaly. like Werner deconvolution, the method uses a dike or contact model.
- physical parameters, physical properties
These are the characteristics of a geological unit. For electromagnetic surveys, the important parameters are conductivity, magnetic permeability (or susceptibility) and dielectric permittivity; for magnetic surveys the parameter is magnetic susceptibility, and for gamma ray spectrometric surveys it is the concentration of the major radioactive elements: potassium, uranium, and thorium.
A term used to describe a sheet-like magnetic source body with limited vertical dimension. That is, its thickness may range from 0.1 to 1.0 times its depth-to-top. Its anomaly character is similar to that of a set of dipoles.
- polarise, polarisation, polarisable
Separation of charge, as in induced polarisation or IP.
The ratio of the void volume of a porous rock to the total volume, usually expressed as a percentage.
- potential field
A field which obeys a differential equation known as Laplace’s Equation. Gravity and magnetic fields are both vector potential fields. Most exploration gravity work utilises the vertical component of the gravity field, while most exploration magnetic work utilises the scalar total intensity of the magnetic field.
The reproducibility of a measurement; the closeness of each of a set of similar measurements to the arithmetic mean of that set.
- primary (magnetic field)
The magnetic field generated by an EM transmitter. May induce a secondary magnetic field.
- primary field
The EM field emitted by a transmitter. This field induces eddy currents in (energises) the conductors in the ground, which then create their own secondary fields.
A term used to describe a magnetic source body which can be considered, for practical purposes, parallelepiped which is semi-infinite in vertical dimension. That is, its depth-to-bottom is at least four times its depth-to-top. Its anomaly character is similar to that of a monopole or line of poles. A two-dimensional prism (semi-infinite normal to the plane of section) is sometimes referred to as a dike model.
Also called sonde or tool; downhole well-logging instrument package.
Geophysically, to change data so as to emphasise certain aspects or correct for known influences, thereby facilitating interpretation.
In geophysics, a survey method whereby an array of sensors is moved along the Earth’s surface without change in its configuration, in order to detect lateral changes in the properties of the subsurface (faults, buried channels, etc.) The alternative is usually a sounding.
The nucleus of a hydrogen atom; a positively charged nuclear particle with a mass of one; see neutron.
An approximation of a gravity field derived from a magnetic field measured at, or transformed to, the magnetic pole. The process requires conversion of susceptibility values to density values and a vertical integration of the magnetic data.
A cross section showing the distribution of a geophysical property, such as seismic travel time, from which the distribution of the geological property of interest (depth to bedrock, for example) can be interpreted.
In time-domain EM surveys, the short period of intense primary field transmission. Most measurements (the off-time) are measured after the pulse. On-time measurements may be made during the pulse.