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


Often used as a name for the time constant.

tau (τ)

Often used as a name for the time constant.


See: time-domain electromagnetic method


See: transient electromagnetic method

terrain conductivity

Geophysical method in which EM methods measure directly the average electrical conductivity of the ground. Operates at low induction number.

tesla (T)

The SI unit of magnetic field B, which is also known as “magnetic flux density” and “magnetic induction”. One tesla is equal to one weber per square meter

thermal neutron

A neutron that is in equilibrium with the surrounding medium such that it will not change energy (average 0.025 eV) until it is captured.

thin sheet

A standard model for electromagnetic geophysical theory. It is usually defined as a thin, flat-lying conductive sheet, infinite in both horizontal directions. See: vertical plate

three dimensional (3D) model

A network or grid of values which models a geologic surface represented as a surface of (gravity) or susceptibility contrast (magnetics). The output of a forward model is based on the calculated gravity or magnetic effect of a specified input surface. The output of an inverse model is the geometry of an appropriate (but non-unique) surface calculated by inverting the input gravity or magnetic field.

tie line

A survey line flown across most of the traverse lines, generally perpendicular to them, to assist in measuring drift and diurnal variation. In the short time required to fly a tie-line it is assumed that the drift and/or diurnal will be minimal, or at least changing at a constant rate.

time channel

In time-domain electromagnetic surveys the decaying secondary field is measured over a period of time, and the divided up into a series of consecutive discrete measurements over that time.

time constant

The time required for an electromagnetic field to decay to a value of 1/e of the original value. In time-domain electromagnetic data, the time constant is proportional to the size and conductance of a tabular conductive body. Also called the decay constant.

tIme domain

In geophysics refers to measurements analysed according to their behaviour in time. The usual alternative is frequency domain measurements.

time domain ectromagnetic method

See: transient electromagnetic method

time domain reflectometry (TDR)

A device, which measures electrical characteristics of wideband transmission systems. Commonly used to measure soil moisture content.


A method for determining the distribution of physical properties within the earth by inverting the results of a large number of measurements made in three dimensions (e.g. seismic, radar, resistivity, EM) between different source and receiver locations.

total energy envelope

The sum of the squares of the three components of the time-domain electromagnetic secondary field. Equivalent to the amplitude of the secondary field.

total magnetic intensity (TMI) anomaly

The total magnetic intensity anomaly field is the resultant field after correcting TF, the total magnetic (observed) field for a regional gradient field, such as an IGRF.

tracer log

Also called tracejector log; a log made for the purpose of measuring fluid movement in a well by means of following a tracer injected into the well bore; tracers can be radioactive or chemical.


Term used for the areas in the American Petroleum Institute log grid that are standard for most large well-logging companies; track 1 is to the left of the depth column, and tracks 2 and 3 are to the right of the depth column, but are not separated.


Any device that converts an input signal to an output signal of a different form; it can be a transmitter or receiver in a logging probe.


Time-varying. Usually used to describe a very short period pulse of electromagnetic field.

transient electromagnetic method

A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. In the last two decades, TEM surveys have become the most popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping.

transmitter (Tx)

The source of the signal to be measured in a geophysical survey. In airborne EM it is most often a coil carrying a time-varying electrical current, transmitting the primary field. See: receiver

traverse line

A normal geophysical survey line. Normally parallel traverse lines are flown across the property in spacing of 50 m to 500 m, and generally perpendicular to the target geology.