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


The degree of homogeneity in Euler’s equation, interpreted physically as the fall-off rate with distance and geophysically as a structural index (SI). Values vary from 1 to 3 according to magnetic or gravity source body geometry.

N electrode

The potential electrode distant from the A electrode in a resistivity device.

nanotesla (nT)

A unit of the magnitude of the magnetic field vector B represented by the number of lines of induction passing through a unit area perpendicular to the vector direction. 1 nanotesla = 1 gamma. See: tesla

natural exposure rate

In radiometric surveys, a calculation of the total exposure rate due to natural-source gamma rays at the ground surface. It is used as a measurement of the concentration of all the natural radioelements at the surface. See: exposure rate

naudy’s method

An automated profile-based depth estimation method wherein anomaly type and location are identified by cross-correlation of the observed magnetic profile with theoretical anomalies. The depth to a dike-like or plate-like source is then estimated from parameters relating source body half-width, depth, and data sampling interval.

neural network

A member of a class of software that is “trained” by presenting it examples of input and the corresponding desired output. For example, the input might be a magnetic anomaly and the required output the depth to the source of that anomaly. Training might be conducted using synthetic data, iterating on the examples until satisfactory depth estimates are obtained. Neural networks are general-purpose programs which have applications outside potential fields, including almost any problem that can be regarded as pattern recognition in some form.

neutron log

Neutrons from an isotopic source are measured at one or several detectors after they migrate through material in, and adjacent to, the borehole. log response primarily results from hydrogen content, but it can be related to saturated porosity and moisture content.


That part of a geophysical measurement that the user does not want. Typically this includes electronic interference from the system, the atmosphere (sferics), and man-made sources. This can be a subjective judgement, as it may include the response from geology other than the target of interest. Commonly the term is used to refer to high frequency (short period) interference. See: drift

non aqueous phase liquid (NAPL)

Elements or compounds in the liquid phase other than water. This phase is immiscible in water. Examples include petroleum hydrocarbons, like gasoline, and solvents such as trichloroethylene.


In geophysical interpretation and mathematical modelling, a problem for which two or more subsurface models satisfy the data equally well.

normal log

A quantitative-resistivity log, made with four electrodes, which employs spacings between 4 and 64 in. to investigate different volumes of material around the borehole; see also long-normal log and short-normal log.

normal moveout (NMO) corrections

Time shift corrections to reflection arrivals because of variation in shotpoint-to-geophone distance (offset). The amount of shift depends on 1) the length of the raypath from shot to reflection point to receiver, and 2) the velocity of the material traversed. Deeper reflections are corrected using velocities indicative of the deeper section.

nuclear log

Well logs using nuclear reactions either measuring response to radiation from sources in the probe or measuring natural radioactivity present in the rocks.