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

observed gravity field

The term “observed gravity” is also often used in lieu of “raw gravity” or “measured gravity”. Incorrectly, but often, the term “observed gravity map” may be posted on the following maps: Bouguer, free-air, regional or residual gravity field.

Occam’s inversion

An inversion process that matches the measured electromagnetic data to a theoretical model of many, thin layers with constant thickness and varying resistivity (Constable et al, 1987a).


In a time-domain electromagnetic survey, the time after the end of the primary field pulse, and before the start of the next pulse.

ohm (Ώ)

The unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.

ohm-metre (Ώm)

Unit of electrical resistivity; the resistivity of 1 m3 of material, which has a resistance of 1 ohm when electrical current flows between opposite faces; the standard unit of measurement for resistivity logs.


In a time-domain electromagnetic survey, the time during the primary field pulse.

open hole

Uncased intervals of a drill hole.

optimum offset

Seismic reflection technique employing optimum window.


In engineering and mineral exploration terms, this most often means the soil on top of the unweathered bedrock. It may be sand, glacial till, or weathered rock.