The background to geomagnetics is the existence of a terrestrial magnetic field. Due to this field all materials (all!) develop magnetic properties through an induction process - in other words - an induced magnetisation. The material's property is called magnetic susceptibility. It can vary by many orders for materials that are important in geophysics (rocks, ores; in environmental sector: iron objects, fired ceramics [bricks!]).
Through induction in the terrestrial magnetic field rock bodies or other objects are turned into a type of magnet themselves with a magnetic field surrounding them. This magnetic field superimposes the inducing (socalled normal) terrestrial field as an interfering field; it produces anomalies in the normal field.
The survey of such anomalies with suitable measuring devices (magnetometer) allows searching for, confining and modelling (position, depth, shape) magnetised bodies and objects.

Special measuring systems take advantage of socalled gradiometers that measure the difference of field values with two (mostly vertically) spaced magnetometer sensors. Like this gradiometers can approximately record the gradient (mostly the vertical gradient of the vertical component) of the terrestrial magnetic field, thus its variation in space. Gradiometers have a high resolution for small range structures. With the help of difference creation temporal fluctuations of the terrestrial magnetic field, which are often irregular and abrupt, can be eliminated; otherwise they would have to be registered with repeated measurements or permanent base stations.