Geomagnetic field anomalies are caused by the presence of rock types and artificial objects with contrasting magnetic properties.
Magnetometry is used, for example, to detect underground objects with different magnetic properties (with ferromagnetic materials, mainly iron and steel), to map lithological types with different magnetic susceptibility, etc. Influence of field time variations, regional anomalies and other disturbing effects (utilities, structures, etc.) can be minimized by so-called gradient measurement (gradiometers).
For the localization of metal underground objects, magnetic measurements are combined with measurements with metal detectors, which are suitable for finding shallowly stored or smaller mainly metal objects.