Magnetic methods may be used to seek specific mineral deposits, map lithologies and structures and detect alteration. The most important magnetic mineral is magnetite, followed by pyrrhotite.
Rocks are magnetised by the presence of the Earth’s magnetic field and often also carry a natural remanent magnetisation (NRM) acquired in the past. The NRM may dominate in some volcanics and chemical sediments.
Magnetic anomalies, mapped in airborne, ground or marine surveys, are caused by local variations in rock magnetism. Their shapes depend on the geomagnetic field orientation as well as the source geometry and magnetisation. Interpretation aims at revealing what is in the ground, from the anomaly patterns.