All materials affect, or are affected by, the earth's magnetic field. The presence of some metallic minerals will create an uneven distribution of this magnetic field resulting in local variations. It is this variation that is used to create the magnetometry survey map. The magnetic field of the earth is measurable and varies due to influences such as the solar wind and proximity to its poles. If there are magnetic minerals buried below the earth the interaction of their magnetic field with that of the earth will create measurable deflections from the earth’s field. 

Magnetometry is the measurement of these deflections and requires two instruments which accurately determines the changing magnetic field of the earth as it is influenced by magnetic mineralization. Due to the natural variations of the earth’s magnetic field the interpretation of results must take into account where on the earth’s surface the measurements are being made.  A magnetometer, then, is like a super-sensitive compass measuring the strength of the earth’s magnetic field and sensing the changes caused by the additional magnetism of of features buried below the surface.