Magnetic prospecting looks for variations in the magnetic field of the earth that are caused by changes in the subsurface geologic structure or by differences in the magnetic properties of near-surface rocks. The inherent magnetism of rocks is called the magnetic susceptibility.

Sedimentary rocks generally have a very small magnetic susceptibility compared with igneous or metamorphic rocks, which tend to have a much higher magnetite (a common magnetic mineral) content. Most magnetic surveys are designed to map the geologic structure on or inside the basement rocks (the crystalline rocks the lie beneath the sedimentary layers) or to detect magnetic minerals directly.

The magnetic method was initially used for petroleum exploration in areas where the structure in oil-bearing sedimentary layers appeared to be controlled by topographic features, such as ridges or faults, on the underlying basement surface. Since the development of aeromagnetic methods, most magnetic surveys undertaken for oil exploration are carried out to ascertain the thickness of the sedimentary section in areas where such information is not otherwise available.