Like gradiometry, the measurement of magnetic susceptibility as a means of identifying areas of past human occupation is dependent on establishing distinctions between naturally produced magnetic variation within geologies and soils and those induced by human intervention.
Intensive occupation tends to increase the magnetic susceptibility of soils. For example, a significant magnetic enhancement can result from burning, by the introduction of fired materials such as brick and tile, or by bacterial decomposition of domestic refuse. Consequently, a localised increase in magnetic strength may be evidence of settlement or industrial activities. Prolonged arable cultivation can produce similar, though less intense variation.
The MS2 Bartington Magnetic Susceptibility Meter with MS2D loop probe attachment temporarily magnetises the ground by creating a low intensity, alternating magnetic field. It then measures the response. The usefulness of this system is confined to the top few centimetres of topsoil, but its wider sampling range enables rapid coverage of large areas (albeit by coarse measurement). This is at the expense of detailed resolution, and is recommended as a preliminary prospecting technique; used to identify target areas for detailed survey using other techniques such as gradiometry or resisitivity. However, on sites where archaeological features may have been completely (or almost entirely) eradicated by subsequent cultivation, the measurement of topsoil magnetic susceptibility measurement may be the only non-intrusive prospection technique that is able to identify traces of past human activity.
10ha survey across prehistoric settlement remains