Magnetic susceptibility survey
Although the gradiometer can be used for rapid scanning on large sites, the effectiveness this technique is limited, and detailed area surveys always produce less subjective results. Additionally, the gradiometer is a 'passive' instrument, measuring magnetic susceptibility by its effect on the Earth's magnetic field. Only magnetically anomalous features will be detectable. For example, the local magnetic distortion induced by thin archaeological horizons may not be sufficient to produce anomalous readings. This problem can exist within topsoils subjected to generations of disturbance (e.g. ploughing). Magnetic discontinuities in the topsoil are often more readily identified by an 'active' instrument, such as the Bartington Magnetic Susceptibility Meter. This instrument temporarily magnetises the ground by creating a low intensity, alternating magnetic field. It then measures the response. This is particularly useful for identifying settlement activity, where soil magnetic susceptibility has been enhanced by intensive and/or prolonged human occupation.
Measurement of magnetic susceptibility is confined to the top few centimetres of topsoil, but its wider range (measurement intervals of up to 30m) enables rapid coverage of large areas. This is, of course, at the expense of detailed resolution, and is recommended primarily as a preliminary prospecting technique used to highlight areas for detailed gradiometry. However, on sites where archaeological features have been completely ploughed out, magnetic susceptibility measurement may produce the only clear evidence of earlier occupation, including sites where traces of archaeological remains have been carried to the surface by earthworm activity (bioturbation).
The susceptibility is measured in SI volume susceptibility units (x 10-5).
10ha magnetic susceptibility survey, with hotspot over prehistoric settlement