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Magnetic Susceptibility Survey

Instruments

Bartington Instruments MS2
Exploranium KT5
Geonics EM38

What is measured & how?

The first two instruments measure using a pair of coincident coils that provide a tightly focused measuurement of a small volume (the halfspace), about 2095 & 57 cubic cm respectively. The EM38 has coils spaced by approximately 1m & is of 'Slingram' form which means the measurement is sampled over a much larger volume. For spot samples the coincident coil instruments are best as (in the case of the KT5) measurements can be as close as 6cm but for area surveys the Slingram format (coils spaced apart) can be better.

Typical targets

Magnetic susceptibility is basically a measure of the ability of a material to become magnetised & in the context of soil these are primarily ferrimagnetic oxides of iron. These are found in many soils altered or exposed to cultural processes, particularly burning & fermentation. For archaeological studies these altered soils tend to become trapped in filled contexts & their enhancement permits diagnosis of their origin through dual-frequency analysis etc.

More information on the uses of magnetic susceptibility can be found at Bartington Instruments here

Common values & sensitivity

For measurements of rock samples a resolution of around 1 x 10^-5SI is required & in general the same applies for archaeological studies. Some approximate volumetric values (KT5) are:
Grey sandstone 2 x 10^-5SI
Red sandstone 5 - 15 x 10^-5SI
Subsoil 5 - 20 x 10^-5SI
Topsoil 15 - 30 x 10^-5SI
Burnt soil 50 - 90 x 10^-5SI
Pottery & tile 100 x 10^-5SI
Smithing debris 200 x 10^-5SI

Sampling

Sampling resolution depends on instrument & purpose & the figures here are for guidance only. If profiles or plans of susceptibility variation are being made using a coincident coil device then the maximum resolution is the diameter of the coil, i.e., about 22cm for the MS2 & 6cm for the KT5. For area surveys using instruments of Slingram form there is little point sampling at closer than the coil spacing, i.e. 1m for the EM38. To collect enough detail for profiling the fills of small ditches then a 10cm resolution is preferable to ensure fine structure is adequately resolved. To simply detect differences between materials then 25cm should be sufficient. If the MS2 is used to delineate areas of cultural debris then intervals of between 10 & 20m are best. It needs to be remembered that for archaeological studies the further apart the samples the less confidence that can be attached to each as susceptibility can vary substantially within a few metres.

Caveats

The use of magnetic susceptibility as a technique for locating archaeological sites has a mixed perception among surveyors & for good reason. It has often been specified as a primary reconnaissance technique without considering the magnetic properties of the soil or whether these vary across the area to be studied. It can of course only detect processes that have led to magnetic enhancement, usually involving heat or burning. The depth of overburden & the modern land use are also important when using coincident coil instruments in particular due to their limited depth of penetration.

Intra-site studies are, however, almost always going to produce useful results if undertaken carefully because one is in part interested in the relative difference between materials rather than absolute susceptibility.