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.
10ha Survey across prehistoric settlement remains
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 resistivity.
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.
Frequently Asked Questions
About Magnetic Susceptibility Surveys
Find out more about magnetic susceptibility surveys from our frequently asked questions. If you require any further information, feel free to get in contact on 01522 704900.
It measures how readily the soil becomes magnetised in an applied magnetic field. Variations across a site can help identify areas where past human activity may have changed the magnetic properties of the topsoil.
Enhanced readings may be associated with settlement, burning or industrial activity. Fired materials such as brick and tile, and the decomposition of domestic refuse, can contribute to changes in soil susceptibility.
The method described here uses a Bartington MS2 meter with an MS2D loop probe. It applies a low-intensity alternating magnetic field and measures the response from the ground at sampling points.
This surface method mainly responds to the top few centimetres of topsoil. It is used to investigate broad patterns near the surface rather than to map the depth of buried structures.
No. Susceptibility measures the soil's response to an applied magnetic field. Gradiometry measures changes in the local magnetic field, and can be used for more detailed investigation of targets highlighted by preliminary prospecting.
Widely spaced sampling can provide a rapid initial view of broad magnetic variation. This can help identify areas for closer investigation, although coarse sampling provides less detail than a closely spaced survey.
No. Geology, natural soil variation and cultivation can also influence the readings. Interpretation considers the background conditions and the pattern of results before suggesting a possible archaeological explanation.
Yes. Topsoil may retain magnetic traces of past activity even where cultivation has damaged the original features. The results can indicate areas of interest, but may not show the original layout of those features.
The broad surface sampling described here is primarily a prospecting method. It is better suited to identifying areas of potential activity than producing a detailed plan of individual buried features.
Depending on the results and the site, gradiometry or resistivity may be suitable for investigating selected areas in greater detail. Contact us with a site plan and your objectives so we can discuss an appropriate approach.
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