Bonekickers

History behind ‘Warriors’: Geophysical Survey

Archaeologists use two types of survey routinely to locate buried sites - resistivity and magnetometry. Both employ specialist equipment, and require the surveyor to walk over the ground in a systematic way taking measurements at regular intervals. Generally, 20 metre grids are used, with reading taken at metre intervals.

Resistivity survey uses the principle that the electrical resistance of the ground depends upon its character and moisture content. Buried ditches for example will have a much lower resistance that for example wall foundations. The method inserts two metal probes, 0.5m apart mounted on a frame then a current is run between them, and the resistance measured and compared to a similar resistance outside the grid. The readings are logged automatically, and can be processed in specialist software to provide a plan based on these readings. The method generally works to a depth of about one metre.

Magnetometry uses the earth’s magnetic field to locate archaeological anomalies. The instrument detects where magnetism has been altered by buried features, as well as where it has been induced in the soil (e.g. through burning). It is also very effective at finding metal objects more deeply buried than the range of metal detectors and is often used for shipwreck survey. It is a very sensitive method, in which the alignment of the instrument is important, and the operator has to be completely ‘demetaled’ or the survey produces spurious results.