Magnetic gradiometric prospecting detects subsurface features in terms of the perturbations or 'anomalies' that they induce in the earth's magnetic field. In contrast to resistivity, seismic or electromagnetic surveying, for example, no energy is injected into the sub-soil and hence this is one of a class of passive geophysical techniques that includes cavity and thermal surveying.
In an archaeological setting two types of magnetic anomalies can be distinguished:-
- Anomalies arising from variations in magnetic susceptibility, which will modulate the component of magnetization indulged in the subsurface by the earth's magnetic field. For most archaeological sites, this is the dominant factor giving rise to geomagnetic anomalies. In general, susceptibility is relatively weak in sediments such as sandstones and enhanced in igneous rocks and soils, especially those which have been burnt or stratified with organic material.
- Anomalies due to large, permanently magnetized structures. Such permanent magnetisation or 'remanence' arises when ferromagnetic minerals are heated to above the 'blocking temperature' of about 600oC and cooled in the geomagnetic field. Thus kilns and hearths are often detected as strong permanent magnets causing high localised anomalies that dominate effects due to background susceptibility variations. Remanence can result from other physical and chemical processes but these give rise to anomalies that are usually unimportant for geophysical prospecting.
The diagram below, based on the geomagnetic field at Tara (55 deg inclined), shows how the anomalies recorded due to a feature are u
sually asymmetric (paired positive - negative) with the main peak displaced to the south of the archaeological feature. Thus, an east/west - aligned ditch filled with a soil of enhanced susceptibility will generate a positive anomaly to the south, mirrored by a weak negative anomaly to the north of the feature.
Alternatively, if the feature comprises an east/west-aligned trackway made of stone with relatively low susceptibility, then the main anomaly will now be negative and again on the south of the feature. When the resulting geophysical data are portrayed as an area map of grey tones these targets will appear to be illuminated from the north, giving a pseudo-relief effect. Although considered visually appealing, this characteristic of the data must be considered when making a rigorous archaeological interpretation. In contrast, north/south- aligned linear structures will generate anomalies centred on their axes, with weaker anomalies of the opposite sign along each side.