Geophysical survey, as employed in a variety of forms at Damerham, simply comprises a range of techniques for seeing what might lie beneath the ground surface without having to dig first. The techniques used so far at Damerham are:
Earth Resistance, or resistivity
This method simply involves measuring how long it takes for an electric current to pass through the ground. The resulting plots highlight areas of higher or lower resistance when compared to the background picture. Put simply, a current will meet greater resistance when encountering more solid material – rubble, road surfaces, or buried wall foundations, for example – than it would when passing through, say, a soil-filled ditch or pit. In practice, of course, the situation is seldom so straightforward and, as with cropmarks, much depends on the moisture content of the soil.
This technique requires a fair amount of physical effort, the probes having to make contact with the ground at a set interval – every metre, say – along a 30 metre transect, this operation being repeated across a grid made up of 30 metre squares. This summer (2009) at Damerham, the frame featured four probes spanning a distance of 2 metres. This meant 15 transects, with 30 readings per transect, for each grid square.
The gradiometer measures tiny variations in magnetic properties of the soil, these hopefully being caused by human activity in the past. It is particularly useful for detecting traces of burning or firing – hearths, kilns etc – but is also pretty good at identifying archaeological features cut into the subsoil or bedrock. The ditches belonging to Dampney Barrow, for example, show up with remarkable clarity on the gradiometer plot.
At Damerham, the same arrangement of grids and transects was used for all forms of geophysical survey, allowing direct comparison between results. However, the gradiometer is less weighty than the earth resistance frame, and merely needs to be carried rather than lifted up and down (although the person wearing the harness does have walk at the right speed to ensure readings are taken at the correct intervals). Consequently, the gradiometer can cover much more ground than earth resistance, the latter being used to target particular areas rather than being used extensively.
One complication that comes with the gradiometer is that anyone using one, or holding the tapes, or who for any reason needs to get within, say 20 metres of one during the survey, should have no metal about them. This includes things like zips on trousers, bra clasps, and piercings. As we quickly realised at Damerham, horses are not metal-free either.
Ground Penetrating Radar (GPR)
Put simply, GPR involves sending pulses of (energy? Sound?) directly into the ground and measuring how long it takes for them to bounce back. The relative depth of features or structures can be estimated by measuring the time taken for the pulses to return. Essentially, each individual transect will produce a cross-section of whatever lies beneath the soil.
At Damerham, the technique has been used so far on the two long barrows. By undertaking numerous transects across the mounds, and then again at right angles to these – along the mounds’ long axes – it should be possible to create a three-dimensional view from the accumulated cross-sections.
Dragging the GPR equipment back and forth repeatedly over uneven ground is clearly quite a laborious process, and the subsequent processing of the data is more complex than dealing with the data from the other techniques used. However, the results should make all that effort more than worthwhile.