The 2006 Google image showed an interesting anomaly which gave us another opportunity to practice our field surveying skills, using the resistance meter.
This is an active geophysical technique which detects features in terms of resistance to the passage of an electric current. Most dry soils and rocks are insulators but, when moist, electric currents can flow through the movement of dissolved ions. Hence electrical resistivity surveying actually maps archaeology in terms of an associated variation in ground moisture.
To record the resistivity, a weak alternating current is injected into the ground through a pair of metal electrodes and the surface voltages are detected between a second pair. The presence of anomalous resistivity modifies the current flow (dotted lines) and the pattern of potential (solid lines). The instrument thus senses a maximum (or minimum) resistance which is centred over the feature.
Through good instrument designs, resistivity surveying is a rapid technique, although the need for soil contact and cables makes this a slower method than undertaking magnetometery surveys.
However, one benefit is that the user can control the sensing depth by changing the electrode spacing on the moving frame.
Measurements are taken at regular intervals on a grid. Both parallel and zig-zag traverse schemes can be used. Only about 0.5ha (1.2 acres) can be surveyed per day by one team, and the technique is physically demanding (as we all discovered!).
Measurements are collected by a data logger on the machine, downloaded onto a computer and processed to create a map of the readings.
We are new to mastering the art of field surveying, and the map results were not too clear. We will need to return to carry out a second survey next year.
