Electrical Resistivity: Principles
This is an active geophysical prospecting technique which detects subsurface features in terms of the resistance they present to the passage of an artificially induced electric current. In the dry state, most soils and rocks are insulators but, when they become moist, electrical currents are able to flow through the movement of ions which are always dissolved in the porewater. As the soil or rock absorbs more water the conductivity increases since more ions become available for the conduction and their mobility is enhanced. Hence electrical resistivity surveying primarily maps the volume concentration of ground moisture, which varies according to lithology, porosity and time of year. Temperature fluctuations can also be important, although in mid-latitudes this effect is insignificant. Experience has shown that, on well-drained soils, mid to late summer is the optimum time for archaeological resistivity survey, when moisture contrasts attain a maximum.
To measure subsoil electrical resistivity an alternating current is injected into the ground through a pair of metal electrodes and the surface potential detected between a second pair. This arrangement is needed to minimise errors arising from contact effects, earth currents (usually of mains origin) and polarisation potentials. For geological resistivity survey it is common practice to insert the four electrodes in Wenner double-dipole or Schlumberger configurations since these can be adapted for area mapping, profiling or deep sounding, depending upon the aims of the investigation. However, for archaeological survey the objective is generally to map subsoil features at similar depths over a large area and thus systems have been developed specifically for this purpose.
In the 1980’s the ‘twin electrode’ scheme became popular and effective for archaeological survey, illustrated above. A mobile frame is used to carry one potential and one current electrode (p2 and c2), which are connected, via the meter to their respective p1 and c1 soil electrodes. Alternating current is passed between c1 and c2 and the potential measured between p1 and p2. The presence of a zone of anomalous resistivity modifies the distribution of current flow (dotted streamlines) and also contours of constant potential (curved solid lines), and is depicted for the case of a high-resistivity structure such as a wall. The instrument thus senses a maximum (or minimum) in the apparent soil resistance which is centred over the feature. A theoretical analysis shows that the distance between the mobile frame and the fixed electrodes must be large compared to the inter-electrode spacing in order for the readings to be independent of geometry. The ‘sensing depth’ is comparable to the distance between the electrodes on the mobile frame (0.5 – 1.0m for 0.5m spacing), while the possible horizontal resolution of the survey increases as this distance decreases. Thus the choice of electrode spacing on the mobile frame is invariably a compromise between maximising the response to target depths while providing usable horizontal resolution. In contrast to geomagnetic data, resistivity anomalies are centred over buried targets. Through good instrument design, resistivity is now a rapid technique, although the need for soil contact and a cable to the remote electrodes makes with a slower method than magnetometry.
Based on extract from written by Mark Noel, published in Appendix 3, Tara: An Archaeological Survey