Electrical resistance is an active geophysical technique. It relies on the principle that many subsurface archaeological features display different electrical properties to those of the host or surrounding soils. These contrasts, where they exist, enable subsurface archaeological features to be detected and mapped.
The technique measures the electrical resistance presented by buried features to the flow of an applied electrical current.
Idealised electrical resistance signal from different buried phenomena.
Pits and ditches, composed of less compacted more porous and permeable soils, normally have larger moisture content than the surrounding soils and tend to exhibit a lower electrical resistance. Building foundations and walls, composed of more compacted less porous and permeable materials, normally have a lower moisture content than the surrounding soils and tend to exhibit a higher electrical resistance.
Electrical resistance surveys are normally carried out using an array of four stainless steel electrodes which are inserted into the ground.
A commonly used array uses a current and a potential electrode spaced 0.5m apart mounted on a mobile frame and a remote current and potential electrode buried in the ground some distance away from the survey grid.
Two electrodes are used to apply an electrical current and two other electrodes are used to measure the resulting potential difference or voltage. The array is systematically moved along lines on a survey grid and measurements are made at fixed intervals along each line. An electrical resistance meter is used to supply the current, measure the voltage, calculate the resistance and store the measurement for later computer processing and analysis.
The depth of investigation of the technique depends on the type and spatial configuration of the array of electrodes. For a mobile frame with current and potential electrodes mounted 0.5m apart the depth of investigation is approximately 0.5m. The detection and recognition of subsurface archaeological features depends on a number of variable factors, including the depth, spaceform and electrical properties of buried features, electrode spacing and configuration, soil moisture conditions and climatic variation.
Topographic contours of Rathcroghan Mound.
Electrical resistance image of the Mound showing a complex pattern of low resistance curving ditches and enclosures (black) cut into higher resistance soils (grey and white). The depth of investigation is about 0.5m.
These factors are taken into consideration in carrying out an electrical resistance survey, processing the measured values and interpretation of the results.