Resistivity is an active technique that uses probes to introduce an electrical current into the soil, measuring the resistance of the soil to the passage of the current (see Clark 2000; Heimmer and De Vore 1995; Somers 2006). In a typical archaeological application, a mobile probe array includes one probe to introduce a current into the ground and a second probe to take a voltage measurement. A pair of stationary probes located at some distance from the mobile probe array provide a return path for the current probe and reference voltage from the voltmeter (Somers 2006:113). The resistance of the soil between the two mobile probes is calculated by taking the ratio of voltage (measured) to current (known). This is converted to a measurement of resistivity by factoring in the distance between the mobile probes (see Somers 2006:113).
The electrical resistance of a soil is related to its “porosity, permeability, saturation, and chemical nature of entrapped fluids” (Heimmer and De Vore 1995:30): higher concentrations of ions allow electrical current to pass more easily through the soil, creating a lower electrical resistance (Clark 2000:27; Somers 2006:111). High resistance soils are often dry, coarse, and have low salinity. Low resistance soils are typically moist, fine, and have a high salinity. Resistivity survey can often be used to identify compacted areas (walkways, floors), buried foundations, and areas that have been previously excavated and filled (ditches, pit features, etc.). A buried stone slab would show up as a high resistance feature, while a ditch filled with moist, organic soil would show up as a low resistance feature. As with any geophysical method, sufficient contrast in resistance between cultural features and the surrounding matrix is required in order for a feature to produce an anomaly.
The spatial resolution of resistivity data depends upon the separation distance of the probes in the mobile array and the sample interval along each transect. Cultural features smaller than the probe separation distance are not likely to produce a distinct anomaly, as multiple readings within a feature are required to define the edges of an anomaly (Somers 2006:112-113). The probe separation distance is also related to the depth at which the instrument “sees” beneath the surface. If the probes are 50 cm apart, the instrument is measuring the resistance of the soil within a semi-circular area extending approximately 50 cm beneath the ground surface (see Somers 2006:113-114; but see also Clark 2000:44). These two factors limit the potential of resistivity survey to detect small features at North American sites that contain a plowzone: a probe spacing of 50 cm is typically required to “see” beneath the plowzone, limiting the ability to detect features less than 50 cm in diameter. Thus resistivity survey is most often successful in the detection of large cultural features such as large pits, ditches, and house foundations.
Resistivity survey using a typical set-up requires constant movement of wires connected the mobile and stationary probe arrays. While resistivity survey is possible in areas with trees, brush, or high crops, open terrain makes survey simpler and faster. The mobile probe array is moved along a series of transects, taking a reading when the probes are inserted into the ground at a constant interval. Data are downloaded from the instrument and used to create resistance maps of the surveyed area. These maps are processed with software to bring out the anomalies of interest and aid in interpretation.
2000 Seeing Beneath the Soil: Prospection Methods in Archaeology. Reprint of 1990 edition. B.T. Batsford, London.
Heimmer, Don H., and Steven L. DeVore
1995 Near-Surface, High Resolution Geophysical Methods for Cultural Resource Management and Archaeological Investigations. Revised edition. National Park Service, Denver, Colorado.
2006 Resistivity Survey. In Remote Sensing in Archaeology, edited by Jay K. Johnson, pp. 109-129. University of Alabama Press, Tuscaloosa.