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Electrical Resistance Survey

How does it work?
Electrical resistance survey with a twin-probe array uses a set of four probes, two stationary & two mobile, to measure the variation of electrical potential across the ground surface, a value directly analogous to electrical resistance at the low frequency of signal (137.5Hz for the RM15) used. Each pair of probes is comprised of one for injection of the electric current, usually set at 1mA or 10mA for archaeological surveys, & one for measurement of the electric potential. In use, the electric current through the soil between the widely-separated pairs of probes (usually 15m or more) forms a half-rugby ball shape in the ground, the flat surface corresponding to the surface of the ground. Along the length of this shape the electrical potential varies as a function of the resistance of the materials through which the current passes. In homogenous ground, lines of equal potential would form a regular pattern at the surface but in reality any buried inhomogeneity causes significant distortion of this. As the mobile pair of probes is moved around, the measuring probe measures the electric potential at 0.5m or 1.0m away from the current injection point. This corresponds to the potential created by electric currents flowing approximately 0.5m or 1.0m beneath the surface. The magnitude of the current flow is known, i.e., 1mA or 10mA & hence the measurement of potential can be used to derive a value of apparent electrical resistance of the material through which this current is flowing. An alternating current (in practice usually a square wave) is used to prevent the build up of charged ions at each probe, which would make them into weak batteries & severely distort the measurements. 137.5Hz (see above) has been chosen to allow interference from UK mains electricity to be filtered out. The value of electrical resistance is entirely dependent upon the configuration of the probe array used & different sorts of arrays have different advantages & disadvantages. For analytical & modelling purposes the measurements are converted to apparent resistivity using geometry factors to remove the effect of the array configuration from the data. Electrical resistance surveys of all forms are strongly influenced by moisture in the topsoil into which the probes are inserted. For this reason the weather before & during a survey must be taken into account for a reliable interpretation to result. Any factor that alters the amount of water in the soil will have an affect upon the data. For example, if survey enters an area of freshly-turned soil, the greater pore size of this soil will permit more water to be contained within it. This will tend to reduce the apparent resistance & can sometimes prevent adequate penetration of the current. Long wet vegetation can have a similar effect. The converse is also true; in cold conditions variations in the amount of frozen water in the soil, e.g., whether the ground is in the shadow of a tree or building or not, will again impact upon the surface electrical resistance & hence the final result. Stony ground, or large stones just beneath the surface, will have a similar effect.
Results
Electrical resistance survey in planar form (i.e., the bulk of commercial surveys for archaeology in the UK) is best suited for locating features that contrast strongly with the surrounding materials. Examples are walls or stone-filled ditches in soil. In addition, negative archaeological features filled with soil can also be detected due to variations in the water-holding ability of the fill. In general, soil filling a pit or ditch will have a larger pore space than natural material around it. This means that it in times of net water loss from the ground, e.g., during dry weather after rain; the fill will remain slightly wetter for a while & hence possess a lower electrical resistance. The anomalous resistance will fade, sometimes fairly rapidly, unless further rainfall occurs. Buried stonework usually appears as regions of higher resistance than the soil around it. In wet conditions, however, it is possible for water to become trapped on top of the masonry & present a low resistance path to electrical current. This means the masonry will appear in the data as a low resistance & not the expected high resistance anomaly. The same effect can also happen when water becomes trapped behind features & the low resistance anomaly this causes can on occasion swamp the resistance values associated with the features themselves. For these reasons it is important that at least a basic understanding of the hydrodynamics of the soil are known prior to survey & that the local effects of weather are understood. Finally, electrical resistance survey can be achieved in a variety of ways & many variables can be changed to adjust the survey to the ground & expected features. These include array type (of which the ‘twin probe’ is only one), probe spacing, magnitude of current, survey resolution & other factors depending upon the instrumentation used.