Area Resistivity Survey

(Geoscan RM15)

Soil Resistance

Area resistivity surveys measure the varying ability of soils to conduct an electrical current which is passed through them. This is called the electrical resistance of the soil or object. Different materials have different resistances which are linked to moisture content, and therefore porosity. Hard, dense features such as rock will give a relatively high resistance while features such as a ditch, which retain moisture, give a relatively low response.

Resistivity surveys are therefore particularly useful when attempting to locate building foundations or other structural features and backfilled ditches and ponds.

The resistivity imaging technique employs a line of metal stakes (electrodes) temporarily inserted in the topsoil through which electrical current is made to flow. Four electrodes are used to make each measurement of the ground resistivity, two conducting current, two other electrodes measuring the electrical potential difference (voltage). By traversing the four electrodes along a line and repeating the process at larger electrode spacings (providing greater depths of investigation), it is possible to build-up a two-dimensional representation of the variation in ground resistivity (measured in Ohm metres, Ohm-m). Complex processing of the measured resistivity data produces the final resistivity image, which may then be used to interpret geological variations.

Resistance Meters and Probe Arrays

Resistance (or its absolute form - resistivity) is measured using a resistance meter connected to a Twin Probe array. These probes are normally mounted 0.5m apart but this can be increased to 1.0m or 1.5m to increase the depth range of the survey from about 1.5m to 2.5m. The associated remote probes are positioned approximately 15m outside the grid.

Readings are recorded on a data logger which permits the data to be recorded as the survey progresses. The data is downloaded onto a computer and processed using Geoplot software. Greyscale plans of the variation in resistance can be plotted. Higher resistance areas are normally presented as dark grey or black and lower resistance as light grey or white. Processing of the data can enhance the results by removing spikes and filtering to remove large area variations.

Applications

• Assessment of archaeology prior to development
• Site assessment allowing targeted excavation of archaeological features
• Definition of the extent of known archaeology for land management purposes
• Archaeological site research.

Advantages

• Responds to a wide range of archaeological features eg. pits, ditches, structural remains etc.
• Excellent response to building remains
• Can provide high definition of archaeological features.

Example of a resistivity survey showing processed data collected on a site in the Walton Basin, Welsh Marches