Geophysical Survey

techniques geophysics intro

Geophysical survey is a general term for a range of non-invasive or non-destructive techniques used to detect features buried beneath the surface without digging. A range of approaches exist using specialist equipment to measure variations in the physical properties below the surface to identify archaeological features. Depending on the type of survey being conducted structural elements, traces of human activity or structures can be identified as they return significant anomalies in the data from the background values. However if no particular contrast can be identified then no archaeological feature can be detected. For this reason it is important to use a survey strategy that includes a number of complementary techniques each measuring a different property of the soil.

Electrical Resistance

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.

techniques geophysics reistance cross section
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.

techniques geophysics reistance fieldwork

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.