Resistivity, sometimes referred to as earth resistance survey, can be an effective geophysical technique suitable for the definition of archaeological features such as former structures and infilled ditches and pits. The resistivity meter records localised variations in current flow and therefore resistance between metal probes inserted into the ground surface. We use the highly stable and sensitive Geoscan RM85 combined with a twin probe frame or MSP25 cart which also allows for the simultaneous collection of magnetometry data if required.
Resistivity survey can be particularly useful for locating structural remains associated with building footings. Surveys are possible over grass, crops and open soil although the results can vary considerably due to weather conditions and other factors that influence ground moisture levels.
Buried features such as walls can affect the moisture distribution and are usually more moisture resistant than other features such as the infill of a ditch. A stone wall will generally give a high resistance response and the moisture retentive content of a ditch can give a low resistance response. In the image of Pillerton Priors Roman villa below, the dark lines represent the foundations of stone walls.
Image of resistivity survey of Pillerton Priors Roman villa
Archaeological Surveys believe resistivity is currently underused due to the generally higher cost per area when compared to magnetometry. However, the technique often provides similar information to magnetometry and frequently can be superior. The rapid data collection possible using cart based systems has resulted in more competitive costs for resistivity; however, these systems require very good surface conditions (short grass, moderate soil moisture) and on many sites are unsuitable. The efficacy of any geophysical technique should be carefully considered on a site by site basis.