Resistivity
Resistivity survey can be
particularly useful for locating structural remains associated with
building footings. In addition, the technique can locate former ditches
and pits. Surveys are possible over grass, crops and open soil.
The electrical resistance or resistivity of the soil depends upon the moisture content and distribution within the soil. 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.
Image of resistivity survey of Pillerton Priors Roman villa
Resistivity surveys are carried out using TR Systems Ltd Resistance Meter TRCIA 1.31 with a mobile Twin Probe array. Surveys are usually carried out collecting data at 1m intervals which gives good resolution with relatively rapid speed of surveying. Survey using 50cm by 50cm spacing can also be carried out with improvement of resolution; however, this quadruples the readings taken in the field, a compromise often used is 1m by 50cm.
Mobile probe separation of 50cm is considered adequate for most archaeological survey work although useful additional depth may be obtained by increasing probe spacing to 1m. Archaeological Surveys have a range of frames and probe separations available for the TRCIA meter.