Resistance survey measures the electrical resistance of the Earth's soil moisture content. A twin probe configuration is normally carried out, which involves the pairing of electrodes (one current and one potential), with one pair remaining in a fixed position (remote probes), whilst the mobile probes measure variations in electrical resistance across the survey grids. Resistance is measured in ohms, and this method generally detects to a depth of 1m.
Features such as wall foundations are usually identified as high resistance anomalies, as well as rubble spreads, made surfaces (ie yards and paths) and metalled roads and trackways. In contrast, low resistance values are normally associated with water-retentive features such as large pits, ditches, drains and gulleys.
The survey is normally undertaken using a Geoscan Research RM15 Resistivity meter (with MPX15 multiplexer) or a TR instrument with twin probe array (pictured below at Hampton Court Palace).
Monastic grange and Bronze Age barrows, Lincolnshire