Resistivity  Detection of underground structures with resistivity methods is possible using the specific electrical conductivity (σ) or the specific electrical resistivity (ρ; ρ = 1 / σ). Hence archaeological features can be mapped when they are of higher or lower resistivity than their surroundings. In general walls and foundations show through an increase in resistivity whereas ditches and pits show through an increase in conductivity.

To define the electrical characteristics of an area several metal probes (electrodes) are placed into the ground. Two probes are used to inserted an electrical current (I) into the earth. Thus the potential difference (U) between two other probes and therewith the electrical resistivity is obtained. Besides that the specific resistivity depends on a geometry factor defined by the exact position of each probe in the measurement.

Various configurations of electrodes and a wide range of geoelectrical investigation methods exist. To detect near surface resistivity anomalies geoelectrical mapping is most efficient. In order to reach a greater depth of penetration multi-electrode-measurements are used. They result either in 2D-vertical sections or 3D-models of the resistivity distribution and give information about the depth of expected underground structures.

Using state-of-the-art multi-electrode systems eastern atlas is able to do 1D sounding, 2D mapping and sounding as well as 3D electrical resistivity tomography (ERT). Furthermore we use the method of induced polarization (IP) - a technique that benefits of the phase shift between electrical current and voltage – for archeometallurgical investigations and in search for buried wooden structures. We also apply inductive electromagnetic measurements for large scaled agricultural prospections. One advantage of the latter is that it does not require direct contact with the ground. Thus the conductivity distribution of large areas is detected and soil categories can be classified quick and easily.

Resistivity field work and data examples:

Resistivity instrument in field		sheme of 2D resistivity imaging		IP (Induced Polarisation) measurements on slag heap		3D IP model of a slag heap (© Th. Günther / C. Rücker)		Resistivity profile on the hill fort of Lossow