We use the TR/CIA multi probe Wenner array kit, the result of an initiative by the Council for Independent Archaeology and TR Systems Ltd. This is relatively easy to use and is quickest with 5 people. It can be done with just one person, but it does take a little longer.
The main problem with the process is that it is very repetative but requires a bit of concentration. If you have ever experienced twin probe resistivity, imagine it without audio cues and with manual logging. Half way through you end up thinking 'did I log that result or not'.
The basic process involves placing 20 or 30 steel pegs in a line at half or one metre intervals to the same depth. Then connect 4 wires in the correct order to the first four pegs and log a reading. Now move the same four wires, keeping the same order, from pegs 1-4 to pegs 2-5. Record the reading and move on until you run out of pegs. Now go back to the beginning and instead of the first 4 pegs (1234) use pegs 1357 ie a spacing of 1. After recording the value move to pegs 2468 and continue until you run out of pegs. Repeat, increasing the spacing by one each time. If you have four people moving the connections and one logging data it ends up looking like a slow line dance!
There is an animation of the process here
Processing the results is a two stage affair because the kit utilises the TR/CIA logger as it was originally configured. Data is downloaded and the software generates the correct format file for Res2DInv software input. The latter software seems to have been produced for commercial geological surveys with a fantastic amount of theoretical basis and detail available from the Geotomo website. As such there are many processing variables which can be altered, so results should be treated as indicative rather than definite. The results below have ground level at the top. The L and D figures give the (L)ength of the array used and the maximum calculated (D)epth, both in metres. Data from nearby excavations suggests that the models may be valid, but we are waiting for a more detailed comparison before getting too excited!
|Very wide Iron Age ditch (L15 D1.73)||Possible brick kiln C16th (L30 D3.46)|
|Conduit near a C16th fountain site (L15 D2.15)||Conduit and wall in a C16th fountain site (L15 D2.15)|