There are two main types of geophysical survey used by archaeologists. Resistivity measures the resistance of the ground to an electrical current. This can reveal buried pits and ditches where, because the fill in looser and moister than the surrounding soil, the current passes through more easily. Walls and roads are dry and therefore do not allow the electrical current to pass through so easily, and so these are detectable by resistivity survey as well. Magnetometry measures the magnetic properties of the soil. High magnetic features include hearths and kilns, because they have been heated. Pits and ditches that have been filled with iron-rich topsoil will also be detected. Both these techniques are non-invasive and under the best conditions can generate plans of buried features and is also used to plan where excavation trenches should be placed.
The resistivity meter has two prongs that are inserted into the ground and an electrical current is passed from one to the other. The results are stored in the built-in datalogger and can be downloaded later. A simpler grid can be laid out for geophysical survey than other types of survey. For resistivity all that is needed is two parallel tapes run out from a baseline and readings are taken at meter intervals. The first tape can then be moved over and repositioned parallel to the second tape on the other side ready for the next set of readings.
The magnetometer is held above the ground. It is held level so that it reads the magnetic property of the soil immediately below it. The geophysicist walks up and down at a steady rate between two parallel lines, similar to the resistivity set up, trying to cover every half-meter. Again the results are stored in the built-in datalogger.
Specialist software is needed to process the results on the computer. Both types of survey produce images showing the anomalies that may be buried archaeological features. These need to be tied into a map of the area and can help pinpoint the location of trenches or more detailed geophysical survey.
Geophysics tends to find large features such as ditches and roads. These kinds of features are rare in certain periods, meaning that geophysics does not work so well when trying to find prehistoric settlement or cemeteries until the Iron Age, or Early Saxon sites in general.
It is quite common for the results to be contaminated by modern features. Recent bonfire sites can give a much higher reading than an ancient hearth and modern service pipes can easily look like walls.
Going the extra mile
Another type of magnetic survey is called magnetic susceptibility. This uses a machine that looks very much like a metal-detector. The grid can be set out using 5x5, 10x10 or 20x20 meter squares, depending on the area to be surveyed and the type of site you are looking for. One or more readings can be taken inside each grid square (make sure this is always in the same part of the square). The readings measure the relative magnetic susceptibility of the soil. The soil should become more magnetic as it gets closer to occupation or industrial sites from which burnt or fired material will have scattered. This is quick work and makes it easy to pinpoint areas of human activity over wide areas.
The equipment for doing all these forms of geophysical survey is expensive to buy, but there are some companies that rent it out. Look on the Current Archaeology website for a directory of suppliers. Specialist software is also important. But, most of all, you need training to do geophysical survey. You can find more information on training at… If there is a geophysical project you want to do, because of the cost of the equipment and training, you may want to apply for funding. Click here for more details on where to get funding.
Clark, A 1990. Seeing Beneath the Soil. London: Routledge.
Gaffney, C, Gater, J and Ovenden, S, 2002. The use of geophysical techniques in archaeological evaluations. Institute of Field Archaeologists Technical Paper No. 6.
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