Resistivity survey at Claydon HouseThere 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.


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.