Ground Penetrating Radar
Ground penetrating radar (GPR) is a variation on conventional radar, which sends an acoustic signal into the soil which is reflected by variations in density below ground. It is very good at detecting the structures of buried masonry structures, and can effectively show holes or voids below ground – it is therefore valuable in finding buried parts of buildings such as crypts, tombs or cellars, and showing up building foundations.
Radar was developed for military purposes during the second world war, and since then it has been adapted for geological and civil engineering uses. GPR is very useful to archaeology where there are voids and substantial deeply-stratified masonry remains (hence it is particularly valuable on urban sites), ditches, cave structures, ancient mines or large landscape features such as dry river channels (known as palaeochannels), but it is rather less use in the softer and more finely-differentiated types of deposit encountered on many rural archaeological sites.
A GPR survey is carried out by dragging a radar scanner across the ground on a wheeled trolley, whilst it takes readings across a survey grid. These are then downloaded and plotted. Plots can be represented horizontally (in plan form), vertically (in section form) or in 3-D. The strength of the signal can be altered to take account of differing densities in below-ground deposits. GPR produces extremely impressive results in the right conditions, but the range of circumstances in which it does not work well, and the expense of mounting a survey, mean that it is less common and take splace over smaller areas than other types of geophysical survey.
