Ground Penetrating Radar is a variation of conventional radar, where a radio signal is transmitted vertically down into the ground,  the reflected signal revealing varitaions in density below the ground surface. The technique is very good at detecting buried masonry, and can also effectively detect buried voids such as cellars or vaults.

One of its advantages over the other main geophysical survey techniques is its ability to not only record responses on a horizontal plane, but also vertically. This allows the creation of “time slices” ie images from different depths, which can be used to create a 3D model of subterranean features. It is generally quite an expensive and often slow method, and there are a range of circumstances where it does not work well, resulting in it tending not to be used perhaps as often as it really ought to be.

Ground Penetrating Radar (GPR)