Ground penetrating radar.

(GPR) is a geophysical method that
uses radar pulses to image the subsurface. This non-destructive method uses UHF/VHF frequencies of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks.