Ground Penetrating Radar (GPR)

An instrument that images the subsurface from reflected radio pulses. GPR finds what carries no signal — plastic pipe, clay tile, concrete, tanks, and voids — and at higher frequencies scans slabs for rebar, conduit, and post-tension tendons.

A GPR antenna transmits short pulses into the ground and records what reflects back from boundaries between materials with different electrical properties. Because it reads a change in material rather than a current on a wire, it reaches targets an electromagnetic locator cannot. Its limit is soil: high-conductivity ground such as wet clay attenuates the signal within a short distance of the surface, and a radar record shows a reflection rather than a labelled pipe, so interpretation is doing real work. GPR and electromagnetic locating are used together because each covers the other's blind spot.