Applied Geophysics


GPR measurement

Ground Penetrating Radar (GPR) uses electromagnetic waves that are emitted by an antenna as short pulses into the subsurface. If the dielectric permittivity of the material changes, parts of the electromagnetic wave are reflected back and received by another antenna. The two-way traveltime (back and forth between antenna and reflector) and the amplitude are digitalized and saved for each scan. These scans are made in short distances (e.g. every 1 cm) while the antennae are pulled along a profile on the surface. If several of these profiles are measured in parallel, a 3D volume is created that can be cut into so-called timeslices. These timeslices sum the absolute reflection amplitudes in a short time interval, e.g. 5 ns, and show reflecting objects in the subsurface, i.e. pipes, archaeological features and geological interfaces, in top view. If larger areas have to be covered, antenna arrays are available that measure several parallel profiles at the same time.

The propagation velocity of electromagnetic waves in the subsurface depends on the dielectric permittivity of the material, which is mainly controlled by the water content in near surface sedimentary layers (high water content means low propagation velocity). If the velocity is known, the traveltime to a reflector can be converted to depth.

The penetration depth of radar signals depends on the electric conductivity of the subsurface and the frequency of the signal. Low frequencies and low conductivities allow the deepest penetration depths.