Stoneley waves and Scholte waves in ground penetrating radar (GPR) investigations.

A well-known saying is that ground penetrating radar (GPR) is seismics with a different excitation: electromagnetic instead of acoustic. The propagation paths are the same, the data processing is mostly identical, and seismic evaluation programs are equally used in the GPR evaluation. Without scale information, radargrams cannot necessarily be distinguished from seismograms. We report on another analogy between seismics and GPR interface waves, long known as Stoneley and Scholte waves in seismics but rarely playing a significant role, which we find are beginning to play an increasingly important role in GPR. Stoneley waves are boundary waves travelling along a solid-solid interface, while Scholte waves are propagating along a solid-fluid interface. Solid-air interface waves are generally known as Rayleigh waves well known in seismics as typical surface waves but may also join the Scholte and Stoneley waves in the context discussed here. The boundary waves especially of the Scholte type came into our view by peculiar observations during diverse GPR measurements over different geological substrates with 200 and 300 MHz antennas and registration depths of the order of 10 m. The phenomenon: Occupation of the radargrams with mostly clearly vertical, mostly sharply defined amplitude stripe patterns from single specimens to regular or irregular cluster formations (see figure). Originally considered as a kind of noise of inexplicable cause, this explanation could be excluded by the fact that stripe patterns could be followed in measurements on parallel GPR profiles, correlating meter by meter, and thus had to have their cause in the geological subsurface. The conclusion from the known Scholte waves in the wall of water-filled boreholes to groundwater-bearing open fractures/fissures in rocks was obvious and short. Meanwhile, the GPR interface waves not only of the Scholte type, but also the Stoneley waves have become an important geological tool in addressing solid boundaries, for example, on faults or in fracture zones. The extent to which Rayleigh waves can play a role in this context as solid-air interface waves for distinguishing dry and water-bearing fractures/fissures/fracture zones remains subject to further investigation.