3-D ground-penetrating radar applied to fracture imaging in gneiss

Three-dimensional, ground-penetrating radar (georadar) techniques suitable for geological engineering applications have been developed and tested. Initial experiments were conducted on the floor of a quarry in southern Switzerland from which ornamental gneissic rock is extracted. During a brief two-day period? constant-offset georadar data were recorded over a 650 m(2) area with a grid cell size of 0.1 m x 0.2 m. Georadar velocities were estimated from the results of expanding spread surveys. All georadar data and associated geometry files were recorded automatically in seismic industry formats. The experimental georadar data set was processed, image-enhanced, and interpreted using 3-D seismic reflection software operating on a workstation. Arbitrary vertical sections, time slices, 3-D images, and animated movies in which the observer ''travels'' through the entire data volume were constructed from the resultant migrated georadar data. Semi-automatic tracking routines allowed continuous subhorizontal reflections to a maximum depth of 30 m to be mapped through the rock mass. These reflections, which are characterized by negative polarity onsets, are probably caused by a system of ubiquitous water-filled fractures, 2-4 cm thick. Volumes of rock bounded by the subhorizontal fractures were estimated from isopach maps and rock quality was assessed on the basis of root-mean-square (rms) amplitudes of reflections. An extension of a steep-dipping fault exposed on a nearby quarry wall was best delineated on maps representing the horizontal gradients of reflection times. To synthesize in a single figure the principal geological results of the study, picked reflection times were presented in the form of shaded relief surfaces, in which remarkably vivid structural details of the subhorizontal fractures and intersecting near-vertical fault could be discerned. It is concluded that 3-D georadar methods have the potential to resolve a wide range of engineering and environmental problems.