Application of seismic processing techniques to discontinuity mapping with ground-penetrating radar in crystalline rock of the Gotthard massif, Switzerland

The Gotthard Pass (2108m above sea level) is one of the main thoroughfares across the Swiss Alps, connecting northern Switzerland to the southern Ticino region. Near the summit of the pass ground-penetrating radar (GPR) data were collected across an accessible "slab" of slightly inclined granitoid rock of 50m length and varying width up to a maximum of 25m. An abandoned quarry face provided depth information on the rock mass down to 15m. The GPR survey conducted by two persons included several single-fold constant-offset profiles and a 14-fold multi-offset profile. The nominal center frequency of the antennae was l00MHz. All data were recorded in digital form with the RAD-SEIS system, based on a GSSI SIR-3 unit and a personal computer. Gained and bandpass-filtered georadar sections show numerous distinct reflections to the maximum recording time of 950ns, corresponding to depths of nearly 60m. Some strong reflections extend the full 50m - length of the recording profiles. Apparent dips of reflections vary from 0 to 20, the principal ones being in the 0 to 5 range. Normal moveout (NMO) velocities, derived by interactive velocity picking of common midpoint gathers, are fairly constant at 0.12m/ns throughout the recorded section. Stacking of the multi-offset profile data reduces noise at large two-way times and increases continuity of the deeper reflections. Migration of the constant-offset profiles and the stacked section focusses the abundant diffraction patterns, yielding much clearer images. Comparison of the GPR sections with the rock face, leads to the conclusion that the principal reflections are due to moisture-filled fractures with widths of less than a centimeter.