Comparison of high‐frequency seismic sources at the Grimsel test site, central Alps, Switzerland

Abstract

In August 1995, various high‐frequency seismic sources were tested at the Grimsel Test Site (GTS), located inside a crystalline rock body in the central Swiss Alps. These source tests were designed to facilitate future tomographic studies of potential radioactive waste disposal sites. The principal objective was to identify borehole and tunnel seismic sources capable of generating powerful high‐frequency signals such that frequencies up to 1000 Hz can be observed over distances of 1000 m in crystalline or consolidated sedimentary rocks. Seismic sources were situated in water‐filled boreholes (sparker, two piezoelectric sources, explosives) and at or near the tunnel wall (accelerated weight drop, minivibrator, bolt gun, buffalo gun, explosives). To evaluate and compare the source characteristics, the direct P-wave generated by the various seismic sources was investigated for the decay of its S/N and dominant frequency with offset and for the maximum distance at which first arrivals could be picked. Of the seismic sources tested, small explosive charges (5–100 g) had the most favorable S/N and frequency characteristics. At GTS, the target distance (∼1000 m) was reached with explosive charges of 50 g or more. None of the sources tested was capable of generating signals that sustained frequencies of 1000 Hz over distances in excess of 100 to 200 m. The unusually strong attenuation implied by this observation is likely due to the fact that the rocks at GTS underwent brittle deformation during the Alpine orogeny and therefore contain numerous fractures and shear zones.