COMPARISON OF HIGH-FREQUENCY SEISMIC SOURCES AT THE GRIMSEL TEST-SITE, CENTRAL ALPS, SWITZERLAND

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 facilitat e future tomographic studies of potential radioactive waste disposal s ites. The principal objective was to identify borehole and tunnel seis mic 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 w ere situated in water-filled boreholes (sparker, two piezoelectric sou rces, explosives) and at or near the tunnel wall (accelerated weight d rop, minivibrator, bolt gun, buffalo gun, explosives). To evaluate and compare the source characteristics, the direct P-wave generated by th e various seismic sources was investigated for the decay of its S/N an d 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 SM and frequency ch aracteristics. At GTS, the target distance (similar to 1000 m) was rea ched with explosive charges of 50 g or more. None of the sources teste d was capable of generating signals that sustained frequencies of 1000 Hz over distances in excess of 100 to 200 m. The unusually strong att enuation implied by this observation is likely due to the fact that th e rocks at GTS underwent brittle deformation during the Alpine orogeny and therefore contain numerous fractures and shear zones.