VERTICAL SEISMIC PROFILE RESULTS FROM THE KOLA SUPERDEEP BOREHOLE, RUSSIA

Multi-offset vertical seismic profiles (VSPs) from the Kola Superdeep Borehole (SG-3), as part of a larger seismic study of the Kola region conducted during the spring of 1992, sample the dipping Pechenga compl ex from 2175 m to 6000 m and contribute to the understanding of reflec tivity in crystalline and Precambrian environments. From the surface t o 6000 m, the SG-3 borehole penetrates interlayered Proterozoic metavo lcanic and metasedimentary units and a mylonitic shear zone ranging fr om greenschist to amphibolite metamorphic grade, respectively The Kola VSPs display a 6% velocity decrease which coincides to a mylonitic sh ear zone located between 4500 m and 5100 m within the SG-3 borehole. S eismic interfaces are identified by mode-converted energy (PS, and SP transmissions and reflections) in addition to primary seismic phases. The VSP shear wave energy is generated at or near the source by vertic al vibrators. P-wave and S-wave reflections are generally detected fro m the same reflecting horizons, but increases in relative S-wave and S P reflection amplitudes originate at 1900 m, 3800 m, 4500 m, and 5100 m depths. These depths coincide with zones of elevated V-p/V-s and may support the presence of free pore fluid which is reported from initia l drilling. For the Proterozoic lithologies sampled by the VSP, reflec tion events result from five mylonitic shear zones and three lithologi c contrasts.