AN ANALYSIS OF SHEAR-WAVES OBSERVED IN VSP DATA FROM THE SUPERDEEP WELL AT KOLA, RUSSIA

Two VSPs have been analysed from the Kola superdeep borehole, located on the Kola Peninsula. The Kola borehole was drilled down to a depth o f 12 260 m and VSPs were recorded in the interval 2150 to 6000 m. The VSPs sample the Proterozoic Pechenga complex, which consists of altern ating metasedimentary and metavolcanic layers that range from greensch ist to amphibolite-grade facies. The structural dip is 28-45 degrees t o the SSW, and a major shear zone (the Luchlompolsky fault) occurs at 4.5 km depth. The VSPS display strong trans itted- and reflected-mode converted energy from structural and lithologic boundaries. A kinemati c (traveltime) ray-tracing modelling of the main compressional- (P-) a nd shear- (S-) wave events was performed to define the seismic boundar ies. P- and S-wave velocities were estimated from the near-offset data , and V-p/V-s ratios were related to the lithology. A significant incr ease in the V-p/V-s ratio is observed in the main shear zone at 4.5 km depth. Shear-wave splitting is identified by traveltime divergence (d ifferent apparent velocites) and orthogonal polarization of S phases i n the far-offset VSP. The inferred polarization direction for the fast shear wave is N160 degrees W, clearly observed below 4400 m depth. Tw o models are suggested to explain the observed shear-wave splitting: i ntrinsic anisotropy caused by aligned hornblende minerals in the amphi bolite-grade facies; and vertical cracks aligned N160 degrees W. The d irection of crack alignment is not consistent with the present-day NW- SE maximum compressive stress field. However, it is consistent with th e direction of the palaeostress, the direction of crack alignment at t he surface and the fast direction obtained from analysis of shear wave s in shallow VSPs. The velocity anisotropy is estimated to be 4-5 per cent with a local increase to 10 per cent in the Luchlompolsky fault z one.