ULTRASONIC VELOCITIES IN CORES FROM THE KOLA SUPERDEEP WELL AND THE NATURE OF SUBHORIZONTAL SEISMIC REFLECTIONS

P wave velocity and orthogonally polarized S wave velocities were meas ured on 12 cores recovered from the Kola superdeep well at depths of 0 to 12 km. Measurements were made along the core axis at a frequency o f 1 MHz, at confining pressures ranging from 2 to 100 MPa, and under d ry and water-saturated conditions. Cores were chosen to sample a varie ty of lithologies and were used to estimate interval velocities based on a simplified geological column of the well. These interval velociti es were then compared with sonic log and vertical seismic profile (VSP ) data. High-pressure lab velocities correlated primarily with rock co mposition and texture. These laboratory velocities are generally in go od agreement with both sonic log and VSP data, suggesting that extreme ly low velocities, as measured in unconfined laboratory samples or at low confining pressure, are the result of drilling and core-recovery-i nduced damage. The magnitude of this microcrack-induced damage general ly increases with depth in a stepwise manner but with a few notable in versions. These inversions are characterized by a relatively small red uction in dry unconfined velocities compared to the in situ velocities . We interpret these inversions to be due to localized in situ stress relief related to faulting, fracturing, and/or hydrothermal alteration . We also observed pronounced S wave splitting in the cores, the analy sis of which suggests that the stress relief microcracks tend to be al igned parallel to the foliation in gneisses and amphibolites (dip angl e 28 degrees-45 degrees) rather then being subhorizontal. These observ ations have important implications for the nature of gently dipping se ismic reflections detected in the immediate vicinity of the Kola well.