US Geological Survey
Date of this Version
1994
Abstract
P wave velocity and orthogonally polarized S wave velocities were
measured 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 of 1 MHz, at confiningp ressures ranging from 2 to 100 MPa, and under dry and water-saturated conditions. Cores were chosen to sample a variety of lithologies and were used to estimate interval velocities based on a simplified geological column of the well. These interval velocities were then compared with sonic log and vertical seismic profile (VSP) data. High-pressure lab velocities correlated primarily with rock composition and texture. These laboratory velocities are generally in good agreement with both sonic log and VSP data, suggesting that extremely low velocities, as measured in unconfined laboratory samples or at low confining pressure, are the result of drilling
and core-recovery-induced damage. The magnitude of this microcrack-induced
damage generally increases with depth in a stepwise manner but with a few notable inversions. These inversions are characterized by a relatively small reduction 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 analysis of which suggests that the stress relief microcracks tend to be aligned parallel to the foliation in gneisses and amphibolites(dip angle 28 °-45 °) rather then being subhorizontal. These observations have important implications for the nature of gently dipping seismic reflections detected in the immediate vicinity of the Kola well.
Comments
Published in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B12, PAGES 24,209-24,219, DECEMBER 10, 1994