Microcrack-Induced Seismic Anisotropy
The failure of brittle rocks during compression is preceded by the formation, growth and coalescence of microcracks. Elastic wave velocities are reduced in the presence of open cracks and may therefore be used to monitor the progressive damage of the rock. In general, these microcracks are not randomly oriented and the rock displays a stress–induced seismic anisotropy which can be expressed in terms of a second–rank and fourth–rank crack density tensor. For open dry cracks the contribution of the fourth–rank crack density tensor to the elastic wave velocities is small.The results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to Berea sandstone. Inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress, in agreement with current rock mechanics theory. A good fit to the data is obtained using only the second–rank crack density tensor even though, at high confining stress, the cracks are expected to be in partial contact along their length; This is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by White (1983).