Velocity anisotropy in granodiorite and quartzite due to the distribution of microcracks

Summary. The spatial anisotropy of P -wave velocities was investigated in spherical samples of granodiorite and quartzite and correlated with the orientation of microcracks and grain boundaries, determined optically by universal stage techniques. The preferred orientation of normals to cleavage cracks of biotite and amphibole in granodiorite correlates well with the direction of maximum velocity increase under hydrostatic pressure, although the minerals form only nine per cent of the volume. Irregular microcracks in an almost monomineral quartzite have only a very small effect on the velocity anisotropy, although they are also preferentially oriented. The nature of the microdiscontinuities is of primary importance for the velocity anisotropy observed at atmospheric and low hydrostatic pressures.