Stress-induced ultrasonic anisotrophy in Berea sandstone

Abstract

Abstract Ultrasonic P- and S-wave velocities have been measured for propagation in three orthogonal directions through a sample of Berea sandstone as a function of maximum compressive stress applied perpendicular to the bedding plane. The sample was a 50mm cube and was stressed to peak in a new triaxial loading frame, with the principal stress components parallel to the bedding plane held constant at 4.1 MPa. The measured velocities are compared with velocities calculated for a medium containing an anisotropic orientation distribution of microcracks. With a recently developed technique, the P- and S-wave measurements can be inverted separately for the microcrack density and orientation distribution, thus enabling a consistency check to be made on current theories of elastic wave scattering by microcracks and fractures. Inversion of the measured velocities to obtain the microcrack orientation distribution function suggests the growth of cracks parallel to the direction of the maximum compressive stress. This supports the current view that failure is preceded by the growth of microcracks parallel to the direction of the maximum principal compressive stress. The large stress-dependent changes in ultrasonic velocities observed suggest the usefulness of ultrasonics for studying damage processes in sedimentary rocks.