The effect of dilatancy on velocity anisotropy in westerly granite

Abstract

Jacketed samples of Westerly granite were fractured at confining pressures up to 1 kbar and at strain rates of 2 × 10−5; s−1 and 3 × 10−7 s−1. Compressional and horizontally as well as vertically polarized shear velocities were measured in orthogonal directions perpendicular to the compression axis. The strains in three orthogonal directions were monitored by strain gauges and compared with the results obtained by optical holography. At approximately 60% of the ultimate axial strain the onset of dilatancy is noted by a decrease in the Vs and Vp velocities. As failure was approached, the compressional velocity across the fault decreased by nearly 40% for the low‐strain‐rate case versus about 22% for the high‐strain‐rate case. In order to characterize the stress‐induced cracks during dilatancy, an analysis of the velocity data was carried out following the approach of Anderson et al. [1974] with the assumptions that flat spheroidal cracks were oriented perpendicular to three orthogonal directions and that the effects of oblique cracks was vectorially divided between the effects of three mutually perpendicular crack types. The comparison between the observed velocities and those calculated from the three crack densities on the basis of the above assumptions showed good agreement. The degree of dilatancy was determined from the differences between the strains measured perpendicularly to the compression axis and the estimated elastic strains in those directions. The aspect ratios for the flat spheroidal cracks were calculated to be 1/400 to 1/800, which are in line with those determined by Hadley [1976].