The change in ultrasonic wave velocities in triaxially stressed brittle rock.

Abstract

The changes in the ultrasonic wave velocities were measured for granodiorite under axial stress up to failure, and confining pressure up to 1kbar. The velocities begin to decrease at the differential stress of 1-2kbar and show a marked decrease at a stress higher than 70% of the failure strength. The observed decrease of P wave velocity in the minimum stress direction is remarkable (25% maximum) and that of S is somewhat smaller (15% maximum). The decrement of P wave velocity in the maximum stress direction is very small compared with that in the minimum stress direction.The normalized velocity Y(≡υ/υmax, a ratio of the velocity to the maximum one for each run) is empirically represented by a function of normalized differential stress X(=Δσ/σSM, ratio of differential stressΔσ to the strength σSM at the same confining pressure), as follows, Y=√1-a-Xbwhere a and b are constants. An elasticity theory for composite materials was used to interpret this relation.