Ultrasonic crosshole assessment of fracture density and stress variations in crystalline rock

Abstract

The potential for application of seismic methods in the evaluation and monitoring of nuclear waste repositories is immense. Past field studies have established that variations in velocity and attenuation indicate changes in the fracture density of the rock mass surrounding a storage facility. A study was undertaken to determine the effects upon velocity and attenuation of varying stress fields surrounding a crystalline repository. Laboratory experiments performed on fractured core samples found that amplitude and rise time are sensitive to stress applied parallel to travel path and velocity is not. Ultrasonic crosshole measurements were taken within the rock mass surrounding an underground research facility. The crosshole method was able to detect a zone of high fracture density caused by blasting near the surface of the room. The blast damage zone was characterized by relatively low velocities and amplitudes and large rise times. Moving outward into the rock mass, velocity stabilized while amplitude and rise time varied significantly indicating the existence of a stress concentration. This coincides well with the estimated stress distribution based on measured in situ stresses. Other significant variations in acoustic parameters were encountered and attributed to changes in fracture densities.