Ultrasonic pulse velocity as a rock stress sensor

A method of measuring with nanosecond precision the times of flight of ultrasonic pulses over distances of a few metres in rock in situ has been developed by using a multiple sampling technique, with pulses repeated at intervals of 20 msec and on-line averaging of many individual readings. In an ideal situation the measurements are sensitive to changes in rock stress smaller than 10-2 bar. The method has been used to seek changes in stress in the rock walls of an underground power station, in which thermal stresses were clearly seen, but without a secular trend, so that mechanical stability of the tunnel is implied