Deformation rate analysis (DRA), although it has a poor theoretical basis, is a useful method for estimating rock stress in the laboratory using core samples. DRA is thought to be closely related to changes in the inner microstructure of rock samples undergoing a de formation process. Ultrasonic measurement is a useful technique for investigating structural changes in rocks. In this study, both methods were applied simultaneously to rock samples to confirm the applicability of ultrasonic measurement for the estimation of rock stress using a system of ultrasonic propagation time with accuracy in 10^-2 nsec.
In the experiments, core samples of sandstone were loaded up to a certain stress in order to artificially input memory of the stress into the samples, and the propagation time in the horizontal direction to the loading axis and the deformation rate were measured simultaneously. Consequently, the propagation time through the specimen was measured with high accuracy during deformation. Variations in the propagation time and lateral strain almost coincided during the loading-up process. Furthermore, the propagation time was drastically changed at the memorized stress corresponding to the deflection point of the deformation rate curve. These experimental results show that highly accurate measurement of the ultrasonic propagation time is useful, as is the DRA method using core samples, for examining changes in the microstructure inside rock samples under stress and applicable for estimating underground rock stress.