Variations in Ultrasonic Signal Spectra for Triaxial Testing of Rock Samples.

Abstract—Experiments on sandstone, basalt and granite were carried out on a controlled hydraulic press under triaxial loading conditions at confining pressure of 5, 10 and 15 MPa. The tests on sandstone and basalt were conducted with the injection of a fluid (water) and subsequent modulation of pore-fluid pressure, and the tests on granite were performed without fluid injection. In the experiments, both the acoustic emission (AE) signals arising during fracture of the material and the ultrasonic (U/S) sounding signals were recorded in 16 directions. Based on the U/S data, a change in the power spectra of recorded U/S signals is revealed at different stages of sample testing. It is shown that the changes in the median of the power spectrum of U/S signals (median frequency f_med) can reach more than 100% depending on the degree of fracture in a rock; besides, compared to elastic velocities, the changes in the spectral content of the U/S signals more dynamically reflect fracture development in samples. The dependence of median frequency on applied axial load is established. It is shown that at fluid injection into a dry sample, f_med shifts strongly towards the low frequencies. The shift of individual spectral components of the U/S signals can be considered as an indicator of changes occurring in the sample. A decline in f_med testifies to the increasing degree of fracture of the material. The effect is most pronounced in highly porous sandstones. [ABSTRACT FROM AUTHOR]