On the effectiveness of elastic wave measurements for monitoring the response of caprocks

This study investigates the effectiveness of elastic wave measurements in tracking the mechanical degradation of a structured clay under monotonic and cyclic loading. Triaxial tests were carried out on both intact and reconstituted specimens monitoring the evolution of the material state through ultrasonic transducers. The S-wave propagation velocity VS is shown to be affected by limited variations (6%). This can be explained considering that material destructuration occurs in a rather narrow shear band. Conversely, the normalised cross-correlation coefficient ρS appears to be a more sensitive indicator of structural degradation, showing a significant decrease – up to 40% – as failure is approaching. Such an effect reflects the changes in signal amplitude and shape associated with the formation of the shear band. The ρS evolution also mimics brittle and ductile responses, demonstrating its potential as a robust parameter for monitoring the in situ state of caprocks in underground storage applications.