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 $ρ^S$ 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 $ρ^S$ 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.