Detection of Seismic Precursors in Converted Ultrasonic Waves to Shear Failure of Natural Sandstone Rock Joints

Geophysical wave measurements have been used as a remote and non-destructive method to monitor the changes in the state of contact, stress, and deformation in rock joints. In recent studies, geophysical precursors were identified as distinct maxima and minima in the amplitude of transmitted and reflected ultrasonic waves propagating through synthetic rock joints prior to the shear failure. However, the ability to detect seismic precursors and to identify their underlying causes under experimental conditions particularly in natural rock joints with rough contact surfaces has been limited. Here in this study, we conducted single direct shear experiments on sandstone rock joints with rough contact surfaces while employing digital image correlation technique and monitoring ultrasonic compressional (P) and shear (S) waves propagating through the rock joint. Our experimental observations show that depending on the surface roughness and the localized physical mechanisms occurring along the rock joint, the transmitted amplitude may either reach a peak prior to the peak shear stress or follow a continuous decreasing trend with shear displacement. An extensive geophysical analysis including wave conversion analysis was conducted to resolve the complexities in the ultrasonic measurements and improve the possibility of detecting ultrasonic precursors to the shear failure. We observed that the changes in the amplitude of converted S–P wave as well as P–S wave show systematic variations and exhibit a clear precursory signature prior to the peak shear stress. Our results indicate that the precursors identified based on the changes in the amplitude of converted waves are consistent with the evolution of failure mechanisms at the joint during inter-seismic and pre-seismic phases. The findings in this study present novel and additional potentials in ultrasonic waves to evaluate deformation and detect slip initiation in fractured rock materials found in rock slopes as well as tectonic faults.