Shear strength along discontinuities plays a crucial role in the stability of rock structures. The development of geophysical methods to remotely monitor and assess changes in shear strength is essential to the identification of rock hazards that can lead to the loss of life and failure of civilian infrastructure. In this study, compressional and shear ultrasonic waves were used to monitor slip along discontinuities (with different surface profiles) during shearing. A series of laboratory direct shear experiments was performed on two gypsum blocks separated by a frictional discontinuity. The gypsum blocks had perfectly matched contact surfaces with a half-cycle sine wave profile that spanned the central third of the discontinuity, surrounded by planar surfaces. The amplitude of the half-cycle sine wave was varied and ranged between 2 to 10 times the height of the asperities. Compressional, P, and shear, S, ultrasonic waves were continuously transmitted and recorded throughout the shearing process, while Digital Image Correlation (DIC) was used to capture surface displacements. At low normal stresses, distinct maxima in the normalized P and S wave transmitted amplitudes occurred before shear failure in regions where dilation was observed. Where dilation was not detected, an increase in transmitted wave amplitude was observed, even after the peak shear stress was achieved. At high normal stresses, dilation was suppressed, which was associated with an increase in wave amplitude with shear stress until the peak, and then a decrease in amplitude. Monitoring changes in transmitted wave amplitude is a potential method for the detection of dilation along rock discontinuities.

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