Ultrasonic Imaging of the Onset and Growth of Fractures within Partially Saturated Whitby Mudstone using Coda-Wave Decorrelation Inversion

Using active ultrasonic source survey data, Coda-wave Decorrelation (CWD) time-lapse imaging during the triaxial compression of Whitby Mudstone cores provides a 3-D description of the evolution and redistribution of inelastic strain concentrations. Acoustic Emissions (AEs) monitoring is also performed between any two consecutive surveys. From these data, we investigate the impact of initial water saturation $S_w$ on the onset, growth, and reactivation of inelastic deformation, compared to the post-deformation fracture network extracted from X-ray tomography scans. Our results indicate for the applied strain-rate and degree of initial water saturation, and within the frequency range of our ultrasonic transducers (0.1 to 1 MHz), that inelastic strain localisation and propagation in the Whitby Mudstone does not radiate AEs of sufficient magnitude to be detected above the average noise level. This is true for both the initial onset of inelasticity (strain localisation), and during macroscopic failure. In cont...