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 Sw on the onset, growth, and reactivation of inelastic deformation, compared to the postdeformation 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 localization 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 localization) and during macroscopic failure. In contrast, the CWD results indicate the onset of what is interpreted as localized regions of inelastic strain at less than 50% of the peak differential stress the Whitby Mudstone can sustain. The seemingly aseismic nature of these clay-rich rocks suggests the gradual development of inelastic strain, from the microscopic diffuse damage, up until the macroscopic shear failure.@en

We all monitor the world around us through waves. After about sixteen weeks in the womb, the ears and eyes of an infant child begin to deliver the first signals of light and sound waves to the brain. In fact, one could argue that consciousness itself is the feeling one receives when processing large amounts of wavefield data. Despite the integral presence of wavefield monitoring in biology, it is only since the dawn of the information age, that society has been able to monitor the world around us with similar fidelity. Where a recorded wavefield can be assumed to have travelled through a simple medium, it is often possible to resolve an image, though where a disordered medium is encountered, and the wavefield is multiply scattered, this becomes more difficult. It is this disordered portion of a wavefield, often referred to as the coda-wave, which this thesis is primarily concerned with. By considering the coda-wave over the coherent arrivals, one loses the ability to resolve the structure of a medium, though in turn gains improved sensitivity to changes within. This makes coda-wave monitoring particularly well suited to problems in which sensitivity to change is a more important quality than the ability to image the medium. On face value, one might consider coda-wave derived monitoring within Early Warning Systems (EWSs), towards hazards such as earthquakes, landslides, or the failure of critical infrastructure. However, operational deployment of such systems must work from simple, robust, and automated alert criteria, and therefore often rely on coherent wavefield observations, typically through passive measurements at the boundary of a region of interest. It is due to this reliance on clear, automated alert criteria derived from passive observation, which limits the lead time provided by EWSs, from only a few tens of seconds for earthquakes, to one day of warning for landslides.@en

The nominally incoherent coda of a scattered wavefield has been shown to be a remarkably sensitive quantitive monitoring tool. Its success is, however, often limited to applications where only moderate or localised changes in the scattering properties of the medium can be assumed. However, the compressional deformation of a relatively homogeneous rock matrix towards failure represents for a monitoring wavefield pronounced changes in both velocity and scattering power often due to a distribution of inelastic changes. A rolling reference wavefield is implemented when applying coda-wave interferometry (CWI) and coda-wave decorrelation (CWD), allowing relative velocity and material scattering power monitoring for such applications. It is demonstrated how this modification enables the qualitative monitoring of stages in material deformation common to unconfined compressive strength tests. In addition, the precursory/subtle onset of material yielding is identifiable in both the CWI and CWD methods, which was not possible when comparing to a fixed reference wavefield. It is, therefore, expected that this approach will enable these coda-based methods to robustly monitor continuous, destructive processes at a variety of scales. Possible applications include critical infrastructure, landslide, and reservoir compaction monitoring where both the subtle continuous and sudden large changes in a material's scattering properties occur.@en