Catastrophic Failure: what controls precursory damage localisation in rocks?

Catastrophic failure is a critically-important phenomenon in the brittle Earth on a variety of scales, from human-induced seismicity to natural landslides, volcanic eruptions and earthquakes. It is invariably associated with the structural concentration of damage in the form of smaller faults and fractures on localised zones of deformation, eventually resulting in system-sized brittle failure along a distinct and emergent fault plane. However, the process of localisation is not well understood - smaller cracks spontaneously self-organise along the incipient fault plane, often immediately before failure, but the precise mechanisms involved have yet to be determined. Many questions remain, including : Q1 - How do cracks, pores and grain boundaries interact locally with the applied stress field to cause catastrophic failure to occur at a specific place, orientation and time?; Q2 what dictates the relative importance of quasi-static and dynamic processes?; and Q3 - why can we detect precursors to catastrophic failure only in some cases?

Here we will address these questions directly by imaging the whole localisation process, using a newly-developed x-ray transparent deformation cell and fast synchrotron x-ray micro-tomography. We will visualise the nature and evolution of the localisation process structurally and seismically together for the first time at high resolution in a synchrotron. We will deliberately slow the process to image its evolution, and to investigate the strain-rate dependence of the underlying mechanisms, using rapid electronic monitoring and feedback control. This will provide unprecedented direct observation of the relevant mechanisms, including the contribution of seismic (local cracking producing acoustic emissions) and aseismic (elastic loading and silent irreversible damage) processes to the outcome. This innovative combination of techniques is timely, feasible, and is likely to transform our understanding of the role of microscopic processes in controlling system-size failure. The results will provide interpretive models for similar processes in natural and human-induced seismicity, including scale-model tests of strategies for managing the risk of large induced events.

The potential for impact is significant. In the main part of the proposal we will combine acoustic data with time-lapse micro-CT images to develop a process-based understanding and new theories for the micro-mechanics of strain localisation. This will increase our understanding of localisation processes and our ability to forecast and possibly to control material failure in Earth materials. This is hugely important for facilitating NERC's mission to "Manage our environment responsibly", from natural resource extraction to building resilience to geological hazards. Our results will feed directly into the study of induced seismicity, where the partition of strain between seismic and total strain, and its implications for supporting operational forecasting of seismic risk for engineering decisions, is a pressing and outstanding research question with direct and immediate practical application (Bourne et al., 2014).

Seismic waves and velocity measurements are central in many applications of geophysics. It is now common in the oil and gas industry to use a combination of field scale seismic information and laboratory-derived physical parameters of different rocks to develop and test models for the subsurface structure, and any changes in the reservoir due to production and injection of fluids. These applications require the same 'time-lapse' approach we will use in the laboratory tests, with the advantage that we will know the deformation field independently. In a recent review of trends and challenges in in the industry the authors conclude that "acceptance and application of 4D seismic techniques in both exploration and production indicates that time-lapse 3D exploration and reservoir monitoring are coming of age as a tool to minimize drilling risk and to maximize the return on investment". However, there is often no way to 'ground-truth' such models, as direct imaging of Earth's subsurface is impossible. The improved understanding on the controls on measured velocity and its changes (including subtle changes detected by coda-wave interferometry) is likely to significantly improve our models of subsurface structure and its evolution, and hence reduce the significant costs associated with drilling risk. For example the current cost of a drilling a well in the North Sea is currently just under $10M.

A better understanding of the velocity structure will also allow us to improve the accuracy of earthquake location algorithms for the quantification of seismic and volcanic hazard and risk. The ground-truth data obtained from our proposed blend of technologies in a high-pressure rock deformation apparatus will provide the first direct means to test location algorithms, in particular their ability to detect more localised correlated structures that may pose a greater risk of generating large events.

More generally, the localisation of deformation is a fundamental physical process with a variety of applications in a diverse range of applications, from developing robust materials, including those used in bio-engineering, to non-destructive testing and monitoring of buildings and infrastructure.