Microfracture network evolution leading to catastrophic failure: hearing and seeing.

Abstract:

The process of strain localisation is critical in the evolution of deformation leading to catastrophic failure. However, the precise mechanisms of how cracks, pores and grain boundaries interact locally under an applied stress field have yet to be determined. To understand the microscopic processes and their control of system-sized failure, a novel x-ray transparent cell was used for deformation experiments of rock samples, which permits integration of acoustic monitoring with fast synchrotron x-ray imaging. To increase temporal characterization of damage beyond the temporal resolution of the fast 3D synchrotron system, acoustic emission (AE) feedback control was used to regulate the applied stress and slow down the deformation processes. As a result, the increment deformation between x-ray scanned states was relatively homogeneous, simplifying comparison between AE events and x-ray scans. In this paper, we present the acoustic data acquisition, conditioning and processing used to characterize the velocity evolution of the rock samples, and the location and characteristics of individual AE events during deformation. Time-lapse velocity measurements are linked to internal stress changes and structural damage corresponding to seismic and aseismic deformation processes, while acoustic emissions are a direct indication of local cracking. We show that we can successfully locate AE events in 3D, using only two sensors on either end of the sample, based on ellipsoid mapping. We further test AE location using Coda Wave Interferometry (CWI), which has only been analysed so far synthetically, with CWI earthquake location techniques previously applied exclusively to field scale. By exploring the kinematic and dynamic signatures of AE events, we characterise and locate seismic deformation processes and examine how those are linked to the strain field in the samples measured with incremental Digital Volume Correlation between pairs of recorded x-ray tomograms.