Seeing and Hearing Precursory Damage Localisation and Catastrophic Failure in a Sandstone

Abstract:

Catastrophic failure in the brittle Earth initiates when smaller-scale faults and fractures localise along a distinct and emergent fault plane. The localisation of such structural damage, then, is a critically-important driving mechanism for such devastating phenomena as earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as large-scale infrastructure collapse. However, due to the speed at which stable crack growth transitions to dynamic rupture at this point, the precise mechanisms involved in strain localisation are yet to be determined. We used time-resolved synchrotron x-ray microtomography to image in-situ the transition at the micron scale and at strain rates consistent with those in the Earth’s crust. By controlling the rate of micro-fracturing events during a triaxial deformation experiment, we deliberately slowed the strain localisation process from seconds to minutes as failure approached. This approach, originally established to indirectly image fault nucleation and propagation with acoustic emissions, is completely novel in synchrotron x-ray microtomography and has enabled us to image directly processes that are normally too transient even for fast synchrotron imaging methods. Moreover, with simultaneous monitoring of acoustic emissions and ultrasonic velocities, we have generated a dataset that allows us to ‘ground-truth’ inferences made from seismic waves; the primary method of detecting deformation at the field-scale where direct imaging of the subsurface is impossible. Here, we first present the experimental apparatus and control system used to acquire the data. We then characterise the developing damage zone in the sample from the 4D x-ray microtomography data and show how it relates to the evolving 4D strain field, measured with incremental Digital Volume Correlation between pairs of recorded x-ray tomographic volumes. Finally, we examine how these are linked to the acoustic emission locations and changing ultrasonic velocities.