Imaging crack and pore anisotropy in deformed rocks: Quantifying the crack fabric tensor

Abstract

Cracks and pores in crustal rocks are often fluid-saturated and subjected to stresses arising from both theoverburden and regional tectonics. These stresses produce changes both in the shape and volume of the rock, andin the shape and volume of the voids i.e. the cracks and pores. Patterns of parallel fluid-filled cracks around majorearthquake prone faults are more compliant to applied stresses compared to randomly oriented cracks and hencecould produce a short-term (i.e. undrained) fluid pressure change along the fault equal to the fault normal stress,allowing the fault to slip in an earthquake [1]. Therefore, developing methodologies to quantify the orientation ofpores and cracks and their mutual arrangement in deformed rocks is of primary importance to understand largeactive fault behaviour during earthquake cycles.X-ray micro-CT analyses, using both intact and laboratory deformed samples, provide high-resolution (micrometre)volumetric scans from which we quantify pore and crack fabrics. Image processing provides adaptivemethods to extract information from such datasets. In particular, we combine Hessian matrix filtering [2] andanisotropic wavelet analysis [3] to extract three-dimensional arrays of pores and cracks with different aspect ratios(i.e. pores and cracks) at different scales of analysis.Observed changes in pore and crack fabrics between the intact and deformed samples allow us to characterisethe multi-scale microstructural variations in anisotropy that form due to the applied stresses. In turn, theinformation gathered at the micro scale can be used to improve our understanding of the fundamental relationshipbetween the theory of anisotropic poroelasticity and the measurable properties of fluid-saturated fault zones.