INVESTIGATION OF FRACTURE INITIATION AND PROPAGATION IN BRITTLE ROCKS USING MECHANICAL-GEOPHYSICAL-OPTICAL VISUALIZATION TECHNIQUES

Monitoring rock fracturing processes in real time with advanced high-resolution imaging techniques in laboratory conditions that simulate the in-situ environments can offer unprecedented opportunities for identifying the underlying causes of crack initiation and coalescence and the accompanying geophysical signals. In our laboratory research work, coupled mechanical-geophysical-optical visualization of fracturing in brittle rocks is used to infer the nature of fractures and their geophysical signals. In this presentation, I will present the results of (a) uniaxial and (b) true-triaxial laboratory compression tests on brittle Barre granite specimens while monitoring the fracturing processes using a combination of acoustic emission (AE), ultrasonic imaging, and digital image correlation (DIC) techniques. These experiments allow for exploring fracturing in rocks under realistic in situ environments and at multiple scales. Our uniaxial compression test results show that the tensile microcracks at the flaw tip form the process zone, and as the applied load to the rock specimen increases, the process zone extends by the accumulation of the tensile, shear, and mixed-mode cracks. The mode of deformation computed from the image-based strain profiles of DIC enabled visual comparison of the initiation, growth and interaction of the tensile and shear cracks with the micro-seismic source mechanism observed by moment tensor inversion of the AE events. Through localization of the detected AEs in our true-triaxial compression experiments, we identified the cracking mechanisms at different stages of the hydraulically induced fracture initiation and propagation in the rock specimens.