Sensing Cracks in the Earth Using Multiple Scattering and Nonlinear Elasticity

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Cracks are increasingly important to our understanding of Earth resources and structures. They hold most of the unconventional oil and gas reserves, are important for geothermal energy, and play a role in conventional oil and gas production, volcanic eruptions and perhaps even earthquakes. In wavefields, they introduce scattering effects, in which waves reflect many times between different cracks in the subsurface, and it is likely cracks that introduce the dependence of elastic paramters on stress and strain, making rocks exhibit strong elastic nonlinearity. The long term goals of this research program are to better understand how to sense and characterize rocks. On the shorter term, we aim to (i) to better understand the role of cracks in the nonlinear elasticity of rocks with the long-term goal of using these parameters to image crack density and orientation and (ii) to use microseismic data sets to characterize scattering from cracks in volcanic regions to characterize cracks through scattering attenuation and fluids through intrinsic attenuation.*** To look at how cracks influence the nonlinearity of rocks, we will work from a recently developed experiment that uses a high-amplitude shear wave to perturb the elastic properties of a rock, and senses those perturbations with a P-wave. We have data indicating that this experiment senses the orientation of cracks in a sample. To better understand why this is the case, and the underlying mechanisms of nonlinear elasticity, we will develop a comprehensive numerical model of this experiment. To verify this model, we will perform a suite of new lab experiments focussing on igneous rocks (our past work has focussed on sedimentary rocks). ***To characterize cracks from microseismic data, we will use a data set from Iceland, collected to monitor seismicity for volcanic and geothermal applications. We will use these data to measure scattering strength and intrinsic attenuation of the region and its variability. We will perform laboratory and numerical experiments, using the techniques developed for the nonlinear elasticity part of the project to better understand our results and will interpret them in terms of local stress changes. *** The novelty of this work is in the use of both techniques to understand cracks, their orientation, density, and properties. The approach of using a combination of numerical modelling, laboratory, and field data will improve our understanding of the physics underlying these phenomena, and will change how we sense cracks in the subsurface. This is important to the oil and gas industry, which plays a significant role in the Canadian economy, as well as to the geothermal industry, which is likely to play a role in future energy supply and has longer term implications for natural hazard mitigation in Canada and elsewhere.**