Sensing Cracks in the Earth Using Multiple Scattering and Nonlinear Elasticity

裂缝对我们了解地球资源和结构越来越重要。它们拥有大部分非常规石油和天然气储量，对地热能很重要，在常规石油和天然气生产、火山爆发甚至地震中发挥着重要作用。在波场中，它们引入了散射效应，其中波在地下不同裂缝之间多次反射，并且很可能是裂缝引入了弹性参数对应力和应变的依赖，使岩石表现出强烈的弹性非线性。这项研究计划的长期目标是更好地了解如何感知和表征岩石。在短期内，我们的目标是(i)更好地理解裂缝在岩石非线性弹性中的作用，长期目标是使用这些参数来成像裂缝密度和方向；（ii）使用微地震数据集来表征火山区域裂缝的散射，通过散射衰减来表征裂缝，通过固有衰减来表征流体。

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.