Sensing Cracks in the Earth Using Multiple Scattering and Nonlinear Elasticity

裂缝对我们理解地球资源和结构越来越重要。它们拥有大多数非常规石油和天然气储量，对地热能非常重要，并在常规石油和天然气生产、火山喷发甚至地震中发挥作用。在波场中，它们引入了散射效应，波在地下不同裂隙之间多次反射，可能是裂隙引入了弹性参数对应力和应变的依赖关系，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)更好地了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，以及(Ii)使用微地震数据集来表征火山区裂缝的散射，以通过散射衰减来表征裂缝，通过本征衰减来表征流体。为了观察裂缝如何影响岩石的非线性，我们将从最近开发的一项实验入手，该实验使用高振幅横波来扰动岩石的弹性性质，并用P波来感知这些扰动。我们有数据表明，这个实验可以检测到样品中裂纹的方向。为了更好地理解为什么会出现这种情况，以及非线性弹性的潜在机制，我们将开发一个全面的实验数值模型。为了验证这一模型，我们将进行一系列新的实验室实验，重点放在火成岩上(我们过去的工作重点是沉积岩)。为了从微地震数据中确定裂缝的特征，我们将使用从冰岛收集的数据集来监测火山和地热应用的地震活动。我们将使用这些数据来测量该区域的散射强度和本征衰减及其变异性。我们将进行实验室和数值实验，使用为项目的非线性弹性部分开发的技术，以更好地理解我们的结果，并将根据局部应力变化来解释它们。这项工作的新奇之处在于使用了这两种技术来了解裂纹及其取向、密度和性质。结合使用数值模型、实验室和现场数据的方法将提高我们对这些现象背后的物理基础的理解，并将改变我们感知地下裂缝的方式。这对石油和天然气行业以及地热行业都很重要，石油和天然气行业在加拿大经济中发挥着重要作用，地热行业很可能在未来的能源供应中发挥作用，并对加拿大和其他地方的自然灾害缓解产生较长期的影响。

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.