Collaborative Research: Multiscale analysis of geological structures that influence crustal seismic anisotropy

该项目是对地壳物质各向异性的研究，重点是宏观结构几何形状及其如何改变岩石组构的地震反应。地震各向异性是地震波传播与各向异性地球物质之间的累积相互作用，它通过地震波速度的方向依赖性表现出来。由于多种因素，如三维地质几何和非均质性、微尺度结构、速度梯度导致的地震射线路径弯曲、可能无法提供全方位覆盖的现场实验、各向异性作为二阶波形或走时特征的观察，在变形的地壳中揭示这种效应是很复杂的。虽然地震各向异性可能源于上部地壳断裂或各向同性物质有组织的精细分层，但物质各向异性也是一个原因，至少涉及四个因素：(1)微观结构特征，包括组成矿物的空间排列、模态丰度、晶体学和形状取向；(2)性质的固有方位变化和使用对称类的近似；(3)不同尺度下材料性质的体积表征（有效介质）；(4)宏观结构的类型和内部几何形状。宏观结构对样品尺度材料各向异性的重定向有其自身的影响。由于材料的原因，地震波会产生一种类型的信号响应；它可以产生不同的响应，因为一组岩石由于结构的几何形状而重新定向。研究人员将使用地震有效介质的概念来表示地震波穿过的地球体积。他们将采用地球体积的表示，允许有效介质的张量表示。这允许通过波动方程的代数张量操作来分离结构几何和组成结构的岩石。该项目的一个主要目标是定义结构对形成有效媒体的贡献。每个结构都有一个几何“脉冲响应”，它将岩石纹理修改为结构的有效介质表示。该项目的第二个目标是了解微尺度岩石织物对给定结构的有效介质的作用。两者结合起来产生传播波响应的净有效介质。他们将对常见的地壳结构几何形状及其如何改变岩石各向异性进行定量和系统的研究，并使用解析几何表面表示结构，并创建严格的综合方法来计算不同尺度的有效介质及其对地震波传播的综合影响。他们还将研究微尺度岩石组构的张拉形式如何对组成矿物的模态组成和统计取向敏感。该项目的成果将用于实际各向异性地震资料的地震解释。该项目汇集了地震学、构造/微结构地质学和理论/计算力学方面的专业知识，以帮助开发一个定量框架，用于分析大陆地壳变形多矿物岩石的物质各向异性和由此产生的地震各向异性。

This project is a study of crustal material anisotropy with a focus on macroscale structural geometries and how they will modify the seismic response of rock fabrics. Seismic anisotropy is the cumulative interplay between propagating seismic waves and anisotropic earth material that manifests itself through the directional dependence of seismic wave speeds. Unraveling this effect in deformed crustal terranes is complex due to several factors, such as 3D geological geometry and heterogeneity, microscale fabric, bending of seismic raypaths due to velocity gradients, field experiments that may not offer full azimuthal coverage, and the observation of anisotropy as second-order waveform or traveltime features. While seismic anisotropy can originate from upper crustal fractures or by organized fine-scale layering of isotropic material, material anisotropy is also a cause and involves at least four factors: (1) microstructural characteristics including spatial arrangement, modal abundances, and crystallographic and shape orientations of constituent minerals, (2) inherent azimuthal variation of properties and approximation using symmetry classes, (3) bulk representation (effective media) of material properties at different scales, and (4) the types and internal geometries of macroscale structures. The reorientation of sample-scale material anisotropy by macroscale structures imparts its own effect. A seismic wave will produce one type of signal response due to material; it can produce a different response due to a package of rocks that are reoriented due to the geometry of a structure. The researchers will use the concept of seismic effective media to represent earth volumes through which seismic waves travel. They will employ a representation of earth volumes that allow for a tensorial representation of effective media. This allows via the wave equation an algebraic tensor manipulation to separate the structural geometry and the rocks composing the structure. A primary goal of the project is to define the contributions of structure to form effective media. Each structure has a geometrical "impulse response" which will modify a rock texture into an effective medium representation of the structure. A second goal of the project is to understand how the role of microscale rock fabrics contribute towards the effective media for given structures. Both combine to produce the net effective medium that a propagating wave responds to. They will conduct a quantitative and systematic study of common crustal structural geometries and how they modify rock anisotropy, and represent structures using analytical geometry surfaces and create a rigorous and integrated methodology to calculate effective media at different scales and their combined effects on seismic wave propagation. They will also examine how the tensorial form of microscale rock fabrics are sensitive to the modal compositions and statistical orientations of constituent minerals. Results of this project will be designed to aid the seismic interpretation of real anisotropic seismic data. This project brings together expertise in seismology, structural/microstructural geology and theoretical/computational mechanics to help develop a quantitative framework for the analysis of material anisotropy and resulting seismic anisotropy in deformed polymineralic rocks of the continental crust.