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

该项目是对地壳材料各向异性的研究，重点是宏观结构几何以及它们将如何改变岩石组构的地震响应。地震各向异性是传播的地震波和各向异性地球材料之间的累积相互作用，它通过地震波速的方向依赖性来表现出来。由于三维地质几何和非均质性、微尺度组构、速度梯度引起的地震射线路径弯曲、可能不能提供完全方位覆盖的野外实验以及作为二阶波形或旅行时特征的各向异性的观察，在变形的地壳地体中解开这种效应是复杂的。虽然地震各向异性可以起源于地壳上部的裂隙或各向同性材料的有组织的精细分层，但材料的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、模式丰度和晶体和形状取向；(2)性质的固有方位变化和使用对称类的近似；(3)材料性质在不同尺度上的整体表示(有效介质)；以及(4)宏观结构的类型和内部几何。宏观结构对样品尺度材料各向异性的重定向具有其自身的效应。由于材料的原因，地震波将产生一种类型的信号响应；由于结构的几何形状，岩石包被重新定向，地震波可能会产生不同的响应。研究人员将使用地震有效介质的概念来表示地震波通过的地球体积。他们将采用地球体积的表示法，以允许有效介质的张量表示。这允许通过波动方程进行代数张量处理来分离结构几何和构成结构的岩石。该项目的一个主要目标是确定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”，它将把岩石纹理修改为结构的有效媒介表示。该项目的第二个目标是了解微尺度岩石组构的作用如何有助于特定结构的有效介质。两者结合在一起，产生传播波所响应的净有效介质。他们将对常见的地壳结构几何及其如何修正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来表示结构，并创建一种严格而完整的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量形式如何对组成矿物的模式组成和统计取向敏感。该项目的成果将用于辅助实际各向异性地震数据的地震解释。该项目汇集了地震学、结构/微构造地质学和理论/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中的材料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.