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.
Seismic anisotropy, or the dependence of seismic velocity on the direction of wave propagation and polarization, is observed throughout the earth and has been used to interpret the earth’s dynamic processes. The objectives of this collaborative project were to develop a rigorous and integrated methodology for studying crustal material anisotropy at different scales and to analyze their combined effects on seismic wave propagation. The major accomplishments of this research project are as follows. 1. The concept of seismic effective media was used to represent earth volumes through which seismic waves travel. An important contribution of this project was the separation of the influence of structural geometry from the representative rock stiffness using the concept of a structural geometry operator (SGO). This separation will allow seismologists to better estimate what types of seismic anisotropy they might interpret in their seismological data. We have developed SGOs for different fold geometries, such as sinusoidal, chevron, parabolic, box and cuspate folds. They can be used to calculate the seismic velocities and anisotropies of common fold geometries. The synthetic seismograms are in good agreement with anticipated wave propagation for our hypothetical folds. Therefore, it might be possible to identify hinge shape, limb angle, and even orientation of subsurface folds in the crust using seismic signals from natural sources detected by appropriate novel array deployments, and in turn obtain information on the kinematics of in situ crustal deformation. 2. An important ramification of our study is that seismologists who interpret or develop analysis methods for crustal seismic anisotropy may need to go beyond viewing the anisotropy as being caused by simple transverse isotropy crustal material. This opens the door to use other geological options (elasticity symmetries) to interpret complicated seismic anisotropy signals. 3. The influence of microstructure on the seismic anisotropy of rocks was investigated using a rigorous computational micromechanics approach. Our results indicate that the anisotropy of seismic wave propagation is strongly in?uenced by the mineralogy and microstructure of rocks. Detailed analyses of phyllosilicate-rich rocks demonstrate that the muscovite grain orientations have a significant influence on the wave speeds. Increasing modal fraction and alignment of muscovite grains leads to greater seismic anisotropy of the rock. Broader Impacts a) An open-source software with a user-friendly graphical user interface has been developed for the calculation of the elastic and seismic properties of rocks using either EBSD data files or computer generated microstructures. The beta version of the software has been made freely available to other researchers and the public through the following web site: http://umaine.edu/mecheng/faculty-and-staff/senthil-vel/software/ESP_Toolbox/. While the computational tools were developed mainly for the Earth Sciences community, it will also enable Materials Scientists to characterize the bulk properties and perform detailed stress analyses of engineered polycrystalline materials such as metallic alloys, ceramics and advanced composite materials. b) This project supported in whole or part the thesis work of four Ph.D. students including three international students. The Ph.D. students received excellent training in both Earth science and computational mechanics disciplines. c) Grant funded research lead to five journal papers including two currently under review and one in preparation, nine abstracts for research presented at international conferences and a User’s manual for our open-source software for calculating the elastic and seismic properties of rocks.
