July 15, 2010-June 30, 2013
Level of Access
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.
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Vel, Senthil and Johnson, Scott E., "Collaborative Research: Multiscale Analysis of Geological Structures That Influence Crustal Seismic Anisotropy" (2013). University of Maine Office of Research Administration: Grant Reports. 357.