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.
In material sciences, anisotropy is a trait wherein a property has different values when measured in different directions. For certain types of earth rocks, the speed at which seismic waves pass through can vary based on direction; these rocks have anisotropic wave speeds. An example is a slate which is a metamorphosed shale. This rock commonly contains an abundance of mica whose platy shapes help form the rock sheets characteristic of a slate. Seismic waves will move faster along these sheets and relatively more slowly across (through) the slate sheets. The behavior for seismic waves that they travel faster or slower in different directions is known as seismic anisotropy. Classification is possible of rocks and minerals based on their degrees of wave speed anisotropy. Some rocks, like those with sheets, have wave speeds that spatially vary in all directions with cylindrical symmetry. Other rocks have speeds whose directional variations describe an irregular ellipsoid. A third set of rocks do not have directional variations and have the same speed in all directions; these rocks are isotropic. The factors that determine which type of wave speed anisotropy exists within a particular rock are the rock's internal minerals and any organized alignment of these minerals. When seismologists analyze seismic waves, they primarily examine two aspects of these waves: (a) the time a wave takes to travel from where it was created to a known observation point, and (b) the shape of the wave. A volume of earth below ground that possesses anisotropy will affect both the time and shape of the wave, and these are examined within analysis methods created by seismologists that allow for the earth anisotropy. However, because the derivation of actual rock anisotropy is difficult from individual types of seismological data, these analysis methods use a simplifying assumption as to the classification of rock anisotropy. For the continental crust, the type of rock anisotropy that is assumed is that associated with rock sheets and is termed transverse isotropy. By analogy, this is similar to a stadium lot that is filled with cars parked head to tail, all pointing in the same direction. To run across the lot parallel to the cars can be quick. In contrast, to run in the direction across the cars can be slower given that cars are of various lengths; one needs to zig-zag some amount between the cars. This speed for running is different based on direction and is hence anisotropic. Seismologists often use this description of anisotropy, transverse isotropy, for studies of the continental crust. To carry the parking lot analogy one step farther, if the parking slots for the cars do not point in one direction but have a large scale pattern, its sense of anisotropy will be different. This larger scale pattern can be if the parked cars follow the shape of the stadium or perhaps show broad undulations due to features of the lot itself. In either case, running in a straight line across the lot in different directions have their own degrees of being slowed down -- their degree of anisotropy is much worse. The topic of this NSF-supported research project was to study the effect that large scale geological structures can have on seismic anisotropy. A large layer filled with what is locally a simple rock anisotropy if bent into long undulatory folds will have a new anisotropy that is of a different classification than that of the local rock. Our project examined the shapes of different fundamental geological structures in order to estimate how much change to the anisotropy could occur. We developed a method that could separate the geometrical shape of the structure from the rock anisotropy that filled the structure in order to quantify the contribution provided by the shape versus the local rock. Our study showed that the simplifying assumption that seismologists make for continental crust (that is, transverse isotropy) might not necessarily be correct and that it is dependent on the geological structures present at a given location. This project provided educational opportunities for doctoral graduate students at two universities. This project also provided new collaborative research opportunities for professors in different scientific disciplines (material sciences and earth sciences).
