Collaborative Research: CMG Research: Statistical Seismic Imaging

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Collaborative Research: CMG Research: Statistical Seismic ImagingA. Levander, W.W. Symes, S. MinkoffBoth the crystalline crust and the shallow near surface ( 100m depth) environment are complex, mechanically heterogeneous regions, often having spatial distributions of mechanical properties that are distinctly different from the relatively smoothly layered sedimentary column. Exploration seismology is currently one of the most important means of exploring the subsurface: for petroleum in the sedimentary column, for geotechnical/environmental purposes in the shallow subsurface, and for understanding of tectonic processes in the crystalline crust. Reflection seismology was developed by the petroleum industry to explore the sedimentary column, i.e., the medium is layered, the preponderance of energy is singly scattered, and scales are dichotomous. Its applications in both the near-surface environment and the deep crust have been very successful but also present problems not usually encountered in petroleum exploration. The near surface is subjected to chemical and mechanical weathering processes that produce a highly heterogeneous, nonlayered medium. Mechanical properties can vary by an order of magnitude within a few wavelengths due to compaction, chemical alteration, and the presence or absence of water. The crystalline crust, although lacking order of magnitude changes in mechanical properties, has a spatially complex, often self-affine (fractal) distribution of seismic velocity and density heterogeneities. For the relatively long path propagation in the crust, the wavefield often falls in a strong scattering regime in which linear imaging systems will produce mediocre or nonsensical results.This research program makes first steps toward a complementary imaging paradigm for complex media, termed statistical seismic imaging, in which statistical properties of the subsurface are mapped using statistical properties of the seismic wavefield. As this is an extremely large topic, the proposal scope will be limited to a few areas of endeavor. The investigations will explore (1) effective models relating subwavelength scale heterogeneity to macro-scale propagation effects (effective mechanical properties, velocity, attenuation, dispersion, etc), (2) the extent to which multiple reflection and refraction can be differentiated and their effects on traditional seismic imaging, and (3) the identification of the onset of strong scattering. The research involves a high level of software development and computational effort, including 3D seismic simulation and nonlinear inversion for material properties and statistics. No single discipline possesses the tools to make significant progress on statistical seismic imaging: expertise in geophysics, applied math, and high performance computing are needed. The principal investigators have expertise in these areas. The research has the potential of improving societally important environmental/geotechnical investigations as well as more fundamental research on the structure and tectonics of the Earth's crust.