This award is an outcome of the NSF 06-504 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation Research (NEESR)" competition. The project team includes Carnegie Mellon University; Georgia Institute of Technology; the University of California at Santa Barbara; the University of Texas, El Paso; and The University of Texas at Austin. The main objective of this research is to develop the capability for estimating the geological structure and mechanical properties both of individual sites and of complete basins, and to demonstrate this capability on the nees@UCSB site at the Garner Valley Downhole Array (GVDA) and the entire Garner Valley. Specifically, this high-fidelity estimation will be based on integrating: (a) in-situ dynamic excitation using the NEES equipment at the University of Texas at Austin (nees@UTexas); (b) earthquake records from new strong-motion and broadband sensor networks; and (c) new inversion methods based on partial-differential-equation (PDE)-constrained optimization. This project represents an unparalleled opportunity to couple state-of-the-art field experimentation with state-of-the-art computational tools for the purpose of imaging the subsurface at resolutions and length scales until recently unattainable. These two NEES sites were selected for the following reasons: (1) The mobile shakers at the nees@UTexas site can apply loads at a wide range of frequencies and loading levels. Coupled with earthquake records, the data collected will permit the estimation of the primary- and shear-wave velocities. In addition, soil damping will be included in the inversion models, to be estimated simultaneously with the two velocities; and (2) The GVDA (nees@UCSB) is a test site located in a narrow valley in a highly seismic region in southern California, which is ideally suited for monitoring ground motion. The hundreds of small earthquakes that have been recorded at the site and at other free surface locations throughout this valley in the last 15 years make it an invaluable source of data for the regional deep structure inversions, and for an independent verification of site inversion through field tests. To increase the fidelity of the inverted models, this dataset will be augmented with data from instruments of the USArray component of EarthScope that will be deployed over periods of time that will overlap the periods of active testing. In-situ tests will provide data for characterizing the upper layers at the GVDA site and throughout the valley. Observations from these tests, both downhole and on the free surface, will be used, along with earthquake observations, for obtaining two-dimensional and three-dimensional, high-fidelity profiles. In addition, these observations will be used with the spectral-analysis-of-surface-waves (SASW) method to obtain the shear-wave velocity profile at the test locations. This velocity profile will be used as part of the blind-test validation suite of the global optimization-based inverse local and regional models. While the integrated methodology will be applied to a specific region in southern California, the results will be applicable to many other regions in the United States and abroad. The new integrated methodology will be equally applicable to similar problems where waves are used for probing, such as those arising in the oil and gas exploration communities, medical imaging applications, other infrastructure condition-assessment problems such as in structural flaw identification problems, assessing the suitability of candidate nuclear waste sites, and even remotely-controlled planetary exploration missions where site characterization is of paramount importance. Because of the critical role that soils play in infrastructure design, the accelerated availability of reliable soil characterization methods will have a direct impact on public safety and welfare.
