A Framework for Online Inversion-Based 3D Site Characterization
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چکیده
Our goal is to develop the capability for characterizing the three-dimensional geological structure and mechanical properties of individual sites and complete basins in earthquake-prone regions. Toward this end we present a framework that integrates in situ field testing, observations of earthquake ground motion, and inversion-based modeling. 1 Geotechnical Site Characterization An important first step for forecasting strong ground motion during future earthquakes in seismically-active regions is to characterize the three-dimensional mechanical properties and geological structure of sites and basins within those regions. Characterizing a site refers to our ability to reconstruct as faithfully as possible the soil profile in terms of a limited set of material parameters, such as shear and compressional wave velocities, density, attenuation, and the slow velocity for poroelastic media. Geological and geotechnical materials, soil and rock, impact the performance of the built environment during earthquakes. They play a critical role in the generation of ground motion, and, consequently, on determining the spatial extent and severity of damage during earthquakes. Yet, they are not well-investigated, even though they are the most variable and least controlled of all materials in the built environment. Since soils cannot be accessed easily, their properties can be inferred only indirectly. Currently, geomaterials are characterized with essentially the same general testing methods that were used 25 years ago. These methods rely on testing a small number of specimens in the laboratory and conducting a limited number of small-strain field tests. There is a critical need to advance beyond current methods to reduce the level of uncertainty that currently exists in the estimation of geological and geotechnical material properties. A variety of techniques for reconstruction of earth properties from noninvasive field tests have been pursued, notably within the gas and oil exploration communities. However, the goals of our work are distinctly different, both in terms of the nature of the problem (e.g. complete material profile reconstruction M. Bubak et al. (Eds.): ICCS 2004, LNCS 3038, pp. 717–724, 2004. c © Springer-Verlag Berlin Heidelberg 2004 718 V. Akçelik et al. vs. estimates of material contrast) and in the models employed (e.g. all elastic waves vs. just acoustic waves). Reconstruction of the soil model results in a time-dependent or timeharmonic wave propagation inverse problem. Solution of this inverse problem represents an enormous challenge from the theoretical, algorithmic, and computational points of view. An added challenge is the lack of a systematic approach to in situ measurements. Such measurements are often decoupled from the needs of the computational process, due to cost considerations, equipment limitations, or the adoption of ad-hoc and simplified analysis procedures. With current test equipment, the volume of soil that can be excited from a single location is somewhat limited because the maximum loads that can be applied are restricted and the response amplitude decays exponentially with distance, frequency, amount of soil damping, and slowness of the propagating waves. The advent of large-scale test equipment makes it possible to apply much larger loads over a wide range of frequencies, and thus excite a larger volume of soil from a single location than has been possible till now. To effectively extract the desired information from the field test data, we need robust, efficient, and scalable forward and inverse three-dimensional wave propagation solvers. We have developed such methods and fine-tuned them for the analysis of earthquake ground motion in large basins. Our forward and inverse modeling methods are overviewed in Sections 2 and 3, respectively. Finally, in Section 4, we present an on-line framework for local site characterization that integrates steerable field experiments with inverse wave propagation. 2 Forward Elastic Wave Propagation Modeling Our forward elastic wave propagation simulations are based on wavelengthadaptive mesh algorithms, which overcome many of the obstacles related to the wide range of length and time scales that characterize wave propagation problems through heterogeneous media [1,2,3,4,5,6,7]. In highly heterogeneous media such as sedimentary basins, seismic wavelengths vary significantly, and wavelength-adaptive meshes result in a tremendous reduction in the number of grid points compared to uniform meshes (e.g. a factor of 2000 in the Los Angeles Basin). Our code includes octree-based trilinear hexahedral elements and local dense element-based data structures, which permit wave propagation simulations to substantially greater resolutions than heretofore possible. We have validated our code using the Southern California Earthquake Center (SCEC) LA Basin model and an idealized model of the 1994 Northridge earthquake. To illustrate the spatial variation of the 1994 Northridge earthquake ground motion, Fig. 1 presents snapshots at different times of an animation of the wave propagation through the basin. The left part of the figure shows a plan view and cross-section of the basin, as defined by the distribution of shear wave velocity. The projections of the fault and hypocenter are also shown. The directivity of the ground motion along strike from the epicenter and the concentration of motion near the fault corners are response patterns of the actual earthquake A Framework for Online Inversion-Based 3D Site Characterization 719 Free Surface Shear Wave Velocity Vs (m/s) A'
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تاریخ انتشار 2004