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Nuclear Treaties Branch

By A. Villaseñor, M. H. Ritzwoller, M. P. Barmin, E. R. Engdahl and A. L. Levshin


Contract No. DTRA01-99-C-0019 We have investigated the performance of a variety of travel time computation methods for application in highly heterogeneous three-dimensional (3-D) velocity structures, such as those found in Central Eurasia. Recent 3-D models of the crust and upper mantle beneath Central Eurasia (e.g., Villaseñor et al., 2000) contain sharp vertical and horizontal velocity contrasts and thin layers which complicate the accurate computation of travel times. Techniques based on ray-perturbation theory or the graph method (e.g., Pulliam and Snieder, 1996; Nolet and Moser, 1993) do not appear to work well when the velocity anomalies are large relative to the reference model. In these cases the reference ray can be very different from the minimum-time ray, and the methods fail to converge rapidly and in some cases do not converge at all to the global minimum. Ray shooting and finite-difference algorithms are more computationally expensive, but provide accurate results in the presence of large velocity contrasts. In the ray-shooting method, the model is parameterized in terms of smooth polynomials in all directions. Ray-shooting methods are normally implemented only in 2- ' H J ýHUYHQê DQ

Year: 2011
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