Analysis and algorithms for a regularized cauchy problem arising from a non-linear elliptic PDE for seismic velocity estimation

Analysis and Algorithms for a Regularized Cauchy Problem arising from a Non-Linear Elliptic PDE for Seismic Velocity Estimation $ M.K. Cameron ,1 , S.B. Fomel ,2 , J.A. Sethian ,3 Abstract In the present work we derive and study a nonlinear elliptic PDE coming from the problem of estimation of sound speed inside the Earth. The physical setting of the PDE allows us to pose only a Cauchy problem, and hence is ill-posed. However we are still able to solve it numerically on a long enough time interval to be of practical use. We used two approaches. The first approach is a finite difference time-marching numerical scheme inspired by the Lax-Friedrichs method. The key features of this scheme is the Lax-Friedrichs averaging and the wide stencil in space. The second approach is a spectral Chebyshev method with truncated series. We show that our schemes work because of (i) the special input corresponding to a positive finite seismic velocity, (ii) special initial conditions corresponding to the image rays, (iii) the fact that our finite-difference scheme contains small error terms which damp the high harmonics; truncation of the Chebyshev series, and (iv) the need to compute the solution only for a short interval of time. We test our numerical scheme on a collection of analytic examples and demonstrate a dramatic improvement in accuracy in the estimation of the sound speed inside the Earth in comparison with the conventional Dix inversion. Our test on the Marmousi example confirms the effectiveness of the proposed approach. This work was supported in part by the Applied Mathematical Science subprogram of the Office of Energy Research, U.S. Department of Energy, under Contract Number DE-AC02-05CH11231 Email addresses: cameron@cims.nyu.edu (M.K. Cameron), sergey.fomel@beg.utexas.edu (S.B. Fomel), sethian@math.berkeley.edu (J.A. Sethian) Courant Institute of Mathematical Science, Department of Mathematics, New York University, 251 Mercer street, New York, NY 10012 Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924 Department of Mathematics, University of California, Berkeley, 940 Evans Hall, Berkeley, CA Preprint submitted to Journal of Computational Physics January 22, 2009