Nonlinear refraction and reflection travel time tomography

We develop a rapid nonlinear travel time tomography method that simultaneou sly inverts refraction and reflection travel times on a regular velocity gr id. For travel time and ray path calculations, we apply a wave front method employing graph theory. The first-arrival refraction travel times are calc ulated on the basis of cell velocities, and the later refraction and reflec tion travel times are computed using both cell velocities and given interfa ces. We solve a regularized nonlinear inverse problem. A Laplacian operator is applied to regularize the model parameters (cell slownesses and reflect or geometry) so that the inverse problem is valid for a continuum. The trav el times are also regularized such that we invert travel time curves rather than travel time points. A conjugate gradient method is applied to minimiz e the nonlinear objective function. After obtaining a solution, we perform nonlinear Monte Carlo inversions for uncertainty analysis and compute the p osterior model covariance. In numerical experiments, we demonstrate that co mbining the first arrival refraction travel times with later reflection tra vel times can better reconstruct the velocity field as well as the reflecto r geometry. This combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust. We ap ply this approach to model the crustal structure of the California Borderla nd using ocean bottom seismometer and land data collected during the Los An geles Region Seismic Experiment along two marine survey lines. Details of o ur image include a high-velocity zone under the Catalina Ridge, but a smoot h gradient zone between Catalina Ridge and San Clemente Ridge. The Moho dep th is about 22 km with lateral variations.