oa Seismic Refraction Shootout: Using Ray-Based Tomographic Inversion

Ray-based Tomographic Inversion is a common technique used to process first-arrival travel times into structural images of the earth. We propose to use a 3-dimensional ray-based travel-time Tomographic Inversion program to obtain a velocity model from the synthetic data for the Seismic Refraction Shootout session at the SAGEEP 2011 meeting. Our code uses a ray-tracing scheme to calculate travel times for the forward problem and an iterative inverse procedure to find incremental changes in model parameters that optimally fit travel-time residuals (observed minus calculated) subject to model smoothness constraints. We first calculate the ray paths by following the finite-difference (eikonal) travel-time gradient from source to receiver to compute travel time residuals and fill the matrix of partial derivatives. We then solve for perturbations in slowness by conjugate gradient least squares. Due to regularization requirements in Tomographic processing, resolution of the data is important to consider especially for complex structures. Resolution tests (checkerboard tests) will be conducted on the data to aide in the Interpretation of the results of the tomogram. Standard error analysis tests (jackknife tests) will also performed on the Tomography model to obtain error estimates. <br>Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.