BACKGROUND VELOCITY ESTIMATION USING NONLINEAR OPTIMIZATION FOR REFLECTION TOMOGRAPHY AND MIGRATION MISFIT

We show that it is possible to estimate the background velocity for pr estack depth migration in 2D laterally varying media using a non-linea r optimization technique called very fast simulated annealing (VFSA). We use cubic splines in the velocity model parametrization and make us e of either successive pairs of shot gathers or several constant-offse t sections as input data for the inversion. A Kirchhoff summation sche me based on first-arrival traveltimes is used to migrate/model the inp ut data during the velocity analysis. We evaluate and compare two diff erent measures of error. The first is defined in the recorded data or (x,t) domain and is based on a reflection-tomography criterion. The se cond is defined in the migrated data or (x,z) domain and is based on a migration-misfit criterion. Depth relaxation is used to improve the c onvergence and quality of the velocity analysis while simultaneously r educing the computational cost. Further, we show that by coarse sampli ng in the offset domain the method is still robust. Our non-linear opt imization approach to migration velocity analysis is evaluated for bot h synthetic and real seismic data. For the velocity-analysis method ba sed on the reflection-tomography criterion, traveltimes do not have to be picked. Similarly, the migration-misfit criterion does not require that depth images be manually compared. Interpreter intervention is r equired only to restrict the search space used in the velocity-analysi s problem. Extension of the proposed schemes to 3D models is straightf orward but practical only for the fastest available computers.