2D multiscale non‐linear velocity inversion

An efficient and robust non‐linear inversion method for velocity optimization combining a global random search followed by a simplex technique is presented. The background velocity field is estimated at different spatial scales by analysing image gathers after iterative prestack depth migrations. First, the global random search is used to determine the main features/trends of the velocity model (large‐scale component). Then, the simplex technique improves the resolution of the velocity field by estimating smaller‐scale features. A measure of the quality of the velocity model (objective function) is based on flattening offset events in depth‐migrated image gathers. To help constrain the solution, the algorithm can incorporate a priori information about the model and a smoothness condition. This 2D velocity estimation offers the benefit of being semi‐automatic (requiring minimal human intervention) as well as providing a global and objective solution (which is a useful approach to an interpretation‐derived velocity‐estimation technique). The method is applied to a real data set where AVO analysis is carried out after prestack depth migration, as structural effects are non‐negligible. It is demonstrated that the method can successfully estimate a laterally inhomogeneous velocity model at a computational cost modest compared with an interpretation‐based iterative prestack depth velocity‐analysis technique.