Coherence-based time migration velocity analysis by the use of supergathers

Abstract

Errors in the migration velocity field are perceived as moveouts in common-image gathers (CIGs). These moveouts depend on discrepancies in the velocity field as well as in the dip of the reflectors. We extend previous works into two main aspects. First, we found an accurate description of the residual moveout, considering the reflector dip, whereas only second-order approximations have been provided so far. Our formula for the moveout is exact, at the cost of being implicit. We evaluate an efficient numerical method to compute it accurately. Examples are provided to confirm that dipping angles far beyond those considered by previous approaches are reliable. Second, our moveout description is valid within a range of offsets and CIG positions, meaning that we can handle more than one CIG at once. In other words, instead of describing a moveout curve on a single CIG, we are able to compute a moveout surface across image gathers. From this larger volume of data, numerical experiments determine that more robust estimations are obtained for the velocity correction factors. Finally, the estimation of the best-fit parameters is carried out by a traditional coherence-based analysis, in which a modern derivative-free optimization method is used. We develop a set of numerical tests to validate the description of the residual moveouts, to compare our strategy with previous work from the literature, as well as to demonstrate that just a few image gathers are sufficient to stabilize the estimation of velocity correction factors. As a by-product, we obtained estimations of the reflector as well.