Velocity Determination for Time Lapse Surveys after Prestack Time Migration - Practical Aspects

After application of prestack time migration to time lapse surveys the interpretation of small moveout differences is considered which are induced at the reservoir base as production of hydrocarbons proceeds: residual moveout analysis have to estimate the moveout perturbations to be expected and inversion schemes have to transform these perturbations into velocity changes in the reservoir. Most important, it has to be ascertained that small errors caused by the imperfect knowledge of the overburden do not obscure the results of the processing. For the application of prestack time migration it is demonstrated that the reservoir velocity distribution of the second survey can be estimated from residual moveout analyses for the two surveys. It is assumed that the reservoir base and the reservoir velocity of the first survey are known. The solution is derived for small offsets and migration apertures without any explicit reference to the overburden. Additional corrections can be computed from a macro model if necessary. Applications to a typical depth model have furnished acceptable results for a particular application of prestack time migration.