Seismic imaging and velocity analysis for an Alberta Foothills seismic survey

Seismic imaging of complex structures from the western Canadian Foothills c all be achieved by applying the closely coupled processes of velocity analy sis and depth migration. For the purposes of defining these structures in t he Shaw Basing area of western Alberta, we performed a series of tests on b oth synthetic and real data to find optimum imaging procedures for handling large topographic relief, near-surface velocity variations, and the comple x structural geology of steeply dipping formations. To better understand th e seismic processing problems, we constructed a typical foothills geologica l model that included thrust faults and duplex structures, computed the mod el responses, and then compared the performance of different migration algo rithms, including the explicit finite difference (f-x) and Kirchhoff integr al methods. When the correct velocity was used in the migration tests, the f-x method was the most effective in migration from topography. In cases wh ere the velocity model was not assumed known, we determined a macrovelocity model by performing migration/velocity analysis by using smiles and frowns in common image gathers and by using depth-focusing analysis. In applying depth imaging to the seismic survey from the Shaw Basing area, we found tha t imaging problems were caused partly by near-surface velocity problems, wh ich were not anticipated in the modeling study Several comparisons of diffe rent migration approaches for these data indicated that prestack depth migr ation from topography provided the best imaging results when nearsurface ve locity information was incorporated, Through iterative and interpretive mig ration/velocity analysis, we built a macrovelocity model for the final pres tack depth migration.