A 2-PASS APPROXIMATION TO 3-D PRESTACK MIGRATION

A two-pass approximation to 3-D Kirchhoff migration simplifies the mig ration procedure by reducing it to a succession of 2-D operations. Thi s approach has proven very successful in the zero-offset case. A two-p ass approximation to 3-D migration is described here for the prestack case. Compared to the one-pass approach, the scheme presented here pro vides significant reduction in computation time and a relatively simpl e data manipulation scheme, The two-pass method was designed using vel ocity independent prestack time migration (DMO-PSI) applied in the cro ssline direction, followed by conventional prestack depth migration in the inline direction. Velocity analysis, an important part of prestac k migration, is also included in the two-pass scheme. It is carried ou t as a 2-D procedure after 3-D effects are removed from the data volum e. The procedure presented here is a practical full volume 3-D prestac k migration. One of its main benefits is a realistic and efficient ite rative velocity analysis procedure in three dimensions, The algorithm was de signed in the frequency domain and the computational scheme was optimized by processing individual frequency slices independently. Ir regular trace distribution, a feature that characterizes most 3-D seis mic surveys, is implicitly accounted for within the two-pass algorithm . A numerical example tests the performance of the two-pass 3-D presta ck migration program in the presence of a vertical velocity gradient. A 3-D land survey from a fold and thrust belt region was used to demon strate the algorithm in a complex geological setting. The results were compared with images from other 2-D and 3-D migration schemes and sho w improved resolution and higher signal content.