Prestack migration by equivalent offsets and CSP gathers

John C. Bancroft, Hugh D. Geiger, Darren S. Foltinek, Shaowu Wang

A method of prestack time migration is presented that is simpler, faster, and more flexible than the conventional methods. The method is based on the principles of prestack Kirchhoff time migration and can be applied to both 2-D and 3-D data. Common Scatter Point (CSP) gathers are created for each output migrated trace, replacing the common midpoint (CMP) gathers of conventional processing. Samples from each input trace are assigned an equivalent offset for each output scatter point position, then copied into the appropriate offset bin of the CSP gather. The time sample position of the input data remains the same when copied to the CSP gather. Data in the CSP gathers may be scaled, filtered, or have noise attenuation process applied. Normal moveout (NMO) and stacking is all that is required to complete the prestack migration process.

The CSP gather is defined at the same position as a CMP gather relative to the output migrated trace, but contains all traces within the migration aperture. The CSP gather also contains larger offsets than the CMP gather, and each bin in the CSP gather may have large fold. These attributes of the CSP gather permit more accurate velocity analysis than conventional methods using super gathers from CMP locations.