Prestack migration by equivalent offsets and CSP gathers: An update
John C. Bancroft, Hugh D. Geiger, Shaowu Wang, Darren S. Foltinek
A method of prestack time migration is presented that is simpler, faster, and provides more information than conventional methods. The method is based on 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 in an intermediate step for each output migrated trace, similar to prestack imaging (PSI) gathers presented by Forel and Gardner (1988). Normal moveout (NMO) and stacking of CSP gathers is all that is required to complete the prestack migration process. This new method allows the CSP gathers to be formed at any arbitrary location for velocity analysis, or prestack migrate a 2-D line from a 3-D volume.
The CSP gather is similar to a CMP gather, as both contain offset traces, and both represent a vertical array of scatter points or reflectors. However, the CSP gather is superior with a greater number of traces, and a larger maximum offset. All the input traces within the migration aperture are used to form the CSP gather. Samples in the input traces are assigned an equivalent offset for each CSP location, then copied into the appropriate offset bin of the CSP gather in an efficient manner. The input time samples remain at the same time when copied to the CSP gather. Data in the CSP gathers may be scaled, filtered, or have noise attenuation processes applied.
The prestack nature, high fold, and large offsets aid in providing a better focus of the semblance plot for an improved velocity analysis. Another benefit of the process is its speed, which combines many prestack arithmetic computations. Additional computation time is saved when amplitude scaling and anti-aliasing filtering are performed on the bins of the CSP gathers.