Maintaining Frequency Content for Long-Offset Seismic Data

A serious drawback with long-offset seismic acquisition is the reduction of frequency content caused by the processing steps of migration and normal moveout. This frequency reduction leads to poor and/or unreliable structural images. With source-receiver offsets greater than three times the target depth, a frequency loss of 100 percent is not uncommon for reflection events after NMO corrections. A processing and interpretation approach has been developed that images all reflections but avoids normal moveout (NMO) stretch. That is, imaged reflections with long offsets have the same frequency content as traces with small offsets. We call this approach Seismic Wide-Angle Processing (SWAP). There are two main steps that are different from conventional processing. The first is a pre-stack Kirchhoff migration in the common-offset domain that maintains the offset separation between source and receiver during migration. That is, the process of NMO is removed from migration. Obliquity and aperture factors different from conventional Kirchhoff migration are introduced to maintain true amplitude. The second step is an NMO correction that is centered on the target horizon. Static shifts based on ray-trace traveltimes to the target horizon provide the NMO corrections; a process often referred to as block NMO. Even though anisotropy is applied in the processing stages, a final adjustment of traveltimes is required to handle lateral velocity changes. Only the target horizon is truly flat in the CMP gathers after applying the NMO static shifts. Depending upon the velocity gradient around the target horizon, interpretation is limited to a time window of approximately 50-200 ms around the target horizon. Initial tests indicate that both structural and amplitude interpretations are improved using data that have offsets greater than twice the depth of the target horizon.