Subsalt imaging is an important process for exploration of offshore basins. These salt basins present a technical challenge for present day imaging methods. Strong lateral velocity variations, irregular salt bodies, and subsalt structure imaging require quality control during the image processing phase. Methodologies and techniques are available for interpreting geophysicists to quality control prestack common image gathers (CIGs) and the velocity model (VM) in depth. The three dimensional (3D) SEG/EAGE Salt Model (Ober et al.,1996) is an ideal dataset to demonstrate the process because it was designed to contain major complex features common to offshore basins.
The SEG/EAGE C3 Narrow Angle Subset Salt Model is used to demonstrate the methodology employed for quality control, CIGs, and the interval velocities in depth used in imaging the data. The methodology involves residual move-out semblance panels based on RHO/percentage (RHO - Greek typography representing percent error) move-out trajectories. Velocity percentages are generated in the image domain to measure the curvature of reflection events on migrated offset image gathers. The curvature of offset image gathers depends strongly on the velocity used in the migration. If the velocity is correct, the reflection events on offset image gathers are horizontally aligned. If the velocity is incorrect, then the reflection events will show depth residual move-out.
Techniques are available for depth imaging that allows the interpreting geophysicist to respect the geologic concepts by controlling the quality control of the CIGs and the VM during the depth imaging processing phase. This includes salt dome basins where the depth imaging technology is most challenged.
Interpreting geophysicists are searching for solutions that balance the divide between turnkey on-time turnaround depth imaging processing and respect of the geology.