The Application of Reverse Time Migration Surface Offset Gathers in Velocity Updating

Reverse time migration (RTM) is a wave equation migration method that has high imaging accuracy and superior imaging effects in complex underground structures and has been extensively employed in industrial production. Surface offset gathers (SOGs) are crucial for migration velocity updating and muting the stretched waveform in the shallow layers of seismic data. However, RTM SOGs cannot be obtained directly. Usually, SOGs obtained through Kirchhoff migration are used for velocity updating, which fails to form a closed-loop process for RTM migration calculation and velocity updating. Attribute migration is a stable, simple, and efficient method to obtain RTM SOGs. Based on attribute migration, this article applied the constrained least-squares inversion instead of division to get a more accurate RTM SOGs and uses residual curvature analysis (RCA) to update the velocity. RTM SOGs can also be obtained by grouping seismic data according to offset and migrating each group separately. By comparison, SOGs obtained by these two methods had identical characteristics. Furthermore, the residual curvature spectrum after velocity analysis had identical characteristics. Therefore, we believe that the attribute migration method using constrained least-squares inversion can be employed for velocity updating and verify its effectiveness with model data and actual data. In summary, this article presents a stable and direct method to efficiently calculate SOGs for RTM and thus form a closed loop of RTM migration calculation and velocity updating.