Sub-salt Velocity Model Building with RTM Angle Gathers from Ocean-bottom Seismic Data

A sub-salt velocity model updating study has been carried out with an ocean bottom seismic data set. Here, residual moveout picked on RTM reflection angle/azimuth gathers is attributed to velocity errors of the sub-salt sediments only, such that tomographic ray-tracing through the complex salt bodies can be avoided. The approach requires that the model above base salt is already of good quality and that residual moveout can indeed be picked on the gathers below the salt, which has been questioned in the past in view of the small illumination aperture. After a single iteration of residual moveout picking and ray-based tomography obvious flattening of the angle gathers and improved image focusing below the salt is observed. The key elements of success in the application of sub-salt tomography on this data set were the ability to efficiently compute RTM angle/azimuth gathers and the full azimuth properties and long recorded offsets of the data.