Subsalt Velocity Analysis By Target-oriented Wavefield Tomography: A 3-D Field-data Example

We apply target-oriented wavefield tomography to a 3-D field data set acquired from the Gulf of Mexico. Instead of using the original surface-recorded data set, we use a new data set synthesized specifically for velocity analysis to update subsalt velocities. The new data set is generated based on an initial unfocused target image and by a novel application of 3-D generalized Born wavefield modeling, which correctly preserves velocity kinematics by modeling non-zero subsurface-offsetdomain images. We show that the target-oriented inversion strategy drastically reduces the data size and the computation domain for 3-D wavefield tomography, greatly improving its efficiency and flexibility. We apply differential semblance optimization (DSO) using the synthesized new data set to optimize subsalt velocities. The updated velocity model significantly improves the continuity of subsalt reflectors and yields flattened angle-domain common-image gathers.