| Popis: |
Tomography has been widely employed for velocity model building. The typical work flow starts with the migration of an initial velocity model. This is followed by picking residual moveouts and then updating the velocity through tomography. The migration process provides common image gathers and a stack. At the tomography stage, a ray tracer is used to trace specular rays from the image points to the surface to set up the system of linear equations for the tomographic inversion by linking valid ray pairs to their corresponding residuals. For narrow azimuth (NAZ) surveys, searching for valid ray pairs is usually limited to a narrow azimuth band. Since neither the gathers nor the stack contain acquisition geometry information, the selected specular ray pairs may not reflect the true ray paths, resulting in inaccurate rays being used in the inversion process. In addition to the problem of “which rays to choose”, we also have the problem of “how many rays to choose”. These problems are even more difficult to handle with acquisition configurations other than NAZ, such as the wide azimuth towed-streamer (WATS) surveys, multi-azimuth (MAZ) surveys, and ocean bottom cable (OBC) surveys. To overcome these problems, we propose a tomography method that incorporates the acquisition geometry information and uses vector offsets to account for both offsets and azimuths. To address the ill-posed nature of the system of equations, we developed an anisotropic Laplacian regularization operator that allows different smoothing along different directions. We validate the method with tests on both synthetic and field data with a WATS geometry. |
