From time to depth imaging with ‘Beyond Dix’

Abstract

In many parts of the world, pre-stack time migration (PSTM) still represents the majority of seismic imaging activity in the industry. The reason for this is the simple efficiency and robustness of time imaging and its ability to focus seismic reflectors for many geological settings. Limitations of PSTM appear in the case of strong lateral velocity variations, where the more rigorous imaging and more accurate velocity models offered by Pre-Stack Depth Migration (PSDM) are required. In areas of moderate complexity, where PSTM begins to struggle we introduce a new, accurate method, ‘Beyond Dix’, to help bridge the gap between PSTM and PSDM. Beyond Dix provides an accurate fast-track PSDM from PSTM outputs. It takes full advantage of the efficiency, good focusing, and high signal-to-noise ratio available from time imaging to jump-start the PSDM process. The name comes from the fact that we are going beyond the limitations of the 1D Dix inversion commonly used to derive depth interval velocities from the PSTM velocities. It uses the full kinematic information available in the time migrated domain to directly build an accurate depth velocity model and bypass the 1D Dix inversion altogether. Not only does this approach extract the maximum value from time imaging, it also adds great flexibility to imaging projects by allowing a seamless and fast transition from PSTM to PSDM and thus avoiding the need to choose at an early stage between a time or depth approach. It is clear that in addition to providing a fast-track PSDM, Beyond Dix has a range of possible applications, including building more accurate initial models for full depth imaging projects. In this paper we explain how the detailed information freely available within the time migrated domain can be used directly to build an accurate depth velocity model. We also illustrate the application of Beyond Dix with two examples from different geological settings. In particular, we demonstrate that the resulting PSDM images converted back to time exhibit significantly improved focusing and structural delineation compared to equivalent PSTM images. Depth velocity model building The estimation of an accurate 3D depth velocity model certainly remains one of the greatest challenges in seismic imaging. Depth velocity model building typically involves: N Initial PSDM N Residual moveout (RMO) picking on common image point (CIP) gathers N Structural dip picking in the migrated domain N Update of the depth velocity model by ray-based tomographic inversion Starting from an initial velocity model, an initial PSDM is performed to build CIP gathers for RMO picking. The objective of the velocity model building/update is to minimize the observed RMO (related to the velocity error) in the migrated domain, flattening the events on the CIP gathers. From the knowledge of the RMO and the structural dip one can implement a linearized solution to tomographically update the velocity model with the objective of reducing RMO in the next PSDM run (Al-Yahya, 1989; Liu and Bleistein, 1995). The tomography is usually repeated through several iterations to solve for complex non-linear effects. To avoid the extra delay involved in running a full PSDM and re-picking RMO and dip after each tomographic update iteration, Guillaume et al. (2001) proposed an efficient kinematic approach. This same kinematic approach is used by Beyond Dix.