Velocity Analysis by Residual Moveout after Migration from VSP Data

摘要

Assessing residual moveout after migration is a useful way to undertake velocity analysis in surface seismic data. In this paper, we apply this concept to walk-away vertical seismic profile (VSP) data. Because of the non-symmetric source and receiver geometry, it is hard to detect the depth residual moveout in conventional common image gathers (CIGs) for VSP data. In this case, we changed the horizontal axis from horizontal offsets from the common image point (CIP) to be receiver depths. We derive a residual moveout function for migrated common receiver gathers assuming a constant velocity model having a single horizontal reflector. We then extend this tool to a layered medium using a layer stripping, V_(rms) approach, which allows us to obtain interval velocities of the model layers. It has better results than the classic depth residual moveout. This is further applied to more complex, laterally varying velocity models with good results. This is valid since with VSP data we are generally imaging within a small (compared to surface seismic) distance away from the borehole and our analysis method allows us to estimate velocity perturbations away from the trial migration velocity model.