Robust 3D scalar imaging condition for elastic RTM

We propose an elastic scalar imaging condition that corrects for the polarity change; however, this imaging condition requires additional prior information which may not be known accurately, or may suffer from interference of waves propagating from opposite sides of a reflector. Using incorrect Pand S-mode velocities causes reflectors in migrated PP, PS, and SP images to be shifted from their true positions. We show that it is more reliable to estimate reflector normals from PS and SP images, computed using conventional imaging methods, instead of from PP images. We also demonstrate that the PS and SP images computed using our elastic imaging condition for waves propagating from opposite sides of a reflector have the same polarity, and therefore they can be stacked over experiments without canceling each other. This analysis, illustrated with numeric experiments, demonstrates the intrinsic robustness of our elastic scalar imaging condition.