Chapter 4: Super-virtual Interferometric Diffractions as Guide Stars

A significant problem with the seismic imaging method is that subsalt reflections are severely defocused by the strong velocity contrasts and the irregular geometries of salt-sediment interfaces. Upgoing reflection energy is present in the data, but cannot be easily detected in the shot records as coherent arrivals with hyperbolic-like moveout trajectories. This means that velocity estimation methods such as traveltime tomography cannot be used and, others, such as migration velocity analysis or full-wave inversion will fail unless an accurate starting velocity model is used. Is there another means for estimating subsalt velocities when the other methods fail?

This paper proposes interferometric extraction of subsalt diffractions, with the possibility that they can also be used as migration operators or for velocity analysis. The key idea is that, similar to surface waves or refractions, 2D subsalt diffractions are associated with stationary source points all along the source line. Thus, application of interferometry can enhance the signal-to-noise ratio of this diffraction energy by $\sqrt {N}$ , where $N$ is the number of source points. This means that undetectable diffractions in the shot records can be enhanced, which can then be used to guide velocity analysis and focusing of subsalt reflections. I refer to such diffractors as guide stars because they, similar to VSP data, can be used as Green's functions to build natural migration operators (Brandsberg-Dahl et al., 2007; Schuster, 2002), or estimate migration velocity (Landa et al., 1987; Berkhout et al., 2001). Similar to guide stars used by astronomers for correcting the optical distortion of the atmosphere, diffraction based migration operators can be used to guide the proper focusing of subsalt reflection energy to their points of origin beneath the salt. Both synthetic and field data records are used to demonstrate the benefits and limitations of this method.