Introduction

Many of the world's giant oil reservoirs discovered in the 21st century are offshore marine fields, and a significant number of them are below salt. For example, deep drilling in the Gulf of Mexico is exclusively below a large salt horizon that blankets the Gulf of Mexico beneath depths of 5 km or more. Another example is offshore Brazil where large scale imaging, drilling, and extraction of subsalt hydrocarbons are carried out. The main challenges with deep subsalt deposits are that they are difficult to identify with the seismic method, and they are extremely expensive to drill and extract. Thus, improving the accuracy of subsalt imaging with the seismic method is an important goal of many large oil companies.

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 chapter 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 (see Figure 4.1) 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.

The first part of this chapter presents the interferometric theory for extracting diffraction energy in seismic records. This is followed by synthetic and field data examples that show both the benefits and limitations of this method, and finally the last section presents a summary.