A joint SEG/EAGE committee was formed in late 1992 specifically for the purpose of creating two 3-D models, which can be used to generate 3-D marine acquisition type datasets. Input for the design of the salt dome model came from 27 geoscientists (experienced in salt tectonics, seismic modeling, and seismic imaging) representing 23 organizations (oil companies, geophysical contractors, and academia). They agreed to base the model on a typical US Gulf Coast salt structure. Special care was taken to ensure that the model was geologically feasible and would be an adequate testing mechanism for seismic imaging algorithms (particularly subsalt).
I will use a program called VIS5D (well known in meteorology circles) to display wavefronts, traveltime sections, and velocity image maps in 3-D. This program is highly convenient for such tasks, and includes impressive slice and dice capabilities. One limitation of this program is its inability to handle large data sets efficiently. On a Linux machine, the grid size should not exceed 150 grid points in each direction.
Figure 3 shows the salt structure, displayed from the top using VIS5D, embedded onto two slices of the velocity field outside the salt body. The P-wave velocity in the salt is about 4482 m/s . This velocity is much higher than the P-wave velocity in the surrounding sediments, resulting in complicated discontinuities in the P-wavefront (when head-waves are excluded). The high-velocity salt body will also result in head-waves, as we will see later, in the Eikonal solution in areas where such head-waves are the fastest.
Figure 4 shows the salt structure from the bottom with the same two slices displayed in Figure 3 shown here as well. From this view, we can appreciate the complexity of this high velocity salt structure. Proper imaging of the bottom of the salt reflection requires accurate calculation of the traveltimes through the salt body; it also requires using the proper traveltime arrival (typically the most energetic arrival) in areas of multiple arrivals. Unfortunately, finite difference solutions of the eikonal equation provide us only with the fastest arrivals, and not necessarily the most energetic ones.