Datuming planar-to-planar datum through an irregular datum, downward

This section describes datuming from a planar datum downward to another planar datum through an irregular velocity contrast interface. By use of a rectangular strip, the sea surface wavefields in Figure 8 are downward datumed to a planar datum below the water bottom. The two sides and the bottom are designed as absorbing boundaries. By using the sea surface wavefields as boundary conditions, and running the wave equation backward in time, I obtain the subsurface reflection wavefields at the planar datum z=0.1, which is shown in Figure 13, and the reflectivity image between the sea surface and this new datum (Shih and Levander 1988; Harris and McMechan, 1992), which is shown in Figure 12. A simple migration scheme can be used to image accurately the zero-offset section from this new datum.

This example demonstrates that we can use the sophisticated though expensive reverse-time migration to migrate across the complex datum of the velocity model. Beyond that, we can use a less sophisticated but fast migration scheme, such as finite-difference paraxial wave equation migration.

 

Strip.H Figure 12 The depth reflectivity section between the sea surface and the lower planar datum. It contains the sea floor reflector.

 

depth.H02 Figure 13 The extrapolated wavefields at the lower planar datum z=0.1 below the water bottom. These wavefields are free from the effects of the water bottom topography.