Datuming from a planar datum to an irregular datum, downward

Datuming from a planar datum downward to an irregular datum has application in marine seismic survey environments, because it removes the effects the irregular seafloor topography has on subsurface reflections. Figure 7 is a depth reflectivity image, containing an irregular seafloor reflector and a deeper flat reflector. The spatial wavelet used is a Ricker wavelet with a fundamental wavenumber of 20 Hz(cycles/km). The velocity in the water layer is 1.5 (km/s), and the velocity in the sediment is 2.5 (km/s). I run the reverse-time migration (McMechan, 1983) in the modeling direction to generate the zero-offset section, which is shown in Figure 8. The planar reflector bears a distinct impression of the water bottom. By the use of a rectangular grid strip that contains the water bottom, the sea surface wavefields are downward datumed to the water bottom. The results are shown in Figure 9. The two sides and the bottom of the rectangular grid strip are designed as absorbing boundaries. Now the water bottom reflection has been flattened. Next, the water bottom wavefields are datumed upward to the sea surface using the constant sediment velocity; the results are shown in Figure 10. The reflection of the subsurface planar reflector is now flat. The whole section has a constant velocity that is equal to the sediment velocity. This process is called velocity layer replacement (Berryhill, 1979). The sea surface section is then migrated with a reverse-time depth migration scheme (McMechan, 1983). The results are shown in Figure 11. This migration image correctly depicts the original input reflectivity model.

 

depth.H Figure 7 A marine model depth reflectivity section.

 

depth.H0 Figure 8 The recorded zero-offset section at the sea surface. The subsurface planar reflector bears a distinct impression of the water bottom.

 

depth.H01 Figure 9 The extrapolated zero-offset section at the surface of water bottom. The reflection of water bottom has been flattened.

 

depth.H010 Figure 10 The new zero-offset section at the sea surface. The water layer velocity has been replaced by constant sediment velocity.

 

depth.H010mig Figure 11 The constant velocity reverse-time migration depth reflectivity section.