Datuming

The regular cube was then datumed to just above the water-bottom to avoid spending migration time on downward continuation through the water layer. Figure 13 shows a comparison of one CMP 3D gather before (left) and after datuming (right). Notice that datuming not only saves propagation time but also allows a significant reduction of the data to be migrated because the offsets decrease as the recording surface is ``moved'' closer to the reflectors, making the positions of sources and receivers closer. Notice also that the holes in both offsets and CMPs have been healed by the propagation.

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Figure 13. Before-after datuming comparison. Panels (a) and (c) correspond to the inline CMP section at CMP-Y=-12.5 m, 150 m inline offset and -25 m crossline offset, before and after datuming, respectively. Panels (b) and (d) correspond to the inline offset gather at CMP-X=8550 m, CMP-Y=-12.5 m and -25 crossline offset, before and after datuming, respectively. Notice that the datuming, besides shifting the data upwards, also healed the holes in the acquisition and reduced the offset coverage.