Shot Profile Migration

In order to efficiently migrate the data with shot profile migration, I removed the time samples before the water-bottom arrival and compensated by applying a linear frequency shift to the source wavelet. The propagation through the water layer was done in two depth steps and from there down the depth sampling was 10 m. For the sake of computer time, only two reference velocities were used to propagate the data at each depth step. These reference velocities were computed with Lloyd's algorithm Clapp (2004). The aperture in the inline direction was 9 km (1.2 km in front of the streamer and 0.6 km at the end of the streamer) and the cross-line aperture was 4.8 km (2.4 km in the up-dip direction and 1.4 km in the down-dip direction). Four hundred frequencies were used from 6 to 40 Hz. Figure  shows an inline and a cross-line sections taken from the migrated cube. The depth axis is with respect to an arbitrary reference. The inline section is at crossline 10240 m and the crossline section is taken at inline 12000 m. The migrated data was filtered in depth and a gain proportional to the depth squared was applied for display purposes.

 

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Figure 8 Shot profile migration. Panel (a) is the inline section at crossline 11440 m. Panel (b) is the crossline section at inline 12000 m.

Since it is not easy to identify the multiples in Figure , I windowed the image below the salt body between 8000 and 16000 m in the inline direction as shown in Figure . The crossing events are an indication of the presence of the migrated multiples and their interference with the legitimate, possibly weak subsalt primaries. Again, it is not immediately obvious which of these reflections are primaries and which are multiples without the help of prestack migrated images as a function of subsurface offset or aperture and azimuth angles. Computing subsurface offsets, however, is very expensive and at the time of this report I have computed only inline subsurface offsets and only for one sail line. The results are presented in the next section.

 

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Figure 9 Close up view of the migrated cube to show the interference between the multiples and the weak subsalt primaries. Panel (a) is the inline section at crossline 11440 m. Panel (b) is the crossline section at inline 12000 m.