Because multiples are so strong in this dataset, multiple migration might unravel some useful information. Again I did not separate multiples from primaries. I muted the up-going wavefield, as with the Gulf of Mexico data, to remove all the data above the first surface-related multiple.
The velocity model for the North Sea dataset is displayed in Figure . I show one example of up- and down-going wavefields in Figure . Notice the strong reverberations of the water column plus other surface-related/internal multiples.
I migrated the North Sea data without large offsets and for a small portion of the survey only. Figure shows the imaging results when primaries (Figure a) and multiples (Figure b) are migrated. It appears that the multiple image is less focused than the primary image. In addition the water-bottom is not at the correct location for both images. What happened ?
The answer is simple: cable feathering. () indicates that cable feathering was quite strong for the subset I migrated. Therefore the shot S, R1 and Rn in Figure are not in the same plane anymore introducing positioning errors in the migration.