A North Sea example

The North Sea dataset illustrates how challenging basalt imaging can be. The data are infested with internal and surface-related multiples. In addition, the internal layering of the basalt layer caused by repetitive basalt flows interlaced with sedimentary deposits make the identification of the bottom of the basalt very difficult. () attempted P-wave imaging with long-offset streamer data. () tried imaging locally-converted shear waves.

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.

 

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Figure 15 Velocity field for the North Sea 2-D line. The top of the basalt corresponds to the top of the white layer. The bottom of the basalt corresponds to the bottom of the next layer, around 2500 meters.

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.

 

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Figure 16 Left: The source function with primaries and multiples. Right: Same as the source with a mute.

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 R_n in Figure are not in the same plane anymore introducing positioning errors in the migration.

 

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Figure 17 (a) Migration of the primaries. (b) Migration of the multiples. The two images are different because of cable feathering.