Friday, September 3, 2010

Marine Multi-channel Seismic Processing (part-3)

(6) NMO & further multiples removal
When the velocity model is ready to go, we can apply NMO to get the data further ready for stack (ProMAX module: Nromal Moveout Correction). But right here, we may have something more to do. That is to further remove multiples. Remember we have applied deconvolution to suppress when pre-process. But it does not often come up the result as good as we want. So at this point, we have the velocity model in hand, we have chance to remove multiples in further extent after NMO. Typically, we would like to employ inside mute, F-K filter or Radon filter to deal with marine seismic multiples. Read the materials about these two kinds of filters, select the proper paramters for the executive modules, then test and compare to find out the best result. Please be patient again, paramter test will need some time.

(7) Stack
When the data are done in CDP domain after applying NMO and filters for multiple removal, they are ready to stack. There is not much work to do except choosing the proper parameters for stack and wait for it done (ProMAX module: CDP/Enmble Stack). But before stack, remember to employ Bandpass Filter to remove the noise generated by the previous processing step, and maybe employ Automitical Gain Control to enhance the deeper reflectors (TBD). Because the multiple removal filter(s) would suppress the primaries signal at the same time. And then, we have to wait for the flow to execute and complete, which will take a long time when we have like 60000 CDPs to stack.

(8) Time migration
Comparing the stacked seismic section plot and the formal near-trace plot at the very beginning, we can easily find that the stacked one has much stronger and clearer signal on the plot, showing more geological details. However, we could still see some diffrations on the stacked plot, which are due to steep and dip change on topography or layering. So it is time to do the migration to correct them. We use Memory F-K Migration with the smoothed velocity model (relative to the velocity model we built up for stack) to make it happen. On the seismic image after migration, we can tell the difference apparently in the dip and steep areas in the seismic section. At this point, most of our work on marine multi-channel seismic data processing are done.

(9) SEG-Y Output & Print
Pick the one with the best effect after stack and migration, and output as a standard SEG-Y file. Then use GMT (command: pssegy) to make a postscript plot of the seismic section and print it out on a big piece of paper in moderate scale with proper vertical exaggeration. Now let the scienctist to tell the geological story when they are pointing at that big paper.

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