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Module 3: Pre-Stack and Post-Stack Migration


MODULE 3: WORKFLOW


MODULE 3: RESULTS & DISCUSSION

Generally, migration consists of pre-stack migration and post-stack migration. Post-stack migration involves the process of correcting normal moveout and dip-moveout on a number of traces, then summing the traces together into a single trace before performing the migration. By using this method, a data volume can be migrated in short time interval and inexpensively. This migration-after-stack procedure has proven over the years to be very successful, especially for imaging the areas with moderate structural complexity. However, for the areas with significant lateral velocity variation such as salt dome structure, we need to do migration before stack. This is due to enormous velocity variations make the post-stack preservation of subsalt reflection events almost impossible. Pre-stack migration, in time or depth, also represents the current commonly applied state of the art. However, this kind of migration is time consuming compared to post-stack.

In this laboratory assignment (Module 3), the students are required to perform both migration on simple dipping model and salt dome model which has been constructed in Module 2. The students are also required to analyse and make the comparison between the output of both processes.




Figure 9: Simple dipping model.


Figure 10: Before (top) and after (below) pre-stack migration.


Figure 11: Before (top) and after (below) post-stack migration.

Based on the image produced after each migration, we can see a better positioning of seismic image compared to the true subsurface geological structure (black box). This is due to the redistribution of the energy to produce clear and true position of subsurface image. Pre-stack migration are usually required to image significant structure complexity while post-stack migration is used for imaging simple to moderate structure complexity. Because of the dipping model as shown in Figure 9 above is a simple structure, thus there is only a small difference between the image produced after each migration.


Figure 12: Salt dome model.


Figure 13: Seismic image of salt dome model before migration.


Figure 14: Salt dome velocity model.


Figure 15: Salt dome model before pre-stack migration.


Figure 16: Salt dome model after pre-stack migration.


Figure 17: Salt dome model before post-stack migration.


Figure 18: Salt dome model after post-stack migration.

Before the migration, we can see that the dipping events are not properly position in the seismic image. The diffraction events are also not collapsed causing overlapping events at the edge of the interface as shown in the black box in Figure 13. Based on the image produced after the pres-stack migration, we can see no more overlapping events in the seismic image because the diffraction events were collapsed during the migration process. The position of the interfaces was also reconstructed and reposition to the true position. Bright colour of amplitude shows that there is high impedance contrast at the interface and this can be used to locate the layer boundary as well as hydrocarbon-water contact.

Based on the post-stack migration results, the stacking process conducted before the migration maybe results in velocity stacking conflicts which produced poor quality image as shown in Figure 18. Besides that, salt dome model is classified as complex geological structure, thus it is not suitable to perform post-stack migration because this type of migration cannot preserve the seismic events of complex structures. Furthermore, we still can observe the diffraction events at the edge of interface which supposed to collapse after the migration takes place.

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