Stacking and migration

The complex structure apparent in this data set calls for prestack depth migration. This requires an accurate interval velocity model. This type of velocity model is usually obtained by integrating well information, geological information, interpreter input, and seismic information into the model building process. Subsequent migration velocity determination is then usually an iterative process involving techniques such as migration velocity analysis and tomography.

An early step in bringing seismic information into the model building loop is to perform conventional velocity analysis and time migration. Wave-equation datuming is applied to data early in the processing flow, so in this section I will demonstrate its effectiveness in improving these early stages of imaging. Wave-equation datuming can not only transform data to a flat processing datum, it can also regularize it onto an evenly sampled computational grid. This is useful for iterative migration velocity estimation because the fastest algorithms are optimized for regularly-sampled data.

Since this data is only 30 fold, conventional CMP velocity analysis is difficult because it is hard to pick the semblance peaks and to maintain geological consistency. As with many low fold land data sets, a better strategy is to generate constant velocity stacks and to pick velocities where laterally continuous and geologically sensible reflection events stack coherently.

When this type of lateral continuity criteria is used to determine velocity using geological constraints, it is advantageous to stack with the data referenced to a flat processing datum. In this way, artificial structure is not introduced by the use of a floating datum. This process can be done using elevation statics, or wave-equation datuming. In Figures  through  I will illustrate the superiority of wave-equation datuming for this task.

Figures  and  are the result of stacking the data after elevation statics and after wave-equation datuming using an approximate stacking velocity. The reflection event at about 2.5 s is the most prominent feature in the data. The stack displays better lateral continuity after wave-equation datuming (Figure ) than after elevation statics (Figure ). This is particularly evident between CMP 1000 and 1200 along the 2.5 s reflector. The dipping reflectors above 1 s and to the right of CMP 1300 are much better defined after wave-equation datuming than after static shift. Two other prominent features that look better after wave-equation datuming are the flat reflection event running across the section at about 2 s, and the dipping event at about 1.5 s, running from CMP 200 to 500. The diffraction events in the middle of the stacked sections are a little harder to evaluate, but it is evident that there are more diffractions at early time in the wave-equation datumed stack.

 

stat1elev3300
Figure 14 Stack after static shift. NMO and stack after static shift.

 

st1wed
Figure 15 Stack after wave-equation datuming. NMO and stack after wave-equation datuming. Movie.

After migration, the superiority of wave-equation datuming before stack over static shift becomes even more evident. The result of migration after static shift is displayed in Figure  and the result of migration after wave-equation datuming is displayed in Figure .

As before, better event continuity is preserved in Figure  after wave-equation datuming. The 2 s and 2.5 s reflectors running along the length of the section are more continuous, and the dipping events in the upper right corner of the image are better imaged. The dipping event extending from CMP 200 to 500 is more continuous after datuming. In Figure , this event can be followed to CMP 1000 at 1.8 s where it pinches out. This interpretation could not be confidently made using the image in Figure .

The migration after static shift does not image the complex structures in the middle of the section as well as the migration after wave-equation datuming. The diffraction events in Figure  have collapsed nicely to image the near-surface structure. One of the most prominent structural features that is imaged in Figure  is the dipping spoon-like event extending from coordinates (750,1.2) to (950, 1.4). The whole structure in this region and above is fairly complex and is much better imaged in Figure  than in Figure 

Figure  is a good starting image that is much easier to interpret than Figure . Geological boundaries could be readily defined and used to build a preliminary interval velocity model which could then be further refined by prestack depth migration.

Better imaging of this data could be achieved by prestack migration or even by DMO before stack. However, the point here is to compare the effects of static shift and wave-equation datuming. The important point is that other than the datum correction step, both images in Figure  and  went through the same processing flow.

 

pmigstat
Figure 16 Post-stack time migration after static shift, NMO, and stack.

 

pmigwed
Figure 17 Post-stack time migration after wave-equation datuming, NMO, and stack. Movie.