--> Applications of Pre-Stack Migration; Richard Postma; Search and Discovery Article #40029 (2001)
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Applications of Pre-Stack Previous HitDepthNext Hit Migration*

By

 Richard Postma1 

 

Search and Discovery Article #40029 (2001)

1Interactive Earth Sciences, Inc., Denver, CO.  

*Adapted for online presentation from article by same author, entitled "Pre-Stack Can Avoid Distortions," in Geophysical Corner, AAPG Explorer, September, 1997. Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.

 

uStatement of problem

uFigure captions

uApplications

uSummary

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uStatement of problem

uFigure captions

uApplications

uSummary

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uStatement of problem

uFigure captions

uApplications

uSummary

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Statement of Problem

 A problem that has always plagued geologists and interpreting geophysicists is the fact that seismic data resemble a cross-section of the earth, but are displayed in time rather than Previous HitdepthNext Hit. To tie well control to seismic, well logs must be scaled to time, Previous HitusingNext Hit check shot surveys or Previous HitvelocityNext Hit Previous HitfunctionsNext Hit derived from other means. The vertical exaggeration changes with Previous HitdepthNext Hit (because Previous HitvelocityNext Hit usually increases with Previous HitdepthNext Hit), thus distorting the perspective and changing the apparent Previous HitdipNext Hit of fault planes, etc. 

These problems, however, are minor compared with the structural distortions that occur when Previous HitvelocityNext Hit varies laterally as well as with Previous HitdepthNext Hit. A solution to these problems exists in the development of pre-stack Previous HitdepthNext Hit migration. 

Figure Captions

Figure 1. A 3-D time-migrated line across two salt domes. Note severe distortion of base salt between S.P. 500 and 700. A well was drilled near S.P. 1520.

 Click here for sequence of Figures 1 and 2.

 

Figure 2. A 2-D prestack migrated line of the same area as in  Figure 1, providing improved imaging beneath the left salt dome, movement of fault image near the well, beneath the right salt dome. The well is now down thrown to the fault.

 Click here for sequence of Figures 1 and 2.

 

Figure 3. Faulting that displaces beds with anomalous Previous HitvelocityNext Hit. Figure 3a shows a fault shadow model; Figure 3b is an example of poststack migration of synthetic data.

 

 

 

Figure 4. Seismic line in South Texas, with poststack time migration, for comparison with same line with prestack Previous HitdepthNext Hit migration (Figure 5).

 Click here for sequence of Figures 4 and 5.

 

Figure 5. Seismic line in South Texas, with prestack Previous HitdepthNext Hit migration, for comparison with same line with poststack time migration (Figure. 4).

 Click here for sequence of Figures 4 and 5.

 

Applications

One of the principal motivators behind development of pre-stack Previous HitdepthNext Hit migration was the desire to image seismic reflectors beneath salt structures. The abrupt Previous HitvelocityNext Hit contrasts between the salt and adjacent sediment – coupled with the sometimes radical structural features associated with salt tectonics – produced severe distortions in the seismic travel times. The result is frequently a very poor stack and time pull-ups in the events that do stack. 

Time migration incorrectly migrates the distorted events because of the rapidly varying lateral velocities. An example of this from the Southern North Sea gas basin is shown in figures 1 and 2. Figure 1 is the 3-D time migrated line from a survey across two salt structures. The objective is the Rotliegendes sand beneath the Zechstein salt. The greatest Previous HitvelocityNext Hit contrast is actually between the Cretaceous Chalk that has been forced upward by the salt movement, and the overlying Tertiary clastics. It is this Tertiary-Cretaceous boundary and the structure on it that produce the greatest distortion. Severe distortions can be seen in the Base Salt/Top Rotliegendes reflector beneath each of the structures. The event actually criss-crosses in a reverse “bow-tie” beneath the structure on the left. An apparent fault is seen beneath the structure on the right.

Prestack Previous HitdepthNext Hit migration, shown in Figure 2, reveals a very different picture. The “bow-tie” under the left structure has been unraveled, revealing a much clearer image that has moved somewhat. Note that the well that was drilled with the intention of reaching the upthrown side of the fault (Figure 1), in fact, entered the downthrown side, as seen in the Previous HitdepthNext Hit image (Figure 2), and reached the base of salt at exactly the Previous HitdepthNext Hit indicated in the Previous HitdepthNext Hit section (about 300 meters low to prognosis, as interpreted from the time section). 

Another more subtle example of the value of prestack Previous HitdepthNext Hit migration is the “fault shadow” problem. Figure 3 illustrates, with model data, what can happen if faulting displaces beds with anomalous Previous HitvelocityNext Hit, thus causing abrupt lateral changes. The raypaths of various offsets passing through the faulted zone are disrupted such that: 

·        They stack poorly. 

·        The stacked traces have severe time distortion. 

Such distortion can easily be interpreted as structure and/or secondary faulting. Figures 4 and 5 show a comparison of a seismic line in South Texas, with time migration and prestack Previous HitdepthNext Hit migration. The effect is most clearly seen in the two circled areas, where the time section indicates folded beds that are much more planar in the Previous HitdepthNext Hit section. False structures could very easily be interpreted on the time data. 

Other problem areas where Previous HitdepthNext Hit migration can help include overthrust faults, channel fills, reefs, and karsted or eroded carbonate in the section above the zone of interest. In short, any time the objective lies beneath strata that have been disrupted by faulting, diapirism, etc. or show significant structural Previous HitdipNext Hit or where there are significant lateral variations in the overburden Previous HitvelocityNext Hit, Previous HitdepthNext Hit migration can be useful. 

Summary

There are two reasons for performing Previous HitdepthNext Hit migration prestack, rather than after stack: 

·        The Previous HitvelocityNext Hit model can be derived directly from the data, usually with more accuracy than from stacking Previous HitvelocityNext Hit or extrapolated well control. 

·        The stack itself is disrupted and degraded beneath Previous HitvelocityNext Hit anomalies. Prestack Previous HitdepthNext Hit migration, with a correct model, can improve the stacked image. 

Deriving and refining the Previous HitvelocityNext Hit model is an iterative process, requiring numerous preliminary migrations and analysis cycles. Because of this, Previous HitdepthTop migration is expensive compared to other data processing procedures. However, it is cheap compared to the cost of drilling dry holes (see Figures 1 and 2)!

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