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Applications of Pre-Stack Depth 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.
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A
problem that has always plagued geologists and interpreting geophysicists
is the fact that seismic These
problems, however, are minor compared with the structural distortions that
occur when 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
Click here for sequence of Figures 4 and 5. Figure 5. Seismic line in South Texas, with prestack depth migration, for comparison with same line with poststack time migration (Figure. 4). Click here for sequence of Figures 4 and 5.
One
of the principal motivators behind development of pre-stack depth
migration was the desire to image seismic reflectors beneath salt
structures. The abrupt 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 Prestack depth 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 depth image (Figure 2), and reached the base of salt at exactly the depth indicated in the depth section (about 300 meters low to prognosis, as interpreted from the time section). Another
more subtle example of the value of prestack depth migration is the
“fault shadow” problem. Figure 3 illustrates, with model · 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 depth 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 depth section.
False structures could very easily be interpreted on the time Other
problem areas where depth 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 dip or where there are significant lateral variations in the
overburden There are two reasons for performing depth migration prestack, rather than after stack: ·
The ·
The stack itself is disrupted and
degraded beneath Deriving and refining the |