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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.
|
A
problem that has always plagued geologists and interpreting geophysicists
is the fact that seismic data resemble a cross-section of the These
problems, however, are minor compared with the structural distortions that
occur when velocity varies laterally as well as with 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 Click here for sequence of Figures 1 and 2.
Click here for sequence of Figures 4 and 5. Figure
5. Seismic line in South Texas, with prestack Click here for sequence of Figures 4 and 5.
One
of the principal motivators behind development of pre-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 velocity 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
Another
more subtle example of the value of prestack · 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 Other
problem areas where There
are two reasons for performing · The velocity model can be derived directly from the data, usually with more accuracy than from stacking velocity or extrapolated well control. ·
The stack itself is disrupted and
degraded beneath velocity anomalies. Prestack Deriving and refining the velocity model is an
iterative process, requiring numerous preliminary migrations and analysis
cycles. Because of this, |