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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 data resemble a cross-section of the earth, but are displayed in time rather than depth. To tie well control to seismic, well logs must be scaled to time, using check shot surveys or velocity functions derived from other means. The vertical exaggeration changes with depth (because velocity usually increases with depth), thus distorting the perspective and changing the apparent dip of fault planes, etc. These
problems, however, are minor compared with the structural distortions that
occur when velocity varies laterally as well as with depth. A solution to
these problems exists in the development of pre-stack depth
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 velocity. Figure 3a shows a
fault shadow model; Figure 3b is an example of poststack
Click here for sequence of Figures 4 and 5. Figure
5. Seismic line in South Texas, with prestack depth Click here for sequence of Figures 4 and 5.
One
of the principal motivators behind development of pre-stack depth
Time
Prestack
depth Another
more subtle example of the value of prestack depth · 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 Other
problem areas where depth There
are two reasons for performing depth · 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 depth Deriving and refining the velocity model is an
iterative process, requiring numerous preliminary migrations and analysis
cycles. Because of this, depth |