Extended pseudo-screen migration with multiple reference velocities Page: 5 of 6
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Extended pseudo-screen migration
reduce alias during migration, a Butterworth filter is
applied in the wavenumber domain and a Hann for
raised cosine) taper is used near the boundaries ex-
cept the upper and lower boundaries of a model.
For migration. the exponential terms and (i 2) in
all above equations must be changed to their complex
conjugates.
Examples
Figure 2(a) is a 2D slice of the SEG/EAEG 3D salt
model provided by Amoco (1995) and Figure 2(b) is
its ideal reflectivity. For migration. both grid spacings
along horizontal and vertical directions are 12.192m.
A finite-difference scheme was used to generate the
zero-offset data set (Figure 3) for the 2D salt model.
The dominant frequency of the Ricker's time history
is 20Hz. The frequency range used in all the follow-
ing migrations are 5-60Hz. Figure 4 shows migration
images by the original and extended split-step Fourier
migrations. For the original split-step Fourier nigra-
tion. the average velocity at each depth level was used
as the only reference velocity for that level. The im-
age with multiple reference velocities in Figure 4(b) is
much better than Figure 4(a). particularly in the re-
gions A and B where the structures are complex and
the lateral velocity variations are large. The pseudo-
screen migration image is shown in Figure 5(a) and
we can see that there are a lot of artifacts due to
the difficulty in the calculations of scattered fields for
large velocity contrasts between the salt body and the
surrounding media. Figure 5(b) is the migration im-
age from the extended pseudo-screen migration with
multiple reference velocities, which clearly images the
lower part of the salt body interface. the regions A and
B I rf. Figure 2(b)), and provides much better images
of the interfaces below the salt body than the original
pseudo-screen migration. The quality of the image in
Figure 5(b) is similar to Figure 4(b) in general. The
differences are the details of the images.
For comparison. the corresponding images cut from
3D migration images by the Kirchhoff migration f Fei
et al. 1996) and FX-migration (Amoco. 1996) are
given in Figure 6(a) and (b), respectively. Compare
Figure 4(b) and Figure 5(b) with Figure 6(a) and
Ib), we can see that both multiple reference velocity
migrations yield much better images than the Kirch-
hoff migration. and give clearer images than the FX-
migration in the region B (cf. Figure 2(b). 1
Conclusions
We have developed an extended pseudo-screen migra-
tion with multiple reference velocities. Different ref-
erence velocities are selected in different regions of a
medium -o that velocity perturbations are small. Noartifacts associated with multiple reference velocities
were observed if the Butterworth filter is applied in
the wavenumber domain. The method can be used
to image complex structures with severe lateral veloc-
ity variations. Computation time increases relative to
the original method but could be significantly reduced
if an inverse Fourier transform algorithm calculating
only the desired part of the output is available.
Acknowledgements
\Ve would like to thank Amoco Production Company for
providing the FX-migration image for the SEG/EAEG 3D
salt model. Tong Fei and the Gulf of Mexico Subsalt Imag-
ing Project for providing their Kirchhoff migration image.
This work is part of the Advanced Computational Tech-
nology Initiative. Funding came from the Department of
Energy Office of Basic Energy Sciences through contract
W-7405-ENG-36.
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Huang, Lian-Jie & Fehler, M. C. Extended pseudo-screen migration with multiple reference velocities, article, November 1, 1997; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc689874/m1/5/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.