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The next example concerns a zero-offset GPR dataset over a lava flow region.
In this situation, diffraction focusing is the only option available
for migration velocity analysis.
The data depicted in LAVAdata show many diffractions
spread over the entire dataset. A few obvious ones are
at x=22 ft, t=27 ns,
at x=28 ft, t=22 ns, and
at x=35 ft, t=23 ns.
LAVAdata
Figure 46
Zero-offset GPR data used for focusing migration
velocity analysis.
I follow the same procedure for migration velocity analysis as the one
described for the preceding example. LAVAimag(a) shows the initial
image obtained by migration with a constant velocity of 0.2 ft/ns, and
LAVAimag(b) shows the final image obtained after velocity update.
We can notice that the image has been vertically compressed, since the
velocity update indicated a faster velocity, and most of the diffractions
have been collapsed.
LAVAimag
Figure 47
Zero-offset migrated images for the data in
LAVAdata using the initial velocity (a)
and the updated velocity (b).
LAVAyoom and LAVAzoom are detailed views
of the initial and final images and slownesses at various locations
of interest.
LAVAyoom shows collapsed diffractions in the left part of the image.
We can also observe features with better continuity in the updated image
than in the original image,
for example at x=20-24 ft and z=2 ft in LAVAyoom(a,c).
Likewise, LAVAzoom shows a better focused image than in the
original,
for example at x=34 ft and z=1.8 ft in LAVAzoom(a,c).
LAVAyoom
Figure 48
Detail of the images depicted in LAVAimag.
Migration with the initial velocity (a),
updated slowness model (b) and
migration with the updated slowness (c).
The window corresponds to x=20-24 ft and z=2 ft.
LAVAzoom
Figure 49
Detail of the images depicted in LAVAimag.
Migration with the initial velocity (a),
updated slowness model (b) and
migration with the updated slowness (c).
The window corresponds to x=34 ft and z=1.8 ft
Next: Discussion
Up: Diffraction-focusing WEMVA example
Previous: Delineation of rough salt
Stanford Exploration Project
11/4/2004