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I present a migration velocity analysis method
using wavefield-extrapolation techniques
that can address the challenges posed by
velocity estimation in complicated media with
sharp contrasts and fine-scale features.
This method is formulated in the migrated image space,
with an objective function aimed at improving the
image quality.
The method is based on a linearization of the downward
continuation operator that relates
perturbations of slowness models to
perturbations of migrated images.
Since the method is based on finite-difference extrapolation
of band-limited waves, it naturally takes into account
the multipathing that characterizes wave propagation
in complex environments with large and sharp velocity
contrasts.
It also takes into account the full wavefield
information, and
not only selectively picked traveltimes, as
it is currently done in state-of-the-art
traveltime tomography.
I use prestack Stolt residual migration (storm)
to define image perturbations by
maximizing focusing and flatness of angle-domain
common image gathers (adcig).
In general, the image perturbations computed with
this method can be too different from the background
image, and we are in danger of subtracting
images that are not in phase, violating the
first-order Born approximation assumption.
I avoid divergence of the inversion procedure
when the velocity perturbations are too large,
by not inverting directly the image perturbations
obtained by residual migration,
but by inverting linearized versions of them.
Thus, I achieve a method which is robust with
respect to large model perturbations, a
crucial step for a practical MVA method.
Next: Acknowledgment
Up: Wave-equation migration velocity analysis
Previous: WEMVA cost
Stanford Exploration Project
11/4/2004