Using fitting goals (\ref{eq:tomo2d}) I estimated a new velocity model.
Figure~\ref{fig:change1} shows the slowness perturbations introduced to the
model.  Note how the changes generally follow structure.
If we add these slowness changes to our initial slowness model and
convert back into depth we get Figure~\ref{fig:vel1}.  Our updated velocity
model is  smoother than our initial model (Figure~\ref{fig:vel0}).
The chalk
layer velocity has been increased and the sediment velocity
along the lower  left flank of the salt has been increased.

\sideplot{change1}{width=3.0in,height=3.0in}{Change in slowness introduced
in iteration one.}
\sideplot{vel1}{width=3.0in,height=3.0in}{Velocity model after first iteration.}


If we migrate using the updated velocity model,
we obtain Figure~\ref{fig:elf-mig1}. Overall the migrated image is improved. 
Comparing the new migration (Figure~\ref{fig:elf-mig1}) to the initial migration
result (Figure~\ref{fig:elf-mig0}) we see that
the chalk (`A') and salt top
reflections (`B') are sharper. The sediments in the upper left portion of
the salt flank are more coherent (`C').  We are beginning to see
reflections along the lower left flank where little coherent events
were present in the initial image (`G').  The right side of the salt shows
similar improvement.  The focusing of the upper events is improved
and we are seeing significantly more of the sediment-salt contact (`D')
than was possible in the initial migration.  The lower portion of
the image has also improved. The salt bottom reflection is more coherent (`E') 
and
we are seeing a greater extent  of  the reflector in the lower left
portion of the image.

\plot{elf-mig1}{width=6.0in,height=7.5 in}{Migration result using
the velocity from Figure~\ref{fig:vel1}. Note that compared to Figure~\ref{fig:elf-mig0} the chalk (`A') and salt top
reflections (`B') are sharper. The sediments in the upper left portion of
the salt flank are more coherent (`C').  We are beginning to see
reflections along the lower left flank where little coherent events
were present in the initial image (`G').  The right side of the salt shows
similar improvement.  The focusing of the upper events is improved
and we are seeing significantly more of the sediment-salt contact (`D')
than were possible in the initial migration.  The lower portion of
the image has also improved. The salt bottom reflection is more coherent (`E')
and
we are seeing a greater extent  of  the reflector in the lower left
portion of the image. }

The CRP gathers (Figure~\ref{fig:moveout-vel1.steer}) and semblance  
(Figure~\ref{fig:elf-sem-ref.vel1.steer}) tell a similar story.
The CRPs are flatter, and much more believable.  The moveout
errors have been decreased and we are now able to see more coherent
events than were initially possible.
      
\plot{moveout-vel1.steer}{width=6.0in,height=8.0in}{CRP gathers from the
velocity  after first iteration. Note how the semblance is flatter (
`A',`B', and `E') and more coherent (`C'and `F') than in the initial
semblance analysis (Figure~\ref{fig:moveout-vel0})}

\plot{elf-sem-ref.vel1.steer}{width=6.0in,height=3.5in}{Semblance panels from
ten of the reflectors used in tomography.}
