%\mhead{TEST ON REAL DATA}
We next decided to test the method on real data.  
For this initial test we decided to work with a relatively
clean data which still had some residual move-out in the
common reflection point (CRP) gathers.
The data is from the Blake Outer Ridge as was used by 
Ecker\shortcite{Ecker.sep.96} to characterize methane hydrate structures.
For our initial velocity model we used Ecker's Dix \shortcite{GEO20-01-00680086}
derived model, Figure~\ref{fig:christine-vel0}.


Our general philosophy was to limit human time as much as possible.
Therefore we chose to do  tau migration 
\cite{Alkhalifah.sep.97.tariq4} using a generic Kirchhoff 
package\cite{Biondi.sep.97.biondo3}.
By using tau rather than depth migration,
we were quickly able to compare CRPs from iteration
to iteration and it allowed us to pick reflector positions
only once.

\plot{christine-vel0}{width=3.5in}{Initial velocity model in depth. Note 
the low velocity zone caused by the gas hydrate starting at approximately
32000 kms and extending to the end of the section.}

After performing the migration we picked six reflectors,
Figure~\ref{fig:stack}. We picked the sea floor, a strong reflector
above the bottom simulating reflector (BSR), the BSR itself, the flat
reflector
below the BSR, and two deeper reflectors.

\plot{stack}{width=6.0in}{Initial stack overlaid by reflectors picked for
tomography.}

Rather than pick move-out differences we decided to create
residual semblance panels at each reflector location, Figure~\ref{fig:sem-vel0}.
The panels indicate that there is significant residual curvature, especially
where the BSR meets the lower reflector.  From these semblance
panels we picked smooth curves
at approximately the maximum semblance at each reflector. 
To check to see if a single parameter adequately described the move-outs we
back projected the picked semblance into our CRP gathers.
Figure~\ref{fig:overlay.vel0} shows that  the semblance
picks did a fairly good job describing the move-out.

\plot{sem-vel0}{width=5.5in,height=3.4in}{Residual semblance panels for the bottom
5 reflectors. The black line in each panel represents the picked maximum.}

\plot{overlay.vel0}{width=6.0in}{Common reflection point gathers from every
2000 meters starting from 28000.
The lines are the result of mapping back
the picked residual slowness values.  Note how the curves do an excellent
job matching the actual reflector move-out.}

We used our picked reflectors to construct our steering filters
and then
applied our tau tomography fitting
goals (\ref{eq:tautomo}). Generally, we have increased velocity, 
Figure~\ref{fig:christine-vel1}, but the changes still keep velocity following
reflector dip.
\plot{christine-vel1}{width=3.5in}{The velocity after 1 iteration of
tau-steering tomography}

The  next step is to see if our new velocity model flattens
our CRP gathers and improves the focusing of the data.  Figures
\ref{fig:overlay.vel1} and \ref{fig:stack-comp} indicate that we
have accomplished both of these goals. Figure~\ref{fig:overlay.vel0} shows
that  all of our reflectors are significantly flatter, with only
significant curvature left along the BSR.  Figure~\ref{fig:stack-comp}
shows a much more continuous BSR reflection along with overall improved
focusing of the section above and below.

\plot{overlay.vel1}{width=6.0in} {CRP gather from 24000-48000 meters. 
Note how they are considerably flatter than Figure~\ref{fig:overlay.vel0}.}


\plot{stack-comp}{width=5.0in}{The stack using our initial velocity and
the velocity after 1 iteration of tau steering tomography.  Note how
the reflectors are generally better focused at a, b, c, and d.}


