\def\figdir{./Figs}
%When trying to solve  a non-linear problem such as tomography obtaining
%a model that a
%This can often be difficult to assess.
With one more iteration of both depth and tau tomography
the velocity model and the resulting depth
migration  improve.
Figure~\ref{fig:iter2-comp-vel}  shows that for both
depth and tau the anticline is better resolved.  The tau result does
a better job in isolating the anticline and has even begun to resolve
the low velocity layer within the anticline structure. 
\activeplot{iter2-comp-vel}{height=3.0in, width=6.0in}{CR} {Velocity model
after two iterations of depth (left) and tau (right) tomography.  The tau
results shows better isolation of the anticline and has even begun
to resolve the low velocity layer.}

Figure~\ref{fig:res.vel2.deltaz} shows the migration corresponding to the depth
derived velocity model.  The second iteration still shows some residual 
moveout in CRP gathers away from the anticline, but an overall improved
positioning of the reflector.  

\activeplot{res.vel2.deltaz}{height=3.0in, width=6.0in}{CR} {Migration result after
two iterations of depth tomography.}

After two iterations we can see more clearly the advantage of the tau
result.  
Figure~\ref{fig:res.vel2.deltatau} shows the migration result after three
iterations of tau tomography.  The reflectors are  well positioned and
we have very little residual moveout in the gathers.

\activeplot{res.vel2.deltatau}{height=3.0in, width=6.0in}{CR} {Migration result
after two iterations of tau tomography.}