\def\figdir{Figs}
In Chapter~\ref{chap:tau}, I introduce the concept of tau (vertical
travel-time) tomography
and derive the operator relating changes in slowness
to moveout errors in tau space.
I show how tau tomography is less sensitive to our initial
guess at reflector position and slowness estimate.  
Figure~\ref{fig:ref-movement} shows the model of Figure~\ref{fig:intro-rays} 
with the correct and initial reflector positions in depth (left) and
tau (right).
\activeplot{ref-movement}{width=6.0in,height=3.0in}{ER}{Reflector movement in
depth (left) and tau (right).  The solid lines are where there reflectors
migrating using the initial v(z) field. The dashed lines are the correct
reflector positions in tau and depth.  Note that correct and initial
positions are much close in the tau case.}

On a synthetic I show how performing tomography
in tau rather than depth better constrains our slowness
changes.  I show that the tau tomography problem converges
significantly faster to a more reasonable result than
its depth counterpart.  The resulting migration are
better focused and the reflectors are better positioned by
doing tau rather than depth tomography.

