\section{MIGRATION TESTS}
To test the migration result  I chose the same two synthetics used
in the previous section. 
Figure~\ref{fig:amoco-mig} shows the result of migrating the
North Sea synthetic using at maximum  four reference velocities.
The top picture shows the result of using a conventional approach
with four reference velocities plus salt. The bottom picture
shows the result of using Lloyd's method with a maximum of four velocities.
The Lloyd result reduced the number of reference velocities needed
at some depth levels, reducing the overall migration 30\% faster and higher quality
than the result with four reference velocities at each depth.
At location `A'  the left edge of the salt
structure is better placed. At locations `B' and `C'  the bottom of the salt
is better focused. 
\plot{amoco-mig}{width=6.0in}{Comparison between regularly sampling the
velocity range (top panel) and using Lloyd's algorithm for velocity selection.At location `A'  the left edge of the salt
structure is better placed. At locations `B' and `C'  the bottom of the salt
is better focused.}
\par
Figure \ref{fig:zig-chosen} shows the correct (left) and selected (right)
velocity for the Sigsbee synthetic. Note how the salt velocity was
correctly selected and the rest of the model well represented.
Figure~\ref{fig:zig-mig} shows the result of migrating
the Gulf of Mexico synthetic. In this case for the standard approach I 
chose five+salt reference velocities. The Lloyd's algorithm selection method resulted
in a 35\% cost saving in compute time.  
The top picture shows the conventional approach while
the bottom picture shows the result of using Lloyd's method
for velocity selection. The bottom of the salt reflection, `A', 
is better focused.  At `B' the right canyon is more accurately positioned
and better focused.  At location `C' we see how the point diffractors
are  collapsed using the Lloyd velocity selection method while using
the more standard approach they are not collapsed.

\plot{zig-chosen}{width=6.0in}{The correct (left) and selected (right)
velocity for the sisbee synthetic. Note how the salt velocity was
correctly selected and the rest of the model well represented.
}
\plot{zig-mig}{width=6.3in}{Comparison between regularly sampling the
velocity range (top panel) and using Lloyd's algorithm for velocity selection.
The bottom of the salt reflection, `A',
is better focused.  At `B' the right canyon is more accurately positioned
and better focused.  At location `C' we see how the point difractors
are  collapsed using the Lloyd velocity selection method while using
the more standard approach they are not collapsed.}

