\par
The North Sea dataset provided to SEP by Elf Aquitaine
% \cite{vaillant.sep.102.louis1,Sava.sep.103.paul2,Fomel.sep.103.sergey1} meets
meets all of these criteria.  
The data is very clean (Figure~\ref{fig:elf-shot}) with strong reflectors
that are generally continuous.  The data contains a chalk layer which
causes a velocity inversion below it.
The initial velocity model (Figure~\ref{fig:vel0}) 
was created by IFP using the
S.M.A.R.T\footnote{Sequential Migration-Aided Reflection Tomography -
KIM (Kinematic Inversion Methods), IFP consortium}
method \cite{smart1,smart2}
The velocity structure shows typical North Sea behavior with velocity following
structural layers.

\sideplot{elf-shot}{width=2.0in,height=4.0in}{A preprocessed shot gather
from the Elf Aquitaine dataset.}
\sideplot{vel0}{width=3.0in,height=3.0in}{The initial velocity
model for the 2-D line. Overlaid are the general geologic units
in the area. Note how the velocity follows structural
dip and  that velocity decrease
sal below the chalk layer in the Liassic.}
\par
The dataset is 3-D marine acquired using three cables with 100 meter
spacing and geophones every 25 meters.  In this chapter I will
be dealing with a 2-D subset of the 3-D dataset.  The line
was chosen to coincide with a 2-D synthetic dataset \cite{Prucha.sep.97.marie1,Malcotti.sep.97.hermes1}.
The  subset  
was created by forming partial stacking and then applying 
Azimuth Moveout (AMO) \cite{GEO63-02-05740588}
to the  CMP gathers.
\par
I will begin this chapter by showing the migration and moveout errors with
the initial velocity.
Next, I will show 
how I built the steering filter operator and some of the issues involved
in dealing with the complex salt structure.  I will then move onto applying
the tomography technique to the dataset.

