The acoustic wave equation in two dimensions relates the spatial
derivatives to the time derivative. Using the acoustic approximation
the wave equation is derived in Section 4.
The dimensionality
of the equation requires a media velocity 
, the
sound speed.
In the two dimensional model of Figure 1
the surface coordinate is x and the depth coordinate
is z with the positive axis down, a tradition in the oil industry.
This half space is defined for 
and for a bounded
.
Figure 1: Geological Model.
