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.