Assume a flat surface and an incident wave
which is propagated by the acoustic wave equation. The finite
difference equation, Equation 56, uses a stencil
which places a fictitious node or nodes outside the model. These nodes
are initially zero and are kept zero for all model time; the superscript
**n** is the time step. This
is illustrated in Figure 14
for a flat surface
boundary and the second order difference stencil.

Figure 14: Boundary Node Stencil.