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.