The earth surface has a density contrast; the density of air is quite different from that of rock, water, and soil. This upper surface has a high reflectivity coefficient and is correctly modeled as a hard surface boundary. The sides and bottom of a model usually are treated as transmissive boundaries. The terminology used for these boundaries is absorbing, infinite, or transmissive, which all mean essentially the same thing, a non-reflecting boundary.
We shall consider two useful types of boundary condition methods for non-reflecting, and one method for reflecting, boundaries. The simplest of these methods is for the reflecting boundary.