I. Introduction
Numerical simulation of wave propagation is a theoretical field of research that began more than three decades ago, in close relationship with development of computer technology and numerical algorithms for solving differential and integral equations of several variables. The improvement of numerical algorithms and computer technology made attainable the running time of more and more realistic wave propagation modeling and decreased the price of necessary hardware. Seismic wave propagation can be described by partial differential equations (PDEs). Solution of PDEs in 3D requires enormous computer memory and computer power. In spite of the improvements of digital computers and numerical algorithms, supercomputers are necessary for efficient solution.