A GEOMETRICAL APPROACH TO THE ELASTIC COMPLEX SCREEN

The elastic complex screen. or ''phase screen'' approach to the forwar d modeling of elastic waves promises significant advantages over other computational schemes, because it is potentially much less intensive numerically than finite difference methods, and yet it models essentia lly the whole useful wave field. In this paper it is shown how the equ ations that describe the technique can be derived through a geometrica l construction. The new formulation is presented in conjunction with a n overview of the earlier derivation of Wu [1994] in order td emphasiz e the similarities and differences between the two approaches. The geo metrical derivation provides for a formulation in terms of interfaces (steps) rather than slabs. It also leads quite naturally to the incorp oration of backscattered energy in the model. An algorithm to implemen t this important capability is developed in some detail and includes a backpropagation technique, which improves storage efficiency by remov ing the need to store the backscattered field at each screen independe ntly. Numerical results are presented for some simple backscattering m odels, first of all to validate the theory, but then more specifically to illustrate the frequency response of the model and the implication s it has for the choice of a suitable source spectrum.