3-D dynamic ray tracing is a forward problem. It requires that a physical model should be known in advance. Constructing a 3-D slowness model is an open topic. Different model gridding schemes have different characteristics. In order to be more general and portable, our dynamic ray tracing program is designed to be independent of the model gridding scheme. It only requires the local velocity information in the vicinity of the central ray be known. We can choose different types of gridding schemes, such as cube, layer, or tetrahedron, according to the characteristics of the geological structure. We use cubic scheme in our application only because it is easy to implement. we use slowness instead of velocity in this paper.
Since the model has already been gridded, the model construction is transferred into an expression of the slowness within each small cube. We select a trilinear interpolation function.