Ray tracing using reciprocity

Conventional ray‐tracing techniques like the shooting method have many difficulties when applied to tomography analysis. For example, (1) head waves are generally not included, (2) a single raypath is generally assumed for each pair of source and receiver, (3) large computation time is required in a cell structure model for many source‐receiver pairs, and (4) it is difficult to find a raypath in a complicated velocity structure. A new ray‐tracing technique can overcome these difficulties. This technique is based on the two well‐known principles: the reciprocity principle and Fermat’s principle. The algorithm is divided into two steps: (1) calculating the traveltime of first break at each grid point, and (2) estimating raypaths using calculated traveltime data. In this second step, we use total traveltime which is the sum of traveltime from a source point and that from a receiver point. A minimum of the total traveltime represents first break raypaths. Therefore, the method can treat not only a single firs...