Two-point ray tracing using Fermat's principle

Ray tracing through models consisting of constant-velocity layers separated by curved interfaces, may be implemented by finding ray-paths whose traveltimes are stationary with respect to changes in ray/interface intersection positions. The amplitude of the resulting event is related to the second derivative of the traveltime with respect to changes in ray/interface intersection position. For complex multilayered models the traveltime functions can be multivalued. Stationary points are then found by decomposition of the function into a number of single valued functions.