Eigenray sonar simulation in two-layer media

In this work, an eigenray finding method for simulating refractive multipath phenomena in an unbounded underwater environment is presented. To simplify the problem, the environment consists of a horizontally-homogeneous two-layer medium in which isogradient sound speed profiles are used in each layer. Eigenrays are found through an iterative Newton-Raphson method on the Snell invariant, and the travel time is found from the computed Snell invariant. A derivation for the maximum number of ray paths that need to be considered to account for all refractive multipaths is included. Given a point source/point target pair, the method will output a launch angle and travel time for the eigenray connecting the source and target. The time accuracy of the eigenray finding method is confirmed using a forward ray-trace. From the travel time of eigenrays, time series data is formed for each receive element. Refractive multipath phenomena are shown using the time series data of a target near the layer boundary. An FFT beamformer is used to beamform the simulated time series data which reproduces an image of the acoustic targets, which shows a time resolution sufficient to support the coherent addition of signals in beamforming.