Striation‐based beamforming using the waveguide invariant for passive sonar.

Consider a horizontal array measuring the coherent field produced by a distant acoustic source. If the resulting acoustic intensity were mapped as a function of position along the array and frequency, the image would exhibit striations, bands of high intensity. It is well known that the slope of the observed striations can be related to bearing of the source and the ratio between the so‐called waveguide invariant and the range to the source. The canonical value of the waveguide invariant in shallow water is 1, but deviations by 30% or more are not uncommon when there is a sharp thermocline. By beamforming the coherent field conventionally at a single frequency, one can determine the bearing of the source and hence the invariant‐to‐range ratio. In the present work, striation‐based beamforming is developed. Rather than pick a single frequency and then beamform, each point on the array is first evaluated at a slightly different frequency with the frequency offset determined by the slope of the observed intensity striations. It is shown that the striation‐based beamformer can be used to produce an estimate for the waveguide invariant that is independent of the source range. Simulation results are presented. [Work supported by the ONR.]