Passive source depth discrimination in deep-water

Abstract

In this work, we address the problem of passive source depth discrimination using a horizontal line array (HLA). The scope is restricted to low-frequency sources (f<500 Hz), broadband signals (a few Hertz bandwidth), deep-water environment (D>1000 m), and distant sources (at least several kilometers). The proposed method is based on the underlying physics driving the propagation. It notably uses the concept of mode trapping and waveguide invariant. In deep water, the waveguide invariant largely depends on source depth, and is thus an interesting input for source depth discrimination. An algorithm is proposed to compute energy ratio in modal groups. The input data for the algorithm are a range-frequency intensity, as measured on a HLA. The modal groups are thus defined based on their respective waveguide invariant values, which is a main difference with existing depth discrimination methods adapted to shallow water context. This idea is explored, and extended to propose a source depth discrimination which is performed as a binary classification problem. As long as the sound speed profile features a surface thermocline, the algorithm does not require detailed knowledge about the environment and it allows one to classify sources under two hypotheses, above or under a threshold depth.