Weak tonal source depth discrimination in shallow ocean using depth-modulated amplitude fluctuation

The difference in tonal amplitude fluctuations between surface and submerged sources in shallow ocean can be used for binary source depth discrimination. However, under low signal-to-noise ratio (SNR) conditions, such fluctuation-based discrimination is often compromised by strong background noise. To improve the depth discrimination capability for weak tonal sources, this paper proposes a source depth discrimination method with a passive horizontal line array (HLA) using depth-modulated amplitude fluctuation. We investigate the physical mechanism of depth-modulated amplitude fluctuation in the beam domain and reveal that existing HLA-based decision metrics are sensitive to the amplitude fluctuation induced by modal interferences. To overcome this limitation, a source range-independent metric: the amplitude fluctuation energy ratio (AFER) via the Fourier transform, is proposed. The AFER leverages the distinct frequency characteristics of modal interference fluctuation, depth-modulated fluctuation, and noise fluctuation in the time domain. The combined array gain and integration gain of the AFER are both utilized to suppress noise. Simulation results demonstrate AFER’s advantages in weak tonal source (SNR at hydrophone equals −5 dB in 1 Hz band) discrimination under unknown source range conditions compared to the existing decision metric built by the ratio of advanced WISPR (Wagstaff’s integration silencing processor) summation (AWSUM). The analysis of SWellEx-96 experimental data further validates the effectiveness of the proposed method.