Using the modified trapped energy ratio for moving source depth discrimination in shallow water with a thermocline

A depth discrimination method is proposed for a moving source in shallow water with a thermocline, based on the received signal at a single fixed hydrophone in its radial direction. Knowing the original signal frequency and the average sound speed at full water depth, the output of synthetic aperture beamforming of the signal shows the energy distribution of normal modes in the elevation angle domain, where a surface source excites trapped modes poorly and a submerged source excites both trapped and non-trapped modes. By integrating the energies of these two mode types, a modified trapped energy ratio between them is introduced to determine the source depth. The integral interval is based on the estimated numbers of these two types depending on the basic environmental details and the decision threshold is decided by the relative magnitude of the energies of these two mode types. This ratio is different from that between the energies of the trapped modes and those of all normal modes, which requires perfect knowledge of the mode matrix or a sufficiently long source travel distance to separate the modes individually. The simulations verify that the discrimination can be done at a shorter distance with few environmental details, and investigate the effects of sound speed profile, synthetic aperture, source frequency, and low signal-to-noise ratio on the performance. Finally, the experimental data analysis validates the proposed method where a surface source and a submerged source of similar frequency are successfully discriminated.