The received sound intensity of bottom-mounted line array varies as the submerged sound source moves in the direct arrival region, which resulting from interference between the direct and surface-reflected propagation paths, modulates with the target depth. In this work, the Fourier integral method from McCargar and Zurk [J. Acoust. Soc. Am. 133, EL320–EL325 (2013)] has been improved for depth estimation with a horizontal line array, and the matched sound intensity structure method from Zheng, Yang, Ma, and Du [J. Acoust. Soc. Am. 148, 347–358 (2020)] is introduced as a comparison. The two methods are verified in a deep ocean experiment.

Adaptive matched field processing (AMFP) has proven effective for source localization in deep-water environments. However, when the target is in motion, the need for numerous snapshot samples can lead to distortion in covariance estimation, degrading AMFP performance. A synthetic AMFP method has been proposed to compensate for the phase of multi-snapshot signals, enhancing AMFP performance. Additionally, a rough estimation of target velocity is obtained. The efficacy of the method has been validated through numerical simulations and experimental data, with results showing that, within a 9 km range, the average localization error is reduced by 1.45 km compared to traditional AMFP.

The horizontal-transverse coherence of low-frequency (300 Hz) and long-range (10–40 km) acoustic fields near the bottom in deep water is investigated based on experimental data obtained from the South China Sea. The results indicate that the horizontal-transverse coherence length exhibits a strong dependence on the source-receiver distance, with fluctuations consistent with sound intensity trends. In high-intensity regions, the horizontal-transverse coherence is relatively high, with a coherence length exceeding 600 λ, where λ is the acoustic wavelength, whereas in low-intensity regions, the horizontal-transverse coherence decreases significantly, with the coherence length shortening to 10–30 λ. The physical mechanisms underlying the horizontal-transverse coherence are analyzed using the ray theory. In high-intensity regions, the energy of the dominant ray (the ray with the highest energy) accounts for over 70% of the total energy of the rays, exerting a decisive influence on the coherence coefficient and leading to stable horizontal-transverse coherence in the received acoustic field. In contrast, in low-intensity regions, the energy distribution is dispersed, and when amplitude and phase disturbances due to spatial inhomogeneity are introduced, the horizontal coherence deteriorates significantly. The numerical simulations are also performed, and the results are consistent with the experimental observations.

In shallow water environments, choosing an appropriate reference phase velocity for direction-of-arrival estimation with a beamformed underwater horizontal line array is very important. The direction of the maximum beamformer output power will deviate from the true source bearing when a mismatched reference phase velocity was used. This Letter analyzed the intrinsic relationship between the reference phase velocity and normal mode amplitude distribution, source bearing, array aperture, and then proposed a multi-parameter weighted reference phase velocity selection method, which has improved the accuracy of source bearing estimation. Numerical simulation and experimental results validated the effectiveness of this method.

The passive localization of dual targets composed of a surface ship and a submerged source located nearby beneath the ship is an intriguing problem. This study develops a passive localization method based on multipath arrival angles for dual targets, with similar source levels in the deep-ocean direct arrival zone, using a horizontal line array. Compared to the classical minimum variance distortionless response method, the sparse Bayesian learning method is used to improve resolution for multipath arrival angles under coherent signal conditions, enhancing both the effective range and localization accuracy. The effectiveness of the proposed method has been validated through simulation and experiment.

In environments with water depth variations, one-way modal solutions involve mode coupling. One approach is to model the depth variation as a series of steps, and use a new set of modes at each step (Evans, JASA, 1983). Higham and Tindle (JASA, 2003) developed an accurate and faster approach using perturbation theory to locally determine the change in mode functions at steps and limit the required number of new sets of mode functions. The method of Higham and Tindle is limited to low frequency (< 250 Hz). We have extended the coupled mode perturbation method so it can apply to higher frequencies. The approach will be described and some examples shown.