This paper reports on the long-term statistics of ambient noise (50–2000 Hz) in the northeast South China Sea. The data were collected from July 2016 to March 2018. The long-term statistics, seasonal, and diel variations of ambient noise are analyzed. There are significant seasonal variations from 500 to 2000 Hz. The ambient noise level at 1000 Hz is 6–10 dB higher in winter than in summer. There is only a small difference between the day and night comparison (less than 1.6 dB). The results are significant to evaluate and improve the performance of underwater acoustic systems in this area.

A high‐range‐resolution imaging method for a multiple‐input–multiple‐output sonar is proposed. In this method, a set of frequency diversity linear frequency modulation (FD‐LFM) pulses is transmitted, with a zero bandgap between two adjacent FD‐LFM pulses to suppress cross‐correlation functions. At the processing end, the synthesised matched filtering, which combines the matched filtering and the frequency band synthesising into one step, is proposed to reduce the computation burden. Moreover, the frequency‐domain weighting is adopted to suppress to the sidelobes of the synthesised auto‐correlation function to further improve the image quality. The effectiveness of the proposed method is validated via the theoretical analysis, numerical simulations and water tank experiments.

A multiple‐input multiple‐output (MIMO) sonar can form a large‐aperture virtual uniform linear array (ULA) from a small number of physical elements. However, the large‐aperture MIMO sonar encounters a dilemma between the computation load and the imaging performance. Since the traditional phase‐shift beamformer (PSBF) and shifted sideband beamformer (SSBF) cannot solve the dilemma individually, the authors proposed a hybrid beamformer (HBF) which combines PSBF and SSBF together. Specifically, the proposed HBF contains the off‐ and on‐line processing. In the off‐line processing SSBF is applied to the transmitting ULA to process transmitting waveforms. In the on‐line processing PSBF is applied to the receiving ULA, and outputs of off‐line SSBFs are utilised to matched filter outputs of PSBFs. Thus, in the proposed HBF the time delays of SSBF is calculated off‐line; and the multi‐beam processing is simplified as PSBFs on the receiving ULA. Consequently, the proposed HBF can reduce the computation load of the large‐aperture MIMO sonar effectively while obtaining a fine imaging performance. Numerical simulations show the effectiveness of the proposed HBF.

Matched-field processing (MFP) for underwater source localization serves as a generalized beamforming approach that assesses the correlation between the received array data and a dictionary of replica vectors. In this study, the processing scheme of MFP is reformulated by computing a statistical metric between two Gaussian probability measures with the cross-spectral density matrices (CSDMs). To achieve this, the Wasserstein metric, a widely used notion of metric in the space of probability measures, is employed for developing the processor to attach the intrinsic properties of CSDMs, expressing the underlying optimal value of the statistic. The Wasserstein processor uses the embedded metric structure to suppress ambiguities, resulting in the ability to distinguish between multiple sources. In this foundation, a multifrequency processor that combines the information at different frequencies is derived, providing improved localization statistics with deficient snapshots. The effectiveness and robustness of the Wasserstein processor are demonstrated using acoustic simulation and the event S5 of the SWellEx-96 experiment data, exhibiting correct localization statistics and a notable reduction in ambiguity. Additionally, this paper presents an approach to derive the averaged Bartlett processor by evaluating the Wasserstein metric between two Dirac measures, providing an innovative perspective for MFP.