A mesoscale eddy is detected and tracked in the western North Pacific region. Within the life cycle of the cyclonic eddies, the intensities of eddies make a difference. Satellite images indicate the oceanic eddy keeps westward-moving until it disappears. Oceanographic and acoustic characteristics of the eddy are studied. The acoustic energy distribution results from the different intensity of both modelled eddy and measured eddy are calculated. With sound propagation through the cyclonic eddy and anticyclonic eddy, the position of convergence zone moves away from and towards the acoustic source compared with the sound propagation through background hydrography. The coupling coefficient of different orders of normal modes changes significantly. The closer to the centre of the eddy, the stronger the coupling coefficient.

Recent studies have illustrated that the Multichannel Analysis of Surface Waves (MASW) method is an effective geoacoustic parameter inversion tool. This particular tool employs the dispersion property of broadband Scholte-type surface wave signals, which propagate along the interface between the sea water and seafloor. It is of critical importance to establish the theoretical Scholte wave dispersion curve computation model. In this typical study, the stiffness matrix method is introduced to compute the phase speed of the Scholte wave in a layered ocean environment with an elastic bottom. By computing the phase velocity in environments with a typical complexly varying seabed, it is observed that the coupling phenomenon occurs among Scholte waves corresponding to the fundamental mode and the first higher-order mode for the model with a low shear-velocity layer. Afterwards, few differences are highlighted, which should be taken into consideration while applying the MASW method in the seabed. Finally, based on the ingeniously developed nonlinear Bayesian inversion theory, the seafloor shear wave velocity profile in the southern Yellow Sea of China is inverted by employing multi-order Scholte wave dispersion curves. These inversion results illustrate that the shear wave speed is below 700 m/s in the upper layers of bottom sediments. Due to the alternation of argillaceous layers and sandy layers in the experimental area, there are several low-shear-wave-velocity layers in the inversion profile.

Oceanic fronts involved by ocean currents led to strong gradients of temperature, density and salinity, which have significant effects on underwater sound propagation. This paper focuses on the impact of the oceanic front on three-dimensional underwater sound propagation. A joint experiment of ocean acoustic and physical oceanography at the western North Pacific fronts is introduced. The measurement data for sound waves passed through the oceanic front is processed. The results are analysed and compared with the numerical simulation. It was found that transmission loss presented some difference when the source was located in the front centre and sound waves propagated towards water mass on opposite sides of the front centre. And when the sound field is excited by the underwater explosion at a depth of 200 m, the effects of the horizontal refraction cannot be neglected. On the other hand, the transmission loss for sound pressure fell sharply and rose rapidly at the side of cold water masses.

In this Letter, a three-dimensional beamforming method is proposed for a miniaturized aperture acoustic vector sensor (AVS) array. This method extracts the multipole modes using the AVS array, and then synthesizes the desired beam pattern according to the relation between multipole modes and spherical harmonics. Compared to the spherical harmonics decomposition of the sound field method, the proposed method achieves comparable array gain with higher white noise gain when the distance of adjacent elements d and the wavelength λ satisfy 0.01≤d/λ≤0.2. Experimental results also demonstrate the effectiveness of the proposed three-dimensional beamforming method at very low frequency.

声波经过声线反转点产生的-$\frac{\pi }{2}$常数相移是高频条件下的理想近似，实际上声波经过反转点时会以非均匀平面波的形式水平传播一段距离再折射返回，这时在声线反转点还应计及一个函数附加相移，当反转点靠近波导界面时又会产生反射相移，定义这两种相移为声衍射相移。经典射线理论忽略了衍射相移的影响，很难解释低频条件下深海会聚区的焦散结构。本文从射线-简正波理论出发，推导了计及衍射相移时声线跨距、传播时延与群速度的表达式，在此基础上给出了一种包含衍射相移影响的深海会聚区焦散结构计算模型。对典型深海声道中第一个上会聚区的仿真研究表明：高频条件下RR（Refracted-Refracted，RR）型会聚区有三条焦散线，RSR（Refracted Surface-Reflected，RSR）型会聚区有四条焦散线。通过与高频结果对比，低频条件下计及衍射相移后发现，界面反射相移引起的水平位移使RR型焦散线向靠近声源的方向水平偏移，使RSR型声线额外多产生数条焦散线，而声波以非均匀波形式传播时产生的位移使RR型焦散线向远离声源的方向水平偏移。频率升高后，声衍射效应减小，焦散结构趋于经典射线理论的计算结果。本文给出的模型物理意义清晰，计算简便准确，弥补了经典射线理论在低频条件下适用性不强的缺陷。

In the direct-arrival zone, the crucial preliminary step for depth estimation by matching the narrowband sound intensity is accurately estimating the elevation angle or the slant range (for calculating replica field). However, the elevation angle estimated by an ocean bottom seismograph may not be satisfactory, leading to mismatch issues for the matching processing algorithm and resulting in inaccurate range and depth estimation results. To overcome this problem, the multidimensional matched sound intensity processing (MD-MSIP) method is proposed. The MD-MSIP is defined as a joint estimation of the initial range, the target speed, the closest point of the approach range, and the source depth. By matching these four preset parameters, the target can be located, which does not require the elevation angle when calculating the replica field, avoiding the influence of the elevation angle error on MSIP. The experimental results demonstrate that the relative errors of the depth and range estimation results can be controlled within 10% and 20% by MD-MSIP, respectively. The target trajectory can also be estimated using the azimuth. In addition, the robustness of MD-MSIP is enhanced during the mismatch of the sound speed profile using the slow phase variation characteristics of the low-frequency signals.