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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to analyze the frequency periodicity characteristics of acoustic field interference and realize acoustic source ranging (ASR), the normal mode model is used to analyze the interference characteristics of the broadband acoustic field under the condition of horizontally layered medium; the broadband received signal field when the broadband pulse signal passes through the acoustic field is also simulated. The variation of interference patterns with frequency is analyzed, and their spatial interference characteristics and mechanisms are analyzed. Based on the interference theory, the relation between the acoustic source range and the frequency periodicity of the broadband acoustic intensity interference is derived. Simulation and experimental results show that this relation can accurately estimate the far-field acoustic source range, and the estimation accuracy and real-time performance are greatly improved compared with previous methods. Besides, simulation shows that the method combined with multiple-receiver ranging obtains high-precision direction of arrival (DOA) estimation as well as ASR. The relation between acoustic source position and broadband acoustic field interference frequency periodicity can be used to improve far-field ASR and DOA estimation, which is of great value for oceanography, marine engineering, and marine military. In addition, this relation can also be extended to that between the modal interference frequency periodicity and other related parameters in other physical fields for parameter inversion. [ABSTRACT FROM AUTHOR]