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甚低频声波因具有强的穿透性，其在海洋环境中的传播特性可受到海底深层地质结构的影响。在已开展的海洋试验中，垂直阵观测到水面航船的甚低频辐射噪声可激发高能量的深层海底声弹射路径，但其激发机理尚不明确，本文针对该现象开展理论基础研究。基于海底的沉积过程构建包含声速梯度的等效海底模型，并利用波数积分数值计算方法模拟声波跨海水-海底-海水的传播过程，深入探究深层海底结构对声传播的影响，进而揭示高能量海底声弹射现象的激发机理和相关特性规律。研究表明，受地质作用影响，海底沉积层中可产生一定的声速梯度，该梯度结构使得入射的甚低频声波在深层海底介质中传播时可发生“声翻转”效应，将大部分能量重新辐射回水声场，从而激发高能量海底声弹射路径。在该过程中，沉积层的厚度和声速结构共同作用影响表层和深层弹射路径的观测特征。本研究深化了深海甚低频声传播机理的认知，为利用海底弹射波进行甚低频目标的声探测应用提供理论支撑。Very-low-frequency (VLF) acoustic waves (≤100 Hz) exhibit special propagation characteristics in the deep sea, owing to strong penetration capability and interaction with deep geological structures. During a deep sea experiment conducted in the South China Sea, a vertical linear array including 64 elements was moored to the bottom (approximately 4360 m depth) to receive the acoustic signal. In the bearing-time record proceed by beamforming,a high-energy bottom bounce path is observed from the ship noise received by the bottom-moored vertical linear array. This shows an abrupt increasing in energy near a 45° grazing angle. However, the physical mechanism causing this phenomenon remains unclear, and we investigate it further in this paper. Based on the data processing, we developed an environmental model of the seabed that incorporates continuous speed gradient, which arises from long-term geological compaction processes, in the sediment. This model is contrasted with a traditional stratified model assuming a uniform sediment layer. The wavenumber integration method is adopted for numerical simulation to accurately calculate the pressure field and analyze the cross-media propagation. The numerical simulations demonstrated that the positive velocity gradient (increasing from 1600 m/s to 2144 m/s) causes an ‘acoustic turning’ effect, which reradiates substantial acoustic energy back into the water column and generates observed high-energy bounce paths. This is supported by theoretical analysis using the WKB approximation, where the calculated reflection coefficient shows a sharp transition in the acoustic turning point, accounting for the energy fluctuations observed in the experimental bearing-time record (BTR). Further analysis shows that the thickness of sediment influences the angular separation between bottom bounce paths, while its sound speed structure dictates the turning angle. These findings provide new insights into VLF acoustic propagation in the deep sea and offer critical evidence supporting a transition from simplified stratified models towards a more realistic model with a continuous gradient structure. Furthermore, the discovery of high-energy bottom bounce paths provides a new way to enhance capabilities of underwater detection, and these observed characteristics also offer reliable pressure field features for the inversion of deep seabed parameters.
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Keywords:
- Very low frequency acoustics detection /
- Continuous sound speed seabed /
- Seabed acoustic bounce /
- Acoustic turning
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