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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) (≤100 Hz) acoustic waves exhibit special propagation characteristics in the deep sea, owing to strong penetration capability and interaction with deep geological structures. In a deep sea experiment conducted in the South China Sea, a vertical linear array including 64 elements is moored to the bottom (approximately 4360 m depth) to receive the acoustic signal. In the bearing-time record (BTR) processed by beamforming, a high-energy bottom bounce path is observed from the ship noise received by the bottom-moored vertical linear array, which shows an abrupt increase in energy near a grazing angle of 45°. However, the physical mechanism causing this phenomenon is still unclear, and we investigate it further in this work. According to the data processing, we develop an environmental model of the seabed by combining continuous speed gradient, which arises from long-term geological compaction processes, in the sediment. This model is compared with a traditional stratified model under the assumption of a uniform sediment layer. The wavenumber integration method is adopted in numerical simulation to accurately calculate the pressure field and analyze the cross-media propagation. The numerical simulations show 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 the observed high-energy bounce paths. This is supported by theoretical analysis in the WKB approximation, where the calculated reflection coefficient shows a sharp transition in the acoustic turning point, explaining the energy fluctuations observed in the experimental BTR. Further analysis shows that the thickness of sediment influences the angular separation between bottom bounce paths, while its sound speed structure determines the turning angle. These findings offer new insights into VLF acoustic propagation in the deep sea and also provide critical evidence for supporting a transition from simplified stratified models to a more realistic model with a continuous gradient structure. Furthermore, the discovery of high-energy bottom bounce paths provides a new way for enhancing the capabilities of underwater detection, and these observed features also provide reliable pressure field characteristics for inverting 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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图 4 时延估计结果(互相关)
Fig. 4. Time delay estimation results from cross-correlation analysis.
图 5 沉积层含声速梯度的声学模型示意图(${\theta _{\mathrm{w}}}, {\theta _{{\mathrm{sed}}}}$分别为海水及沉积层中的掠射角; ${\theta _{\mathrm{c}}}$为表层海底全反射临界角, $h$为沉积层厚度, ${c_i}(i = {\mathrm{w}}, {\mathrm{sed}}\text{-}{\mathrm{top}}, {\mathrm{sed}}\text{-}{\mathrm{bot}}, {\mathrm{b}})$分别为海水, 沉积层上、下界面及基底层中的声速, ${\rho _i}(i = {\mathrm{w}}, {\mathrm{sed}}, {\mathrm{b}})$分别为海水, 沉积层及基底层中的密度)
Fig. 5. Schematic diagram of the acoustic model with a varying velocity in the sediment layer (${\theta _{\mathrm{w}}}, {\theta _{{\mathrm{sed}}}}$ are the grazing angles in the seawater and the sediment layer; ${\theta _{\mathrm{c}}}$ is the critical angle at the seabed surface; $h$ is the thickness of the sediment layer; ${c_i}(i = {\mathrm{w}}, {\mathrm{sed}}\text{-}{\mathrm{top}}, {\mathrm{sed}}\text{-}{\mathrm{bot}}, {\mathrm{b}})$ are the sound speeds in the seawater, at the upper/lower interfaces of the sediment layer, and in the basement layer; ${\rho _i}(i = {\mathrm{w}}, {\mathrm{sed}}, {\mathrm{b}})$ are the densities of the seawater, the sediment layer, and the basement layer).
图 6 沉积层为均匀声速的声学模型示意图
Fig. 6. Schematic diagram of the acoustic model with a constant velocity in the sediment layer.
图 7 均匀沉积层模型下的深层海底路径反射系数
Fig. 7. Seabed reflection coefficient for the model with a constant velocity in the sediment layer.
图 8 沉积层含声速梯度模型下的深层海底路径反射系数
Fig. 8. Seabed reflection coefficient for the model with a varying velocity in the sediment layer.
图 9 声场传播损失(仿真) (a) 沉积层含声速梯度的仿真结果; (b) 均匀沉积层模型的仿真结果
Fig. 9. Acoustic transmission loss field by numerical simulation: (a) Simulation results for the model with a varying velocity in the sediment layer; (b) simulation results for the model with constant velocity in the sediment layer.
图 10 宽带波束形成的仿真结果 (a) 沉积层含声速梯度的沉积仿真结果; (b) 均匀沉积层模型仿真结果
Fig. 10. Broadband beamforming by numerical simulation: (a) Simulation results for the model with a varying velocity in the sediment layer; (b) simulation results for the model with constant velocity in the sediment layer.
图 11 不同条件下深层海底路径俯仰角
Fig. 11. Beam angle of deep bottom path under different conditions.
图 13 模型1和模型2的仿真结果 (a) 模型1的宽带波束输出(20—100 Hz); (b) 模型1的声场传播损失; (c) 模型2的宽带波束输出(20—100 Hz); (d) 模型2的声场传播损失
Fig. 13. Simulation results for model 1 and model 2: (a) Broadband beam output (20–100 Hz) for model 1; (b) acoustic transmission loss field for model 1; (c) broadband beam output (20–100 Hz) for model 2; (d) acoustic transmission loss field for model 2.
图 14 模型3的仿真结果 (a) 模型3的宽带波束输出(20—100 Hz); (b) 模型3的声场传播损失
Fig. 14. Simulation results for model 3: (a) Broadband beam output (20–100 Hz) for model 3; (b) acoustic transmission loss field for model 3.
表 1 模型所用环境参数
Table 1. Environmental parameters used in models.
h/m ${c_{{\mathrm{sed}}}}$/
(${\mathrm{m}} \cdot {{\mathrm{s}}^{ - 1}}$)${\rho _{{\mathrm{sed}}}}$/
(${\mathrm{g}} \cdot {\mathrm{cm}}^{ - 3}$)${c_{\mathrm{b}}}$/
(${\mathrm{m}} \cdot {{\mathrm{s}}^{ - 1}}$)${\rho _{\mathrm{b}}}$/
(${\mathrm{g}} \cdot {\mathrm{cm}}^{ - 3}$)声速连续
模型450 1600—2144 1.1 2144 1.7 声速均匀
模型450 1600 1.1 2144 1.7 
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表 2 仿真中模型所用环境参数
Table 2. Environmental parameters used in simulation models.
$h$/${\mathrm{m}}$ ${c_{{\mathrm{sed}}}}$/
(${\mathrm{m}} \cdot {{\mathrm{s}}^{ - 1}}$)${\rho _{{\mathrm{sed}}}}$/
(${\mathrm{g}} \cdot {\mathrm{cm}}^{ - 3}$)${c_{\mathrm{b}}}$/
(${\mathrm{m}} \cdot {{\mathrm{s}}^{ - 1}}$)${\rho _{\mathrm{b}}}$/
(${\mathrm{g}} \cdot {\mathrm{cm}}^{ - 3}$)模型1 450 1600—1800 1.1 2144 1.7 模型2 450 1600—2500 1.1 2500 1.7 模型3 50 1600—2144 1.1 2144 1.7
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