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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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