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Shear waves and sound attenuation in underwater waveguides
1Department of Physics, Naval Postgraduate School, 833 Dyer Road, Monterey, California 93943-5216, USA.
The Journal of the Acoustical Society of America
|July 9, 2021
Summary
Sound attenuation in soft seabed sediments is influenced by shear waves. This study reveals a stronger, first-order shear-wave contribution to sound attenuation than previously thought, impacting acoustic energy dissipation models.
Area of Science:
- Geophysics
- Acoustics
- Oceanography
Background:
- Bottom-interacting acoustic modes are attenuated by energy dissipation in the seabed and shear wave radiation into soft sediments.
- The shear wave speed in soft sediments is significantly lower than the sound speed in water, leading to distinct frequency dependencies in attenuation.
- Previous research indicated a third-order dependence of shear-wave attenuation on the ratio of shear to sound speeds.
Purpose of the Study:
- To analyze the coupling of compressional and shear waves in layered soft sediments.
- To identify and quantify the contributions of shear waves to acoustic mode attenuation.
- To investigate the impact of sediment stratification and shear wave interference on frequency-dependent attenuation.
Main Methods:
- Analysis of compressional and shear wave coupling in layered soft sediment models.
- Investigation of wave conversion at the water-sediment interface and internal sediment interfaces.
- Examination of the influence of sediment density stratification and internal shear wave reflections.
Main Results:
- A significant first-order contribution to acoustic attenuation from compressional-to-shear wave conversion at internal sediment interfaces was identified.
- This first-order contribution is stronger than the previously recognized third-order contribution from shear wave generation at the water-sediment interface.
- First-order effects of weak shear on acoustic mode travel times were also detected.
- Sediment stratification and shear wave interference were found to control the frequency dependence of shear wave attenuation.
Conclusions:
- The shear-wave contribution to sound attenuation in soft sediments is potentially larger than previously estimated.
- The findings necessitate a re-evaluation of the effect of shear waves on experimentally measured frequency-dependent sound dissipation.
- Accurate modeling of acoustic propagation in marine sediments requires considering these enhanced shear wave effects.
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