Over the past several decades, physical experiments and theoretical approaches have been used to explore the effects of seamounts on the sound propagation in deep water.
However, the phenomena of acoustic propagation around seamounts are not well comprehended due to the complexities and uncertainties from oceanographic variability and geo-acoustic property of the sea bottom.
It is worthy to experimentally investigate the effects of seamounts on sound propagation in deep water further.
Recently, researchers from the Institute of Acoustics of the Chinese Academy and General Staff MET-HYD Department cooperated to have conducted a sound propagation experiment with the presence of seamounts in the South China Sea. Results show seamounts have reflecting and shadowing effects on sound propagation in deep water.
The transmission losses along two propagation tracks with and without seamounts were compared. The numerical transmission losses were simulated to compare with experiment data. Besides, the arrival pluses and ray diagram were analyzed to show the effects and mechanisms of seamounts on sound propagation in deep water.
In the experiment, the receiving array made by 27 distributed underwater signal recorder from 130m to 1800m with different intervals was moored at the bottom. The sample rate of hydrophones was 8000Hz. And the wide band signals charged with1kg TNT were dropped from Chinese R/V Shi Yan 1 from the Institute of Acoustics of the Chinese Academy of Sciences along two propagation tracks with and without seamounts. The nominal detonation depth of wide band signals was 200m. Besides, the measured discrete spectrums were averaged in 1/3-octave bandwidth with frequency varying from 50Hz to 2000Hz.
After processing and analyzing the data, obvious transmission loss differences for propagation in the environments with and without seamounts were observed.
For conditions of seamount located in the first shadow zone, the transmission losses decreased up to 7 dB for the ranges before the top of seamount due to reflection of bathymetry. This abnormal transmission losses and pulse arrival structures were explained by using ray theory.
It was found that there were three main kinds of rays. The first arrival pulse underwent one bottom reflection (1BR), the second arrival pulse underwent 1BR and one surface reflection (1SR) or 2SR, and the third pulse underwent 2BR and 1SR. The amount of rays that underwent 1BR for the environment with seamount was twice of the rays in flat bottom environment. Moreover, the rays undergoing same reflection had approximate amplitude and phase. Other rays which underwent more than 1BR could also reach to the receiver.
The convergence zone structure appeared in the deep water with a flat bottom environment might be destroyed by the direct blockage of seamount, and transmission losses increased more than 30 dB after passing the seamount. Numerical arrival pulse could roughly match with the experimental results. In fact, the geo-acoustic properties and slope bathymetry of the first seamount played an important role in the reflection. Small errors for these parameters in the model could cause the differences.
In the future, statistical approach will be used to explain some of sound propagation phenomenon after the seamounts.
Funding for the research came from the National Nature Science Foundation of China under Grant Nos 11434012 and 11174312.
References:
LI Wen, LI Zhenglin, ZHANG Renhe, QIN Jixing, LI Jun, NAN Mingxing. The Effects of Seamounts on Sound Propagation in Deep Water. Chinese Physics Letters (Vol. 32, No. 06: 064302, 13 May 2015). DOI: 10.1088/0256-307X/32/6/064302
Contact:
LI WEN
Institute of Acoustics, Chinese Academy of Sciences, 100190 Beijing, China
Email: cocolee.lw@163.com