Experimental observations of diurnal acoustic propagation effects in seagrass meadows on the Dongsha Atoll.

Large regions with seagrass meadows have been discovered on Dongsha Atoll located in the South China Sea. The aim of this study is the estimation of acoustic propagation loss in the frequency range of individual bubble resonance due to the gas content of seagrass in seagrass meadows. Specifically, this study seeks to (a) investigate the attenuation of the mid-frequency (2-8 kHz) signal induced by the gas content in a seagrass meadow and (b) present the diurnal variation of sound attenuation. The results demonstrate that >40 dB diurnal variations of mid-frequency propagation loss was observed in shallow seagrass meadow region.

Resonant interaction of acoustic waves with subaqueous bedforms: Sand dunes in the South China Sea.

The large subaqueous sand dunes in the South China Sea are expected to produce the coupling of energy between acoustic normal modes. In this letter, resonant interaction between acoustic propagating modes and subaqueous bedforms are numerically investigated as a function of bedform wavelength, acoustic frequency and bedform packet length. The results demonstrate that bedform wavelength impacts acoustic mode coupling behavior, with the principal transfer of energy occurring between acoustic modes whose eigenvalue difference is equal to the peak value in the bedform wavenumber spectrum. The observed effect of wavelength is greater than that of acoustic frequency and bedform packet length.

Normal incidence measurement in a subaqueous sand dune field in the South China Sea.

Regions with subaqueous sand dunes have been discovered on the upper continental slope of the northern South China Sea. These large subaqueous sand dunes are expected to cause errors in the measurement of normal incidence reflection. This letter presents experiment results of two normal incidence survey tracks conducted in 2013, and the errors in reflection coefficient estimation and the resulting sediment properties induced by sand dune bedforms. The results demonstrate that the reflected energy is focused and scattered by different parts of sand dune bedforms and that they produce significant variation in the estimated reflection coefficients and the inverted geoacoustic properties.