Estimating Geoacoustic Properties of Surficial Sediments in the Northern South China Sea

Abstract

To clarify the acoustic propagations and seabed properties in the northern South China Sea, the South China Sea Upper-Slope Sand Dunes Experiments were conducted in 2013-2014. To estimate the geacoustic properties of the surficial sediments in the experimental area, geoacoustic inversion with chirp sonar data, predictive model, and matched field technique were applied and compared with one another. Firstly, a predictive model based on sediment samples and the Biot theory with fluid approximation, the effective density fluid model (EDFM), was presented and utilized. The sediment samples were collected using a Shipek sediment sampler and were analyzed using a GEOTEK Multi-Sensor Core Logger and a Beckman Coulter LS particle size analyzer. The relationship between porosities and mean grain sizes in this area with shoaling internal waves has been discussed and compared to the Bachman's regression equation for SAX99. Propagating shoaling internal waves with huge amplitudes observed in the medium over the slope, shelf break, plateau, and continental shelf in the South China Sea are expected to cause strong amplification of near - bottom velocities. Hence, it may cause movement of an exposed bed particle on the bottom sediment, resulting in different grain-packing configurations. The measured porosities and mean grain sizes are inputted to the geoacoustic model to estimate the properties of the surficial sediments. The estimated bulk density is verified by core sample analysis, and the estimated sound speed and attenuation are verified by the match field geoacoustic inversion using the acoustic propagation data. Furthermore, the regression between the measured porosities and mean grain sizes is utilized in the geoacoustic inversion model, and the geoacoustic properties along the chirp sonar survey track are obtained.