Seafloor sediments of different geographical areas in the southern South China Sea (continental shelf, continental slope, and Okinawa Trough) were gravity cored at 21 locations. Sound velocities (V) of the samples were measured at 15-cm increments immediately upon retrieval, and porosity, wet bulk density, and mean grain size were measured later in the laboratory. Empirical equations from previous studies were applied to predict V of sediment samples from the measured physical properties and it was found that the sound velocities derived from the existing equations did not closely match the measured sound velocities. Therefore empirical equations were reconstructed based on the measured data that represent the relationships between physical and acoustic properties of the different geographical area in the study area. Possible explanations for the discrepancies between the measured data and those of previous studies were investigated and found that physical properties, sediment types, geographical area, etc. are important factors that influence sound velocity. The empirical equations of this report should be preferred for prediction of sediment sound velocity for high-frequency acoustic experiments.

Based on the core samples collected in different geological tectonic units in the southern South China Sea, the double parameter equations between sediment sound speed and physical parameters were developed. According to the double parameter equations, the influences of physical parameters (porosity, mean grain size, wet bulk density, and clay content) on sediment sound speed were analyzed. To analyze the sensitivity of each parameter in the double parameter equations, the error norm method was introduced. The most influencing physical parameters with relative order were analyzed using the error norm method. These results show that porosity has a major influence on the sound speed, and the most influential physical parameters on sound speed are as follows: porosity > wet bulk density > clay content > mean grain size.

The Drag-type In-Situ Acoustic Measurement System (DISAMS) is a new instrument designed for rapid measurement of seabed sediment acoustic parameters, including the sediment sound velocity and acoustic attenuation coefficient. The DISAMS consists of six independent acoustic probes arranged symmetrically such that each side has one transmitting probe and two receiving probes. The entire operation is controlled and monitored in real time by a deck control unit on board, and the center frequency of the DISAMS is 30 kHz. The DISAMS can record full waveforms to determine the sound velocity and attenuation coefficient in the seabed sediment. In addition to site measurements, the DISAMS can also carry out towing measurements, resulting in improved efficiency compared with existing in-situ acoustic measurement systems. This article presents the configuration, electronics, and tests of the DISAMS in detail. Laboratory tests were conducted in a sediment pool, sea trials were carried out in the Zhoushan Islands, and sound velocity and attenuation coefficient data were obtained. The test results demonstrated that the DISAMS performed well and was able to measure the sound velocity and attenuation coefficient rapidly and accurately in both site measurements and towing measurements.