The acoustic characteristics of three fine-grained sediments (silty sand, silt, silty clay) in the South China Sea (SCS) were measured and analyzed at high frequency range of 27–247 kHz. The measurement results show that the sound speed dispersion is a positive linear relation at the measured frequency range, and the attenuation follows nonlinear frequency dependence, α = kfⁿ, where n ranges from 0.59 to 0.85 for the three different sediments in the SCS. The frequency dependence of sound speed and attenuation were compared with the published literature. It was found that for silty clay, clayey silt, silt, and silty sand, the dispersion characteristics of these four sediments are basically consistent; in general, the dispersion of coarse particles is significant, and that of fine particles is weak, and permeability is the key parameter that determines the inflection point of high frequency to low frequency. By modeling these sediments with the Biot–Stoll model, it was found that the Biot–Stoll model can better predict the frequency-dependent characteristics of sound attenuation in a high-frequency band under the matching constraints of sound speed dispersion characteristics, indicating that the Biot–Stoll model has good applicability to different types of sediments in a high-frequency band.

The acoustic properties of seafloor sediments on continental slopes play a crucial role in underwater acoustic propagation, communication, and detection. To investigate the acoustic characteristics and spatial distribution patterns of sediments on the continental slope, a geoacoustic experiment was conducted in the northwestern South China Sea. The experiment covered two sections: one crossing the shelf and slope in the downslope direction, and the other near the shelf break in the along-slope direction. In situ techniques, sediment sampling, and laboratory measurements were used to acquire data on sediment acoustic properties (such as sound speed and attenuation) and physical properties (including particle composition, density, porosity, and mean grain size). The experimental findings revealed several key points: (1) Acoustic properties of shallow water coarse-grained sediments and deep-sea sediments were higher when measured in the laboratory compared to in situ measurements. (2) Relationships between measured attenuation and physical properties, as well as between sound speed and mean grain size, showed deviations from previous empirical equations. (3) Sediment acoustic and physical properties exhibited significant variations in the downslope direction, while showing gradual variations in the along-slope direction. These variations can be attributed to sedimentary environmental factors such as material sources, hydrodynamic conditions, and water depth.

SUMMARY: The crustal structure is a window to understand the tectonic evolution of an area. Through the first large-scale passive-source ocean bottom seismometer (OBS) array observation experiment in the Eastern Subbasin of the South China Sea (ESSCS), we calculated and obtained the respective receiver functions of these stations. As OBS works on the seafloor, where the working environment is different from that for its land-based counterpart, the effects of the sea-water layer and the oceanic low-velocity sediment layer on the seismic signals must be evaluated. Receiver functions’ synthetic test demonstrates the following: the presence of the sea-water layer produces a strong negative-amplitude phase (Pwp) in the receiver function, and the arrival time of this phase is delayed as the thickness of the sea-water layer increases; the presence of the low-velocity sediment layer produces a strong positive-amplitude phase (Pseds) which causes a delay in the arrival time of the Moho-related phases of the receiver function and makes them difficult to distinguish. On the basis of phase identification and synthetic tests, we estimate crustal thickness beneath each of the stations using two approaches. The first approach utilizes the observed arrival time of Moho phases, and the second approach finds the optimal crustal thickness by comparing the synthetic and observed receiver functions, which leads to more reliable results. The results of the second method show that the thickness of the sediment in the study area is mainly controlled by seafloor topography, the thickness of the crust in the seamount area is affected by the magma supply during the expansion stage and the post-spreading magmatism, and the crust in the flat basin is only affected by the magma supply during the expansion period. Moreover, the crust in the area affected by magmatic activity after the expansion stage is thicker than that in the area not affected.

Based on multibeam bathymetric data and high-resolution shallow sub-bottom profiles acquired during China’s 10th Arctic Scientific Expedition Cruise in 2019, a sediment wave field was found on the western slope of the Chukchi Rise, in the Arctic Ocean. This sediment wave field developed on the lower slope with water depths of between 1200 m and 1800 m and stretched 15 km in the downslope direction. It comprised several parallel sediment waves, with wavelengths ranging from 700 m to 3400 m and wave heights from 12 m to 70 m. In the vertical direction, well-stratified deposits, tens of meters thick, were affected by sediment waves, which exhibit asymmetric waveforms and upslope migration trends. The morphological and seismostratigraphic characteristics of the sediment waves suggested their genesis as a result of the interaction between the bottom current and seafloor morphology, which was also supported by hydrographical data adjacent to the sediment wave field. It was infered that this bottom current was related to the Arctic Circumpolar Boundary Current, which many researchers suggest flows through the study area.