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The acoustic properties of seafloor coral sand sediments are of significant meaning for accurately modeling underwater sound propagation in sea areas around coral reef islands. To ultrasonically measure coral sand sediments with different grain sizes, sediment samples from coral islands were screened and remolded into five types of samples with different grain sizes. This study measured the sound speed and attenuation in the frequency range of 27–247 kHz and analyzed the frequency dependence of the acoustic properties. The results showed that the sound speed increased significantly with frequency, suggesting an approximately linear relation. The sound speed dispersion in the coarse-grained sediments was larger than that in the fine-grained sediments. The measured attenuation data showed a variable frequency dependence among the different sediment types. The measured data were compared with Biot-Stoll model (BS model) predictions, and the fitting results showed that intergranular pores can lead to sound speed data-model mismatches. The attenuation data-model comparisons revealed that sediments with different grain sizes have different sound wave energy loss mechanisms. The BS model with grain contact squirt flow and shear drag (BICSQS model) can match the attenuation data more consistently than the BS model. The results show that scattering is an important factor in sound attenuation in coral sand sediment and the attenuation scattering factors for different grain size sediments were provided. • Acoustic properties of coral sand sediments with different grain size were measured at the frequency of 27–247 kHz. • The measured velocity and attenuation present a positive dispersion with frequency. Attenuation scattering factors for different grain size sediments were provided. • Scattering is an important factor in sound attenuation in coral sand sediment. [ABSTRACT FROM AUTHOR]

Measuring the in situ acoustic properties of the sediments on continental slopes is essential not only for model construction but also for sedimentary environment and slope stability assessments. To acquire the acoustic properties of the fine-grained sediments on the northern slope of the South China Sea (SCS), this study conducted an experiment using the deep-water hydraulic-driven in situ sediment acoustic measurement system (HISAMS). There were four experimental stations in this study, one on the upper slope and the other three on the lower slope, the sound speed and attenuation coefficient values were obtained at different depths within 1.4 m below the seabed. The results showed extraordinarily low and vertically uniform acoustic properties in the fine-grained sediment on the SCS northern slope. Combined with other in situ data previously reported near the study region, the spatial distribution characteristics of the sediment acoustic properties were analyzed, and the results revealed a downslope variation trend and an along-slope uniformity. The distribution pattern of the sediment sound speed was related to the water depth, and the sedimentary environment might be the key constraint on the vertical variation feature of the sediment acoustic properties on the continental slope. • The acoustic properties in fine grained sediments were in-situ measured on the northern slope of South China Sea. • The sound speeds have a vertically uniform characteristics at different depth within 1.4m below the seabed. • The acoustic properties distribution trends are related to the water depth and the sedimentary environment. [ABSTRACT FROM AUTHOR]

Based on a substantial number of seabed sediment samples and corresponding acoustic characteristic and physical property test data collected in the continental shelf area of the northwestern South China Sea, this study employed a sound speed ratio correction model to analyze the correlation between the sound speed ratio of seabed sediments and parameters such as porosity, density, mean grain size, and water content. This study resulted in the development of both single-parameter and double-parameter empirical formulas for the sound speed ratio of seabed sediments in this marine region. The determination coefficients (r2) for the single-parameter empirical formulas based on physical properties, excluding the mean grain size (r2 ≤ 0.75), were all above 0.78, indicating a strong correlation. Additionally, the fitting curve at 100 kHz was slightly greater than that at 50 kHz, consistent with the acoustic dispersion characteristics of sediments. To assess the applicability of the empirical formulas to different maritime areas, the formulas derived from the South Yellow Sea, East China Sea, and South China Sea were compared with those of the study area. The results indicated that the empirical formulas for the sound speed ratio established in this study have general applicability, especially for sediments with low porosity, high density, and low mean grain size. In addition, the determination coefficient of the dual-parameter empirical formula for the sound speed ratio–porosity–mean grain size is superior to that of the single-parameter empirical formula for the sound speed ratio–mean grain size. Through comparative studies with other marine areas, it was found that there are certain differences in the relationships between porosity and mean grain size established in different marine areas. Therefore, based on the rough sphere model, this study transforms the GS model into a single-parameter model built on either porosity or mean grain size. The error analysis results indicate that the single-parameter GS model has a smaller error than the measured data. However, the GS model controlled by a single parameter performs poorly compared to the predicted curves of the sound speed ratio–porosity and the sound speed ratio–mean grain size fitted in this study. This discrepancy may be attributed to the fact that the input parameters of the GS model are reference values, emphasizing the need for further research to determine suitable input parameters for the GS model in this marine area. • The empirical equations for the sound speed ratio were developed in this study in different marine regions. • The established empirical equations for the study area were compared with those for the South Yellow Sea, East China Sea, and other marine regions. • This study compared the empirical equations with the grain shearing (GS) theoretical model. [ABSTRACT FROM AUTHOR]

Abstract The thermal stability and non-isothermal decomposition kinetics of the terpolymer poly (lactide-co-propylene carbonate) (PLAPC) were studied by thermogravimetric and derivative thermogravimetric analyses with multiple heating rates (2.5, 5, 10, 20 °C min−1) under nitrogen gas atmosphere. The microstructure and molecule thermal transition property of PLAPC were examined by 1H NMR, FTIR and differential scanning calorimetry (DSC) respectively. The decomposition kinetic parameters of PLAPC were calculated using "model-free" integral method (Flynn-Wall-Ozawa), "model-free" differential method (Kissinger) and "model-fitting" method (Tang). Its average activation energy thus obtained with different methods ranges from 96.62 to 104.91 kJ mol−1 12 kinetic models were employed to check the most probable decomposition mechanism. The results showed R2 model, F1 (A1) model and R3 model are the most probable to fit the decomposition mechanism. Highlights • Thermal decomposition parameters of poly(lactide-co-propylene carbonate) were evaluated using "model-free" differential method, "model-free" integral method and "model-fitting" method. • 12 comparative models were used to analyze the decomposition mechanism of poly(lactide-co-propylene carbonate). • R2 cylinder symmetry mechanism was considered as the most suitable for poly(lactide-co-propylene carbonate). • Tang method is the best to descript the decomposition kinetic of poly(lactide-co-propylene carbonate). [ABSTRACT FROM AUTHOR]

The frequency dependence of the sound speed and attenuation in seafloor sediments was measured in a laboratory by using a probe method at the frequency range of 25–250 kHz. The standard deviation of repeated sound speed measurement in fresh water is about 2.82 m/s. Repeated measurement of attenuation shows a minimum standard deviation of 0.58 dB at 25 kHz and a maximum standard deviation of 3.06 dB/m at 250 kHz. The measurement results indicate that the sound speeds vary from 1638 to 1658 m/s in sandy silt sediments and from 1548 to 1571 m/s in clayey silt sediments. The attenuation increases from 14.2 to 51.9 dB/m in sandy silt sediments and from 7.8 to 31.7 dB/m in clayey silt sediments. The measured sound speed dispersion in sandy silt sediment agrees reasonably with the predictions of the Biot-Stoll model, the Grain-Shearing model, and the Effective Density Fluid model (EDFM). The measured sound speed in clayey silt sediment agrees better with the predictions of the GS model than that of the Biot-Stoll model and the EDFM. The measured attenuation is more consistent with f 1/2 dependence than the f 1 dependence, and is similar to the predictions of Biot-Stoll model and the EDFM. [ABSTRACT FROM AUTHOR]

Highlights: [•] The in situ acoustic properties are acquired using a completely new apparatus. [•] Relations between acoustic properties and the geotechnical parameters are obtained. [•] Reasons contribute to the relations are analyzed. [•] Velocity shows good correlation with geotechnical parameters. [•] Attenuation shows weak correlation with geotechnical parameters. [Copyright &y& Elsevier]