Study on the acoustical measurements of surficial marine sediments using acoustical reflection

1.理论分析利用OASES（Biot-Stoll模型）计算了表层海洋沉积物过渡层反射系数的频率特性。在计算中，过渡层被划分为200层，这是很好的近似连续层。从计算结果可以看出，截止频率和反射系数对频率的斜率随孔隙度的深度依赖性的变化而变化。2.水槽实验在清水港采样的淤泥被用作沉积物模型。该模型设置在圆柱形水箱的底部。使用了谐振频率为500 kHz的宽带压电换能器。对水-沉积物模型界面反射信号和水-空气界面反射信号进行傅立叶变换。利用这些变换结果，得到反射系数的频率特性。实验结果表明，反射系数频率特性的测量结果与理论计算结果基本吻合。3.原位实验首先，进行了原位声反射测量系统的设计和制作。在清水港进行了现场实验。测量系统是从一艘小船上取下来，放在海底的。使用了谐振频率为100 kHz的宽带压电换能器。在使用GS型岩心取样器取样后，从沉积物顶部开始测量每个1cm长的样品的孔隙度。反射系数频率特性的实测结果与计算结果基本一致，表明利用反射系数频率特性的实测结果估算孔隙度随深度的分布是可行的。

1.Theoretical analysesThe frequency characteristics of the reflection coefficients form the transition layer of surficial marine sediments were calculated using OASES (Biot-Stoll model). In this calculation, the transition layer was divided into 200 layers which was good approximation to the continuous layer. From the calculated results, it was seen that the cut-off frequency and the slope of the reflection coefficients to the frequency varies with the variation of the depth dependence of the porosity.2.Tank experimentsSilt sampled at the Shimizu harbor was used as a sediment model. This model was set in the bottom of the cylindrical tank. The wideband piezoelectric transducer with the resonant frequency of 500 kHz was used. The Fourier transforms of the reflected signal form the water-sediment model interface and that from the water-air interface. Using these transformed results, the frequency characteristics of the reflection coefficients were obtained. It was seen that the measured results of the frequency characteristics of the reflection coefficients was almost agrees with the calculated results.3.In situ experimentsFirst, the design and fabrication of the system for the in situ acoustic reflection measurements were performed. In situ experiments were performed in the Shimizu harbor. The measuring system was taken down from a small ship and set on the sea bottom. The wideband piezoelectric transducer with the resonant frequency of 100 kHz was used. The porosity was measured for each 1-cm-long sample from the top of the sediment, after taking sample using GS-type core sampler. It was seen that the measured results of the frequency characteristics of the reflection coefficients was almost consistent with the calculated results.From the above-mentioned results, it was indicated that the estimation of the porosity profile with the depth using the measured results of the frequency characteristics of the reflection coefficients are possible.