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

1.理论分析利用OASS(Biot-Stoll模型)计算了海洋表层沉积物过渡层反射系数的频率特性。在此计算中，过渡层被划分为200层，这与连续层非常接近。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深度的变化而变化。2.水槽试验以清水港采集的泥沙为泥沙模型。这个模型被放置在圆柱形水箱的底部。采用的是谐振频率为500 kHz的宽带压电换能器。反射信号的傅里叶变换形成水-泥沙模型界面和水-空气界面。利用这些变换结果，得到了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计算结果基本吻合。3.现场实验首先，设计并制作了原位声反射测量系统。现场试验在清水港进行。测量系统是从一艘小船上取下来放在海底的。采用的是谐振频率为100 kHz的宽带压电换能器。用GS-型岩心取样器从沉积物顶部采集每个1厘米长的样品，测量孔隙度。结果表明，反射系数频率特性的测量结果与计算结果基本一致，说明利用反射系数频率特性的测量结果估算孔隙度随深度变化的剖面是可行的。

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.