In Situ Acoustic Experiment And Properties Study In Marine Sediments

The in situ acoustic experimental technology, principle and propagation features inmarine sediments are described in this dissertation. This research includes the designand developing of the new in situ acoustic instrument, the experiment, engineeringanalysis, data processing, comparison of the different acoustic properties of USA andChinese sediments, multi-parameters inversion of Biot-Stoll model and BISQ model,and the finite-difference modeling.Supported by 863 project, a real time control, multi-frequency and multi-depth in situacoustic system was developed with intellectual asset in the form of copyright. It hasreal time monitoring and on deck power supply, multi-point high resolutionmeasurement functions.With four frequencies, which are 8, 10, 12 and 15 kHz, in situ acoustic data have beenacquired in 8 different depths at Zhapu bay, Zhejiang, China. By using antitheses, newtechnology of getting velocity and attenuation was used. Velocity dispersion in Zhapusediments is observed. The velocity and attenuation increase as the frequencyincreases. A linear relation between frequency and velocity as well as attenuation isfound while water depth less than 2 meters and frequency below 15 kHz. Thesediment is so soft that the attenuation is low. The dominate frequency in sediment is300Hz lower than that in water because of attenuation.In situ compressional wave velocity and attenuation measurements were made at 2carbonate sediment sites over a frequency interval between 20 and 100 kHz. Velocitydispersion, while slight, appears in the data. Effective attenuation ranges from 15 to75 dB/m at H3 and from 22 to 62 dB/m at H4. Parameters determined by coreanalysis were fixed in Biot-Stoll models and inversion of velocity and attenuation datawas used to evaluate the 6 un-measurable parameters. An increase in the lowfrequency logarithmic decrement well above Stoll's recommended values allowed fitsto both velocity and attenuation very well.The multi-parameter BISQ inversion method is achieved for Zhapu marine sediments.It is found that the BISQ model could be used to describe the marine sediment'scharacteristics of velocity dispersion and attenuation.The lnα=alnf~b relationship for attenuation and frequency is approved in marinesediments. The slope b divides into three segments: b≈1 for higher frequency (f > 40kHz); 0≦ b≦0.5 for intermediate frequency (16-18 kHz); b≈2 for lower frequency(<16 kHz). The constant a depends on the characteristics of the sediments.By comparing the high-order staggered finite-difference modeling based on the Biotmodel, with the in situ wave characteristics, it is indicated that the system is welldesigned in avoiding head wave inference and sound absorbing. This modeling hassignificance for upgrading the system.