A new geo-acoustic model for gas-bearing sediment is proposed based on the work of Dvorkin and Prasad,and Biot theory.Only five geophysical parameters:sediment mineral composition,free gas saturation,tortuosity(also known as the structure factor),permeability,and porosity,are considered in the model.A benefit of this model is that we need only five parameters instead of ten parameters in the Biot's formulas for acoustic velocity and attenuation calculation.Here the model is demonstrated with the in-situ experimental data collected from the Hangzhou Bay,China.The results of this study suggest that free gas content in sediment is the most critical condition resulting in a low acoustic velocity(compressional wave).The respective contributions of the other four parameters in the model are also discussed.
Compared with the laboratory acoustic measurement of sediment samples,the in-situ acoustic measurement in marine sediment is considered more accurate and reliable,because it covers all of the surrounding environment factors and avoids the disturbance during the course of sampling and transporting of sediment samples.A new multi-frequency in-situ geo-acoustic measurement system (MFISGAMS) has been developed.The system can provide acoustic velocity (compressional wave) and attenuation profiles of the uppermost 4～8 m sediment in the seafloor.It consists of 8 channels with 12 frequencies (multi-frequencies) and 0.5～2 MHz sampling rates.The data collected can be transmiuted in real-time.Associated with inclinometer and altimeter,it can provide the data for depth emendation.Acoustic velocity and attenuation data have been obtained from two in-situ experiments conducted in the Hangzhou Bay.
