Study on Sound-Speed Dispersion in A Sandy Sediment at Frequency Ranges of 0.5–3 kHz and 90–170 kHz

In order to study the properties of sound-speed dispersion in a sandy sediment, the sound speed was measured both at high frequency （90-170 kHz） and low frequency （0.5-3 kHz） in laboratory environments. At high frequency, a sampling measurement was conducted with boiled and uncooked sand samples collected from the bottom of a large water tank. The sound speed was directly obtained through transmission measurement using single source and single hydrophone. At low frequency, an in situ measurement was conducted in the water tank, where the sandy sediment had been homogeneously paved at the bottom for a long time. The sound speed was indirectly inverted according to the traveling time of signals received by three buried hydrophones in the sandy sediment and the geometry in experiment. The results show that the mean sound speed is approximate 1710-1713 m/s with a weak positive gradient in the sand sample after being boiled （as a method to eliminate bubbles as much as possible） at high frequency, which agrees well with the predictions of Biot theory, the effective density fluid model （EDFM） and Buckingham＇s theory. However, the sound speed in the uncooked sandy sediment obviously decreases （about 80%） both at high frequency and low frequency due to plenty of bubbles in existence. And the sound-speed dispersion performs a weak negative gradient at high frequency. Finally, a water-unsaturated Biot model is presented for trying to explain the decrease of sound speed in the sandy sediment with plenty of bubbles.

Study of A Geo-Acoustic Model of Gas-Bearing Sediment and Its Application in Sediment with Low Acoustic Veloctiy

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 Blot＇ 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.

Effects of sediment parameters on the low frequency acoustic wave propagation in shallow water

The integral expression for acoustic field due to a point source in shallow waterwhile sediment is either a liquid or a solid is derived. The synthetic full waveforms are simulatedusing real axis integration and FFF method. The effects of the seabed sediment parametersand center frequency of the source on the low frequency acoustic wave propagation in shallowwater are investigated. The conclusion is that the wave groups received in far field are thoseof the mode waves of the source center frequency. The possibility for inversely deducing thecompressional and shear sound speeds of sediment using the least square optimum through themeasured group velocities of a selected mode at different frequencies is discussed.

Theoretical analysis of acoustic suspended sediment observation

A theoretical analysis of designing an acoustic backscatter system for observations of suspended sediments has been made through a derived sonar equation. Some formulae and data for reference in determining several main technical parameters of the system, such as optimum frequency, sound transmission power, aperture of transducer etc., airs provided. And some key problems on the acoustic observation of suspended sediments, such as data processing of a time--Varying stochastic process, compensation of sound propagation loss in complicated conditions and in-situ calibration of sediment concentration, are discussed in detail. The results of the analysis could be helpful to the development and application of a specific acoustic backscatter instrument (e.g. the acoustic sediment concentration profiler, the acoustic Doppler current profiler etc.).