The Research On Investigating The Properties Of Sound Absorption And Sound Scattering In Turbid Seawater

There exist a lot of suspended sediment particles in the seawater close to the most part of the coast of our country, since the sediment carried by the rivers pours into the sea. This kind of seawater is also characterized as turbid seawater. The domestic ocean specialists have been doing a lot of research on offshore turbid seawater. But their research is mainly done on the process of suspended sediment particles' deposition and transport. Only a little research has been done on the properties of sound absorption and sound scattering in turbid seawater. The sound absorption is one of the reasons why sound energy attenuates, when sound wave propagates in turbid seawater, the other reason is sound scattering. The paper is focused on investigating the properties of sound absorption and sound scattering, namely, how much suspended sediment particles has the effect on sound absorption and sound scattering in the seawater. It is very useful to model sound propagation in shallow water, design the sonar system, and detect the submerged objects, etc.Based on the basic principle of sound absorption, the composition of the seawater, and measurement method of sound absorption, the method of investigating the property of sound absorption in turbid seawater was settled down. Moreover, sound field of cylindrical resonator (reverberation barrel, hereafter) was investigated. The eigenfrequency, the distribution of normal-mode frequency, and the transient effect, namely, reverberation, of the barrel were illustrated. The basic principle for measuring sound absorption coefficient of water medium by the barrel was given. The effect of air-bubble and transducers on sound absorption, as well as the method of measuring reverberation time on the measurement error had been verified by the experiment. The effect of temperature, salinity and pressure on sound absorption in turbid seawater was investigated by the simulation. After that, a system for measuring sound absorption was designed and built up in laboratory, which had complete design of vacuum degassing, sound isolation, vibration isolation, temperature isolation, etc. The maximum measurement error was no more than 15%, after the system was calibrated by standard solution. The measured frequency could be lowered at 20 kHz. The property of sound absorption in turbid seawater between 20～60 kHz, of which the concentration was in the range from 40 to 490 mg/L, was measured by the reverberation method. It filled up the investigation of sound absorption property in turbid seawater at low frequency and low concentration of suspended sediment particles. It was investigated that when the concentration of suspended sediment particles is less than 110 mg/L, the coefficient difference between turbid seawater and clear seawater is very little, sound absorption in turbid seawater is mainly caused by the electrolyte, magnesium sulfate. If the concentration of suspended sediment particles is above 140 mg/L, sound absorption caused by suspended sediment particles is remarkable, of which maximum coefficient is twice as that in clear seawater. The results show that the coefficient of sound absorption in turbid seawater is approximately proportional to the concentration of suspended sediment particles and the square of the frequency.The shape of suspended sediment particles in turbid seawater is so complex that the coefficient of sound absorption can't be calculated directly and accurately. The method, that the coefficient of viscous sound absorption was calculated by median diameter of suspended sediment particles, was proposed. And the coefficient of the electrolyte's relaxation sound absorption could be calculated by clear seawater's sound absorption formula. The total sound absorption coefficient calculated by the combination of these two methods agrees very well with that measured by reverberation method. So, if the salinity, temperature, median diameter of suspended sediment particles, etc, are known, the coefficient of sound absorption in turbid seawater could be predicted by the combination of clear seawater's sound absorption formula and the proposed method of viscous sound absorption calculation.Generally speaking, sound scattering along anti-incident direction is characterized as volume reverberation, which is the main background interference to active sonar system. However, the volume reverberation measurements can't be fulfilled by conventional underwater acoustic equipments in lower frequency bands, due to the characteristic of shallow water environment and the property of volume reverberation. Because there are no programmable switch and time-variable-gain device in conventional underwater acoustic equipments, circuit obstruction usually happens in the measurements. In order to solve the problem, a data acquisition system, composed of analog receiving board, control board and PCI bus communication board, was developed. Then, a system for measuring sound scattering was designed and built up. The system had been tested in the sea area outside Yangtze River estuary, China. The law is firstly summarized that volume backscattering intensity changes with frequency, the concentration of suspended sediment particles, turbidity, etc. The results demonstrate that volume backscattering intensity might be changed by 30 dB, due to the variations of suspended sediment particles'concentration and temperature.