【作者】 刘永伟；

【导师】 李琪；

【作者基本信息】 哈尔滨工程大学 ， 水声工程， 2011， 博士

【摘要】 在近岸海域,由于沿海和港湾附近的江河携带大量泥沙的注入,使得海水中悬浮有大量的泥沙颗粒,此类海水也称为混浊海水。海洋界曾对近岸混浊海水进行过大量的调查研究,但多是针对悬浮泥沙颗粒的沉积、输运过程等机理研究,对其声吸收和声散射特性的研究相对较少。声波在混浊海水中传播时,声吸收是引起声能衰减的主要原因之-,而声散射则是另外的一个原因。本文主要研究混浊海水的声吸收与声散射特性,即悬浮泥沙颗粒对海水声吸收和声散射特性的影响,这对浅海声传播建模、声纳系统设计、沉底目标探测等都是十分有意义的。本文从声吸收基本原理、海水化学组成、声吸收测量方法出发,以明确如何对混浊海水声吸收特性进行实验研究。其次研究圆柱形共振器(以下简称混响桶)内的声场,理论上阐明了混响桶的本征频率、简正频率的分布以及瞬态效应(混响)。研究了混响桶测量水介质声吸收系数的基本原理。利用实验研究气泡、传感器对声吸收测量的影响,以及混响时间的测量方法所引起的声吸收测量误差,仿真计算了温度、盐度、压强对混浊海水声吸收的影响。在此基础上,设计并构建了一套混响法声吸收测量实验系统,具备完善的真空除气、隔声、隔振、保温等设计。经标准溶液校验,测量误差最大不超过15%,且测量频率可低至20kHz。测量了悬浮泥沙颗粒浓度为40-490mg/L混浊海水在20～60kHz的声吸收系数,填补了低频段、低悬浮泥沙颗粒浓度混浊海水声吸收研究的空白。当混浊海水中悬浮泥沙颗粒的浓度小于110mg/L时,混浊海水的声吸收系数与纯净海水(盐度35‰)的声吸收系数差别较小,混浊海水中引起声吸收的主要成份是电解质硫酸镁；当悬浮泥沙颗粒的浓度超过140mg/L时,悬浮泥沙颗粒引起的声吸收已非常明显,混浊海水的声吸收系数最大为纯净海水声吸收系数的2倍。实验结果表明：混浊海水的声吸收系数近似与悬浮泥沙颗粒的浓度成正比,与频率的平方成正比。由于混浊海水中悬浮的泥沙颗粒形状极其复杂,使得直接、准确计算混浊海水的声吸收系数几乎是不可能的。提出了采用悬浮泥沙颗粒的中值粒径去计算其粘滞声吸收系数的方法。而利用纯净海水声吸收公式可计算混浊海水中电解质的弛豫声吸收系数。利用该方法计算的混浊海水的声吸收系数与混响法测量的结果符合较好!因此,若已知混浊海水的盐度、温度、悬浮泥沙颗粒的中值粒径等信息时,则可利用纯净海水的声吸收公式和采用悬浮泥沙颗粒的中值粒径计算其粘滞声吸收系数的方法来预报混浊海水的声吸收系数。通常把逆入射方向的声散射称为体积混响,它是主动声纳的主要背景干扰。由于浅海环境的特征及体积混响的特性,很难利用常规的水声设备进行低频段体积混响的试验测量。这是因为常规的水声设备一般不具备程控开关和时间可变增益器件,极易造成水声设备的电路阻塞。针对这一问题,研制了一套由模拟接收板、控制板和PCI总线通信板组成的数据采集器。在此基础上,设计并构建了一套混浊海水声散射特性测量实验系统。该系统在长江口外海域进行了试验。首次揭示了混浊海水的体积反向散射强度随频率、悬浮泥沙颗粒浓度、浊度等的变化规律,发现体积反向散射强度随悬浮泥沙颗粒浓度、温度的改变可有高达30dB的变化。更多还原

【Abstract】 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.更多还原