基于非线性声学效应的海洋气泡群参数测量与反演

There are many kinds of bubbles observed in the ocean, such as surface and sub-surface bubble population, the wake of vessels with lots of bubbles and so on. The bubbles (especially the bubbles in resonance) are one of most efficient sound scatters and have strong acoustic effects on the sound propagation and scattering. Meanwhile, the parameters of Bubble population, such as size, void Fraction, distributions and so on, in the ocean are of considerable importance in physical oceanography. So, It is necessary to note that how to measure and inverse the parameters of bubbles by using acoustical methods is the most hot topic in underwater acoustic and oceanography.With a wide range of high-power underwater source applications, more and more complex acoustical phenomena during the wave propagation, such as the appearance of sub-harmonic, higher harmonic and combined frequency components and so on, can not be described by the classical acoustic theory. Because the nonlinear acoustic propagation and scattering in oceanic bubbles can provide critical information only about the parameter of bubbles, it is important to study the bubbles parameters by using nonlinear acoustic methods. Compared with the measurement and inversion innovative method based on linear scattering and attenuation, we mainly study how to measure and inverse the bubbles information by using nonlinear acoustical methods.Based on the sound propagation and scattering characteristics influenced by oceanic bubbles, the core research in this project is to establish the theoretical system about acoustic measurement and inversion of oceanic bubble population. The results obtained by this research can improve the measurement accurate of oceanic bubbles, and can provide the innovative method for analyzing the influence on underwater acoustic propagation caused by surface and sub-surface bubble layers and detecting and identifying the underwater targets by the wake with lots of bubbles.

海洋中普遍存在大量的气泡及气核，它们主要出现在海洋表面附近以及舰艇和水下各类航行器的尾流中。由于气泡（尤其是处于共振状态的气泡）是一种强散射体，因此对水下声场声传播和水下声探测产生明显的影响。到目前为止，含气泡海水介质的复杂声学特性仍是海洋及水声领域中最为引人瞩目的课题之一。随着水下大功率声源的发展和应用，适用于小振幅声波的线性声学方法无法解释气泡非线性振动下的次谐波、高次谐波和组合新频率声波的出现及传播规律，而这些特征恰恰与气泡群参数紧密相关，亟待对有限振幅声波在气泡群中的非线性传播规律及相关的气泡群参数测量手段进行丰富和完善。本项目创新性利用气泡独有的非线性声学效应并发展相应的非线性测量手段，获取气泡群中尺寸、分布及孔隙率等重要参数，建立较为完整的海水气泡群参数测量与反演理论体系，为今后解决气泡群对海洋声传播的影响，以及以尾流为探测目标的水中目标探测与识别等提供基础理论支撑。

通过对海面气泡层和行船及水下航行体的尾流气泡群等的非线性声学特性进行研究，以获得气泡群对声传播及声散射特性的影响为基础，通过声学测量方法，反演气泡参数，建立起较为完整的海水气泡群参数声学线性及非线性测量及反演理论体系，为今后解决气泡群对海洋声传播的影响，以及以尾流为探测目标的水中目标探测与识别等提供基础理论支持和创新性方法。本项目的核心是声波在含气泡海水介质中的非线性传播及散射问题，研究中综合考虑声波在该复杂非均匀介质中的非线性效应、耗散效应、衍射效应、频散效应以及弛豫吸收等诸多因素，明确气泡的存在对声波传播衰减及新频率成分滋生的影响，基本掌握了有限振幅声波在该介质中的复杂声学特征及规律；相对的，有针对性的利用非线性声传播特性与气泡参数之间特有的紧密联系，综合利用有限振幅声波及恰当的气泡共振条件，更有针对性和有效性的捕捉的气泡群参数。本项目创新得到了含气核气泡层海水介质中非线性声场特性的振动-波动耦合模型，提出了基于Westervelt频域波矢量模型的声传播分析方法，对目前的含气泡海水介质中的非线性传播模型和方法进行了修正和完善。创新研究从有限振幅波理论出发，提取仅与气泡尺寸相关的非线性声场信息，作为反演的依据和模型，提出了基于非线性散射截面和非线性参数的非线性声学反演方法，有效提高在实际复杂海洋环境中的适用性。