基于视场可调节激光雷达探测海洋光学特性多参数垂直结构的关键问题研究

The advantage of Lidar is that it can obtain the high-resolution vertical structure of optical properties efficiently in the subsurface water. However, according to the Lidar theory, conventional Lidar can only measure a single parameter of optical properties, i.e. the absorption coefficient, the scattering coefficient or the diffuse attenuation coefficient, owing to the limitation of single field-of-view (FOV) and the inhomogeneous structure of water. There is still no relevant report on an oceanographic Lidar that can be utilized to measure multiple parameters of optical properties in multi-band. Thus, the main objective of this project is to address how to retrieve multiple optical parameters of seawater at multiple detection wavelengths by using backscattering of laser radiation and fluorescence received from multiple FOVs regarding multiple scattering. The study includes the following four aspects. 1) Based on multiple scattering distribution function, experiments will be implemented to study the quantitative relationship of FOVs with optical properties of a homogeneous water medium. 2) By comparing the measured data of seawater optical properties and the results of inversion model, the accuracy of multiple scattering model applied to oceanographic Lidar will be analyzed. 3) In an inhomogeneous water medium which can be considered as a mixture of homogeneous water layers, the scattering effect in adjacent layers will be used to retrieve optical properties by a recursive method. 4) After a deconvolution for removing the domain superposition of fluorescence, attenuation characteristics of fluorescence will be analyzed to retrieve seawater optical properties at fluorescence wavelengths. This project will contribute to understanding the mechanism for detecting the seawater optical properties by Lidar, and then this theoretical study is very important to oceanographic Lidar applications.

激光雷达的优势在于可快速遥感测量高分辨率的表层海水光学特性垂直结构，但受视场和非均匀水体的影响，根据海洋激光雷达理论仅实现了激光波长处单一参数的测量，多波长处多个海洋光学特征参数的激光雷达探测方法尚无报道。本项目采用视场可调节的探测方式，基于多次散射理论，结合弹性散射和荧光机制，研究激光及其诱导荧光在海水中的衰减特性，目的反演多波长处表层海水光学特征参数（吸收、散射和漫射衰减系数）的垂直分布。研究内容包括：基于多次散射分布函数，实验研究均匀水体情况下激光雷达视场和反演参数间的定量关系；通过实测和模型反演结果的对比，分析反演方法中准确的多次散射模型形式；分析非均匀水体中相邻水层间的散射影响，利用递推数值模型反演非均匀水体光学特性；通过退卷积去除水体荧光寿命造成的时域信号叠加影响，利用荧光衰减特性反演荧光波长处海水光学特征参数。本研究将丰富激光雷达探测的海水光学特征参数，为其应用奠定理论基础。

海洋光学特性，是海洋生态环境、渔业资源开发和水文灾害预警等方面的重要研究对象。目前，被动海洋光学遥感已无法满足人类社会活动对获取水表以下水体剖面上各种参数的需求。激光雷达的优势在于能够高效探测表层海水的后向散射回波，但存在探测波长较少的不足，因此，本项目基于多次散射理论，结合了激光与海水作用的弹性和非弹性散射机制，研究了多波长处表层海水光学特征参数的垂直分布反演方法。研究内容包括：基于多次散射分布函数，实验研究均匀水体情况下激光雷达视场和反演参数间的定量关系；通过实测和模型反演结果的对比，分析反演方法中准确的多次散射模型形式；分析非均匀水体中相邻水层间的散射影响，利用递推数值模型反演非均匀水体光学特性；通过退卷积去除水体荧光寿命造成的时域信号叠加影响，利用荧光衰减特性反演荧光波长处海水光学特征参数。.研究首先基于辐射传输理论，模拟水下光场的多次散射分布情况，并结合均匀水体的实验测量，分析激光雷达接收结构因子和反演参数间的定量关系。其次，对比涉及视场的多次散射模型和多次前向-单次后向散射模型，分析了这两种模型中多次散射因子的等效性，讨论了利用这两个模型反演水体光学特性参数的异同。另外，假设非均匀水体中相邻水层间的后向散射函数一致，利用递推数值模型反演非均匀水体光学特性剖面。最后，结合水体荧光信号退卷积方法，利用荧光衰减特性反演荧光波长处海水光学特征参数。本项目研究结果表明：对于固定水体的测量，海洋激光雷达衰减系数的变化与接收结构因子相关，利用涉及多次散射分布的物理模型，能够通过海洋激光雷达衰减系数反演水体的光学特性垂直分布，进一步结合激光诱导荧光机制，可拓展荧光波长处水体光学特性垂直分布的反演。本项目成果可以用于海洋光学卫星的辐射定标和数据检验、以及海洋环境的理化特征研究，能够为海洋光学研究中多个波长处海水光学特性的获取提供新的尝试，也为主动光学遥感探测海水光学特征参数的研究探索了一种新的方法。