浅海底质高光谱反射率测量系统的设计及应用

Design and Application of Reflectance Measurement System for Sea Bottom in Optically Shallow Water

海底光谱反射率是光学浅水中太阳辐射传输的重要组成部分,影响着海水表面离水辐亮度的光谱特性,因此底质光谱信息的准确获取对于浅海遥感工作的开展至关重要。专门设计了一套海底光谱反射率测量系统填补了国际上在这方面的空白。采用可自由伸缩并旋转角度的参考白板贴近目标物测量,以消除探头到目标物之间水体吸收衰减的影响,双光路采集系统同步测量的设计避免了水下光场迅速变化对辐射测量的影响。于2018年9月3日—8日,用该系统在三亚珊瑚礁保护区进行原位海底反射率测量试验,测量对象包括珊瑚、海草、泥沙、沙滩等多种底质。各底质类型之间具有光谱可分性,具体表现为,在波长大于580 nm的长波段,浅海沙子底质与岸上沙滩光谱反射率特征差异明显,表明相对于空气中,水体和水中微藻介质的吸收散射作用严重影响着水下光谱辐射的测量,证实了空气中测量的目标光谱不可替代水中的结果。珊瑚和水草的光谱反射率特征主要区别在于海草反射率光谱在540~600 nm波段有一个宽反射峰,而珊瑚的典型特征是在575,600和650 nm附近有三个特征反射峰。此外,珊瑚、沙子和沙滩三种碳酸盐质底质在395,430,490和520 nm存在反射峰,485和585 nm处有一个小吸收峰,而海草则相反,在395,430,490和520 nm存在吸收峰,485和585 nm处显示反射峰。以上数据为将来利用底质反射率提取底栖物质组成信息奠定了基础,同时其结果也能够证实系统的可靠性和有效性。

The spectral reflectance of the sea bottom plays an important role in radiative transfer signal of optically shallow water,affecting the spectral characteristics of water-leaving radiance in sea surface.Therefore,the precise information of the substrate is particularly significant in the study of the coastal remote sensing.In order to provide an accurate and convenient on-site information extraction of optically shallow water bottom,a set of sea bottom reflectance measurement system was designed,which is characterized by designing a reference whiteboard that can be freely stretched and rotated in order to eliminate the influence of the absorption attenuation of water between the probe and the target.And the design of the dual optical path for simultaneous measurement solved the interference of the spatial and temporal variability of water optical properties.Optical in-situ bottom reflectance of various substrates include corals,seagrass,sediment and beach collected in Sanya Coral Reefs Reserve during September 3—8,2018 were used to study the feasibility of the system.As expected,the various types of substrate have distinct spectral separability.The spectral reflectance feature of the sand bottom and onshore beach are different over the range 580~700 nm,which suggests that the absorption and scattering of water and the presence of microalgae strongly affect the measurement of under water radiation.The difference between seagrass and coral is obvious that there is positive reflectance feature at 540~600 nm of seagrass,whereas the typical characteristic of coral is that there are three positive features around 575,600 and 650 nm.In addition,the three carbonate substrates of coral,sand bottom and sandy beach have reflection peaks at 395,430,490 and 520 nm,and a small absorption peak at 485 and 585 nm,while seagrass has an absorption peak at 395,430,490 and 520 nm,a reflection peak at 485 and 585 nm.The above data results laid the foundation for the future extraction of benthic composition information,and also confirmed the reliability and real validity of the system design.