Pushbroom Underwater Hyperspectral Imaging System For Deep Sea Exploration

Underwater hyperspectral imaging(UHI)is an extension of spectral imaging technology in underwater optical detection.UHI systems are usually deployed on underwater platforms such as submersibles and landers,so that interference from the water-air interface and the atmospheric environment can be avoided during imaging.By using the active light source,the UHI system can also break through the imaging depth limitation caused by the attenuation of the water columns and achieve high-precision,high-resolution observation and detection of seabed biogeochemical targets.At present,UHI technology has been used in marine ecological environment surveys and underwater archaeology in shallow water areas,and it has also played an important role in detecting deep sea minerals,hydrothermal vents,and cold springs.Under the impetus of the national ocean strategy,the deep sea has become a new focus for marine exploration due to its resource value and strategic significance.Therefore,it is necessary to carry out research on UHI technology for deep sea exploration.In this dissertation,we designed and developed a pushbroom UHI system for deep sea resource exploration.The solutions to stable imaging and autofocus for pushbroom systems in deep sea environments were proposed through module design and method research.Based on the designed pushbroom UHI system,the laboratory and deep sea field tests were conducted,and preliminary results were obtained.The main contents of this dissertation are as follows:1.The pushbroom UHI system for deep sea exploration was developed based on the modular design idea.The system consisted of the imaging,sensor,and data acquisition and control modules.The core imaging module adopted a combination of prism-grating-prism(PGP)type hyperspectral imager and color camera.According to low-power consumption and miniaturization requirements,the sensor module and data acquisition and control module were designed and developed.On this basis,the application program of the UHI system was designed to achieve automatic control and real-time communication with the submersible during underwater operation.The pressure vessel for the UHI system was designed through theoretical calculations and finite element analysis,and the integration of the UHI system was completed.After the calibration and hydraulic test,the spectral range of the designed system was 400 nm-700 nm,and the spectral resolution was 2.6 nm4.7 nm.The system was 152 mm in diameter and 426 mm in length,with a maximum operating depth of 6000 m.2.The built-in scanning module based on dual-drive scanning mode was designed according to the functional characteristics of the pushbroom UHI system,which achieved hyperspectral imaging with dual-drive scanning from external platforms and an internal rotational mirror.The built-in scanning module adopted primary and secondary mirrors,with a scanning field of view of 36°.The geometric deformation correction method was proposed through theoretical analysis to address the problems of geometric deformation in internal-drive scanning.The relationship between object plane coordinates and detector pixel coordinates was established,and the effectiveness of the geometric deformation correction method was verified through experiments.The designed built-in scanning module can reduce the pushbroom system’s requirements on platform stability,positioning and navigation accuracy to a certain extent.3.Aiming at the autofocus problem of UHI,an autofocus strategy for UHI based on the liquid lenses focusing transfer method was proposed.The color camera served as an auxiliary device to realize the focusing transfer between two liquid lenses of the color camera and hyperspectral imager.The relationship between focusing voltage and focusing distance was determined through theoretical analysis and calibration,and the focusing transfer coefficient between two liquid lenses was obtained.Based on the focusing transfer method,an autofocus experiment was carried out in the laboratory to verify the effectiveness of the autofocus strategy.The proposed autofocus strategy can achieve fast and accurate autofocus for underwater hyperspectral imaging without affecting the continuous acquisition of hyperspectral images.4.Based on the developed pushbroom UHI system,the laboratory and deep sea field tests were carried out.The detection process of deep sea minerals was simulated in the laboratory,and the hyperspectral data of ore samples were obtained.Several methods,such as support vector machine(SVM),spectral angle mapper(SAM),and convolutional neural network(CNN),were used to classify and identify ore samples,and some of them achieved good classification results.The deep sea field test was carried out in the Tianxiu hydrothermal field and Ninety east ridge of the Northwest Indian Ocean,verifying that the UHI system has a good condition in the deep sea environment.The hyperspectral data of the seabed sediments were obtained during testing.The main innovations of this dissertation are as follows:1.An internal and external dual-drive scanning method suitable for both moving and fixedpoint underwater operations was proposed,and a miniaturized built-in scanning module based on rotating mirror scanning was developed to realize 36° wide field of view scanning hyperspectral imaging on an underwater fixed platform.2.An autofocus strategy based on the liquid lenses focusing transfer method was proposed,which solved the autofocus problem of pushbroom underwater hyperspectral imaging by pre-focusing and focusing transfer coefficient calibration.3.A pushbroom underwater hyperspectral imaging system for deep sea exploration has been developed.The spectral range of the system covers 400 nm-700 nm,the spectral resolution is 2.6 nm-4.7 nm,and the maximum working depth is 6000 m.The system has been deployed on "Jiaolong" submersible to carry out the first domestic deep sea in-situ test based on underwater hyperspectral imaging technology in the northwest Indian Ocean.