Experimental Study On Polarized Optical Remote Sensing Of Marine Oil Spills Under Sunglint

Marine oil spills cause great damage to ocean ecological environment,human health and social development.According to oil spills weathering process,oil spills visual characteristics and optical response process,marine oil spills detection targets for optical remote sensing are black oils,oil films and oil emulsifications.Microwave Radar remote sensing,optical remote sensing,thermal infrared remote sensing and laser-induced fluorescence remote sensing are broadly studied in marine oil spills detection,and have demonstrated their own characteristics.Polarized optical remote sensing is a new earth observation technology,which has been used for detection,recognition and quantitative inversion of aerosols,clouds and water bodies,and it can also provides new ideas and solutions for marine oil spill monitoring.The use of polarization technology can enhance the reflectance contrast between oil and background seawater,but there lacks of a systematic research on marine oil spills polarization detection mechanism,model construction,application and analysis of satellite-borne polarized remote sensing of marine oil spills.Sunglint can be seen at the direction where the incident sun light is reflected off the ocean surface,and there exists obvious polarization characteristic difference between the sunglint off the oil surface and the ocean surface,which implies the physical property differences between oil and seawater.Based on marine oil spills polarization detection mechanism,a marine oil spills sunglint polarization model is built,and an indoor simulation experiment is designed to test the model.Multi-angle polarized optical remote sensing images of a typical oil spill accident are selected for analysis of multi-angle,multi-spectrum polarized optical remote sensing of marine oil spills.The main contents are as follows(1)Based on polarization optics theory,Cox-Munk model,and Fresnel's law,the marine oil spill sunglint polarization model is built.The model is used to simulate the degree of polarization(DOP)of sunglint reflected from seawater and oils'(with different refractive indexes)surface under different observation geometries(solar zenith,sensor zenith and relative azimuth).The optimal detection geometry for oil spills polarized optical detection is analyzed from the simulation.Critical angle exists in optical remote sensing images.The model is used to simulate DOPs of oil and seawater surface's sunglint and evaluate the ability of polarized optical detection of marine oil spills in critical angle area.(2)An indoor multi-angle polarization CCD imaging experiment is designed to test the results from the model simulation and analysis,in which a full-range tungsten lamp with a narrow-band filter is used to simulate quasi-monochromatic parallel unpolarized light(635 nm),an array CCD is used to detect multi-viewing-angle(viewing zenith equals light source zenith,both of which range from 15° to 55°,with an interval of 5°)mirror-reflected radiance in different polarization directions,and Stokes vector(I,Q,U)as well as DOPs are calculated from those radiances.That DOPs of the light reflected off the water surface and oil film and the differences in DOP between them change with angles is analyzed to verify the results of the model analysis.(3)Based on PARASOL remote sensing images,analysis of multi-angle optical polarization remote sensing of marine oil spill was carried out for a typical marine oil spill accident.The coincidence trends of model simulation and satellite observation under the conditions of sunglint and non-sunglint were tested to assess the simulation performance of the constructed model.Analyze the differences in polarization characteristics between marine oil spills and background seawater at multiple angles,clarify the detectability of polarized remote sensing of oil spills under sunglint,and assess the influence of atmospheric scattering on multi-angle optical polarization remote sensing of oil spills.According to the research described above,the main findings are as follows.(1)The marine oil spill sunglint polarization model is applicable to where there exists sunglint while not applicable to none-sunglint conditions.(2)DOPs of oil are lower than that of background seawater.DOPs of oil spills and seawater as well as difference between them are sensitive to angles.DOPs of oil spill and seawater contrasts are most obvious when the sum of solar zenith and sensor zenith is 82.6° with the relative azimuth of 180°.Such a geometry is the optimal polarization detection angle for oil detection.(3)Oil spills can be detected by polarized optical remote sensing under sunglint.And the stronger the sunglint,the better detection performance.(2)Polarized optical remote sensing demonstrates its detectability in marine oil spills detection at critical angle area.