Inversion of the refractive index of marine spilled oil using multi-angle sun glitter images acquired by the ASTER sensor

Oil spills in the ocean pollute and harm the marine environment. Remote sensing has been widely used to examine and analyze marine oil spills. Refractive index is an important parameter for monitoring, evaluating, and mitigating marine oil spills, but its quantitative detection through remote sensing is still in the exploratory stages. In this study, we developed a quantitative inversion model at a pixel-scale to determine the equivalent refractive index (ERI) of the sea surface. The proposed model involved two important steps. First, we performed simulation experiments and found that the multi-angle Fresnel reflection coefficient ratio is not sensitive to the refractive index, which was further verified via real image geometry. Second, we proposed a sun glitter (SG) correction method by reanalyzing wind data and seawater refractive index in the background seawater area as constraints. High-resolution multi-angle sun glitter (MSG) images of the oil spill in the Gulf of Mexico acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer sensor were used to estimate the ERI. The ERI images with the resolution same as that of the original images (15 m) exhibited significant spatial variability. The estimated ERI exhibited a reasonable range as compared to the existing oil spill refractive index data in relevant literature, which were derived from laboratory measurements and through remote sensing inversion. Finally, the sensitivities of the SG correction model and multi-angle Fresnel ratio model were discussed. The results show that the MSG images can estimate high-resolution ERI of sea surface that plays a significant role in the observations of marine oil spills.