Simultaneous estimation of the refractive index and thickness of marine oil slick from the degree of linear polarization of the sun-glint reflection

Abstract

—Airborne and spaceborne optical remote sensing is an important means for monitoring oil slicks on ocean surface. However, it is still a major challenge to determine both the category (related to a specific value of reflective index) and thickness of the marine oil slick with existing methods, particularly when the oil slick is too thin to obtain significant fluorescence signal with a laser induced fluorescence method. Sun-glint is usually harmful to optical remote sensing of an ocean target. In this work we utilize the polarized sun-glint reflection to monitor oil slicks on a rough ocean surface. The degree of linear polarization (DOLP) of the sun-glint reflection contains the characteristics information of the oil slick with different physical properties. Combining the polarized optical remote sensing and the inversion theory based on a thin-film optical model, we analyze the variation trend of the DOLP with the parameters of solar zenith angle, sensor zenith angle, relative azimuth angle, refractive index and thickness of the oil slick. Different types and thicknesses of the oil slicks give different Fresnel’s reflection coefficients of polarized sun-glint reflections and consequently different Stokes parameters, which lead to different DOLP. We analyze the DOLP of the sun-glint reflection at the wavelength of 532 nm, and determine simultaneously the refractive index and thickness of marine oil slick from the DOLP values measured by a remote detector at two different zenith angles.