Using remote sensing to detect the polarized sunglint reflected from oil slicks beyond the critical angle

Abstract

The critical angle at which the brightness of oil slicks and oil‐free seawater is reversed occurs under sunglint and is often shown as an area of uncertainty due to different roughness and surface Fresnel reflection parameters. Consequently, differentiating oil slicks from the seawater in these areas using optical sensors is a challenge. Polarized optical remote sensing techniques provide complementary information for intensity imagery with different physical properties and, thus, possess the ability to resolve this difficult problem. In the polarized reflectance model, the degree of linear polarization (DOLP) of sunglint depends on accurately knowing the Stokes parameter for the reflected light, and varies with the refractive index of the surface layer and viewing angles. For the polarized detection of oil slicks, the highest sensitivity of the DOLP to the refractive index is located within the specular reflection direction where the sum of the solar and sensor zenith angles is 82.6°. The modeled results clearly indicate that the DOLP of oil slicks is weaker in comparison with oil‐free seawater under sunglint. Using measurements from the space‐borne Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) over the Deepwater Horizon oil spill in the Gulf of Mexico, we illustrate that the PARASOL‐derived DOLP difference between the oil spill and seawater is obvious and is in accordance with the modeled results. These preliminary results suggest that the potential of multiangle measurement and feasibility of deriving refractive index of ocean surface from space‐borne sensors need further researches.

Modeling Atmosphere-Ocean Radiative Transfer: A PACE Mission Perspective

“…Remote sensing observations of sunglint can therefore provide information on aerosol properties (Kaufman et al, 2002;Ottaviani et al, 2008Ottaviani et al, , 2013 and on ocean surface slicks when polarimetric measurements are available (Ottaviani et al, 2012). Similar results for oil slick properties were subsequently obtained by Lu et al (2017) from spaceborne polarimetric observations by the POLDER (Polarization and Directionality of the Earth's Reflectances) instrument.…”

“…These reductions in measurement capabilities lead to a decrease in the retrieval accuracy of surface refractive indices from sunglint DoLP data, even in absence of aerosol scattering. Nevertheless, DoLP measurements of sunglint that were obtained by POLDER at a coarser accuracy (∼1%) in the NIR (870 nm) for ≤ 14 viewing angles have still proven useful in distinguishing between surface refractive indices retrieved for clean ocean waters and for oilslick covers (Lu et al, 2017). Hence, while PACE DoLP measurements of sunglint will not lead to the same retrieval accuracies for surface refractive index as RSP, they will likely contain information that can be used to not only detect and but also identify substances covering large sections of the ocean surface.…”

Degree of Linear Polarization of Land Surfaces: Analyses Using POLDER/PARASOL Measurements

“…In this case, this study provides complementary information to compensate or enhance normal non-polarized remote sensing on the earth surface. For example, remote monitoring and classification of crop structure parameters [7], sunglint detection on oceanic oil slicks [32] , and detection of land surface types with varying contamination [33] have been proven effective using DOLP.…”

Harnessing remote sensing to address critical science questions on ocean-atmosphere interactions

“…The presence of natural or anthropogenic surfactants in the microlayer therefore affects both the intensity and the polarisation state of the sun-glint signal. Recent studies showed that space-borne polarimetric measurements of sun glint can be used to estimate the presence of oil slicks from the change in refractive index between oiled and oil-free waters (Ottaviani et al, 2012;Lu et al, 2017). However, further studies are required to demonstrate whether this method could be applied to the detection of natural surfactants in order to provide estimations of the surfactant distributions and their impacts on the air-sea exchange processes at the global scale.…”