Spectral radiative properties of seawater-in-oil emulsions in visible-infrared region

Abstract

Optical identification, classification and quantification of different oil slicks play important roles in oil-slick monitoring and detection. Under the actions of wind, waves and other marine forces, oil slicks on the sea surface mix with seawater to form oil-seawater emulsions. Herein, we focus on the spectral radiative properties of seawater-in-oil emulsions in the visible-infrared region. The complex refractive indices of crude oil are precisely measured using the combined ellipsometry-transmission method, after which the radiative properties of seawater droplets embedded in crude oil and seawater-in-oil emulsions obtained using the traditional and improved Lorenz-Mie theory are compared and analyzed to illustrate the effects of the absorption of host medium. The directional-hemispherical emittance and reflectance for seawater-in-oil emulsions are calculated using the Monte Carlo method. Results show that even though the traditional Mie-calculated radiative properties deviate obviously from the improved Lorenz-Mie theory results, directional-hemispherical reflectance and emittance of seawater-in-oil emulsions show relatively small difference. Besides, increasing the droplet volume fraction and film thickness may generally lead to enhanced scattering of seawater droplets, further leading to enhanced light reflection and reduced light emission. With the increase in both droplet size and absorption index of seawater and crude oil, the effects of seawater droplet absorption and oil absorption on overall attenuation of emulsions increase considerably; they may weaken the reflection and strengthen the emission. Obtaining the key radiative parameters of different oil species and oil emulsions will be helpful for promoting the practical application of quantitative optical remote sensing of oil slicks on sea surface.