Automatic Detection of Marine Surfactants by MODIS Sunglint Imagery: A Study Case of Biogenic Films off the Southeastern Coast of Brazil

摘要

Global monitoring of marine surfactants is important for understanding the heat, particle, momentum, and gas exchanges between the ocean and atmosphere. Observing the sunglint, i.e., the specular reflection between Sun, sea surface, and sensor, which intensity depends on the geometry of acquisition, has the potential to improve surfactants detection. The basis of the approach is that surfactants modify surface roughness, therefore the probability distribution of observing reflection by wave facets in the sensor field-of-view. An automatic method to detect surfactants from optical imagery under Sun glint conditions is presented. The method consists in comparing modeled normalized radiance (for clean water and surfactant contaminated water) with observed values. Application to MODIS imagery acquired off the Southwest Coast of Brazil in January 2003 demonstrates the method's feasibility, revealing the presence of biogenic films formed by Trichodesmium colonies. The presence of the N_2 fixing cyanobacteria was verified independently from in-situ measurements and ocean color remote sensing. An advantage of the glint-based method over detection methods using SAR imagery is the good spatial resolution (250 m) and the capability to detect surfactants at low wind speeds (＜ 2 m/s), impractical to SAR sensors due to the absence of Bragg scattering. Discriminating the type of surfactant, i.e., biogenic versus mineral, requires additional information, such as spectral marine reflectance.