Thermal-infrared spectral and angular characterization of crude oil and seawater emissivities for oil slick identification

摘要

Previous work has shown that crude oil emissivity is lower than that of seawater in the thermal-infrared (TIR) spectrum. Thus, oil slicks cause an emissivity decrease relative to seawater in that region. The aim of this paper was to carry out experimental measurements to characterize crude oil and seawater emissivity spectral and angular variations. The results showed that crude oil emissivity is lower than seawater emissivity and essentially flat in the 8 - 13 μm atmospheric window. Crude oil emissivity has a marked emissivity decrease with angle (from 0.956±0.005 at 15º to 0.873±0.007 at 65º), even higher than that of seawater, and thus the seawater-crude emissivity difference increases with angle (from +0.030±0.007 at close-to-nadir angles up to +0.068±0.010 in average at 65º). In addition, the experimental results were checked by using the dual-angle viewing capability of the ENVISAT-AATSR images (i.e., 0º-22º and 53º-55º for nadir and forward views respectively), with data acquired during the BP Deepwater Horizon oil slick in 2010. The objective was to explore the applicability to satellite observations. Nadir-forward emissivity differences of +0.028 and +0.017 were obtained for the oil slick and surrounding clean seawater respectively. Emissivity differences between the seawater and oil slick were +0.035 and +0.046 for nadir and forward views respectively, in agreement with the experimental data. The increase of seawater-crude emissivity difference with angle gives significant differences for off-nadir observation angles, showing a new chance of crude oil slick identification from satellite TIR data.