Crude and Seawater 8-12µm Emissivity Measurements for Oil Slick Detection
Niclos, Raquel; Doņa, Carolina; Valor, Enric; Bisquert, Maria del Mar
University of Valencia, SPAIN
Previous works have shown that crude emissivity is lower than that of seawater in the thermal-infrared (TIR) spectrum and oil slicks cause an emissivity decrease relative to seawaters in that region.
The aim of this work was measuring crude and seawater emissivities to characterize their spectral and angular variations. Angular measurements of crude and seawater samples were collected with Cimel Electronique CE 312 6-band TIR radiometers. Crude emissivity measurements showed a featureless grey-body spectrum within the 8 - 12 µm atmospheric window and a marked emissivity decrease with angle (from 0.956±0.005 at 0° to 0.873±0.007 at 65°). The crude angular emissivity decrease was even larger than that of seawaters. This work also quantified the emissivity difference between crude and seawater with the aim to make possible crude oil slick detection from satellite TIR data. Crude emissivity was lower than that of seawater, as previous works had pointed out. Crude-seawater emissivity differences were quantified at different observation angles; the differences ranging from -0.030±0.007 at close-to-nadir angles to -0.068±0.010 at 65°.
The experimental conclusions were checked by using the dual-angle viewing capability of the ENVISAT-AASTR images registered during the BP Deepwater Horizon oil spill in 2010. Differences between the oil slick and surrounding seawater emissivities of -0.035 and -0.046 were shown for nadir and forward respectively, following the experimental results. Nadir-forward emissivity differences of +0.028 and +0.017 were obtained for oil slick and seawater respectively, in agreement with the measurements, and our previous work results for seawater emissivities.
Therefore, crude oil slicks can be identified from satellite TIR data. Crude emissivity decrease with angle, even larger than that for seawaters, makes crude oil slicks be more easily identified by comparing with seawater response from satellite TIR data at large observation angles.