On the combined use of sun glint Modis and Meris signatures and SAR data to detect oil slicks

SAR spaceborne capability to detect marine oil spills through damping of short gravity-capillary waves has been extensively demonstrated during past years. In contrast, it has not yet been found the optimal use of VIS/NIR imaging sensors for detection and monitoring of oil polluted areas. We propose the use of Modis images acquired in sun glint conditions to reveal smoothed regions such as those affected by oil pollution. According to Cox and Munk model, the physical mechanism that allows detection of oil slicks under sun glint imaging of clear sea surface is based on the modification of the wind-generated wave slopes distribution due the action of mineral oils. The methodology is demonstrated for a number of case studies occurred in the Mediterranean Sea and North Atlantic from 2001 to 2004. For each case, the oil slicks were detected by ERS SAR imaging and the corresponding Modis images were acquired within a few hours the SAR passage under sun glint conditions. The implemented procedure compares the water-leaving Cox and Munk sun glint reflectance with the reflectance measured by Modis at the top of the atmosphere (TOA). To accomplish the task, the Modis imaging parameters and an estimate of the wind vector are provided as input. The ECMWF analysis wind fields are considered for the purpose. It was found that the ratio between the TOA reflectance and the C&M reflectance enhances the capability to detect oil slicks. Moreover, an extensive analysis of the atmospheric effects on oil slick detection has been carried out by performing simulations using the 6S code. Preliminary results show that atmosphere contribution to the reflectance has little impact on oil slick detection, so that implementation of a surveillance procedure could be envisaged.