# A MODIS-Based Robust Satellite Technique (RST) for Timely Detection of Oil Spilled Areas
## 1 Introduction  
Oil spills cause the contamination of seawater, shores, and beaches, which may persist for several months, representing a serious hazard for marine resources and the ecosystem [1]. Besides natural crude-oil seeps, the biggest contribution to this type of sea pollution comes from tanker operative discharge that spreads in the sea, equivalent to one full tanker disaster every week [2, 3]. Satellite remote sensing might effectively contribute to mitigating the environmental impact of oil spills, provided that reliable and effective detection techniques are used, and relevant information and products may be quickly delivered and shared. The large number of satellite data used and methodologies implemented/developed during the Deepwater Horizon spill event affecting the Gulfof Mexico in April-May 2010 have been a clear representation of the importance of using satellite remote sensing in response to oil spills [4; 5]. Synthetic Aperture Radar (SAR) currently represents the most widely used satellite system for oil spill detection, due to its capability to identify oil spills with a good level of reliability and high spatial resolution (from tens of meters up to 1 m) with all-weather and all-day detection [6; 7; 8; 9; 10; 11; 12; 13]. Unfortunately, besides the relatively high cost and a non-negligible rate of false alarms due to the variation of wind conditions, the presence of look-alikes, etc., the main limitation of SAR systems within an operational continuous monitoring system is represented by their low revisiting time, ranging from several days to dozens of days at mid-latitudes [13; 14; 15; 16]. Even the full implementation of the COSMO-SkyMed satellite mission (an Italian mission with both military and civilian purposes) with four X-band SAR satellites in constellation [17] failed to guarantee an observation frequency suitable for operational surveillance activities. Some issues related to military restrictions, data distribution and access policies have, indeed, reduced their actual use for limited time and/or areas [18]. Neither is the full deployment of the Copernicus ESA Sentinel-1 mission [19], finalized in April 2016 with the launch of the second satellite (i.e., Sentinel 1B), able to assure a temporal resolution better than three days at the equator. In this context, passive instruments working in the optical band (visible and infrared) aboard weather satellites may be a viable alternative or at least a complement to SAR-based techniques [5]. Despite their lower spatial resolutions (from 30 m to 3 km) with respect to SAR, they show temporal resolutions (from 6 h of polar satellites to 5 min of geostationary ones) and coverage continuity that is more suitable for real-time monitoring of those areas exposed to oil pollution risk. Obviously, such data, which are freely available and easily accessible on the Internet, must be processed using automated methods of analysis able to ensure both adequate response times with respect to the observation frequency and reliable results regardless of the considered areas or sensors [20]. Several techniques have already been proposed for oil spill detection and monitoring using satellite data acquired in the optical band [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. Most of them are based on the analysis of satellite radiances acquired in the middle (MIR) and thermal (TIR) infrared spectral regions where the contrast between the emittance of the oil-polluted area and the surrounding clean seawater is more appreciable. Due to the thermal inertia of oil, which is lower than seawater, oil-polluted areas usually have the opposite behavior compared with the surrounding oil-free seawater, with higher brightness temperatures in TIR images collected in daytime and lower brightness temperatures during the nighttime [35; 36].