# A MODIS-Based Robust Satellite Technique (RST) for Timely Detection of Oil Spilled Areas
## 3 Results and Discussion
### Discussion  
Timely identification of oil spill events can be fundamental for reducing their environmental impacts, implementing adequate countermeasures and providing detailed information about their evolution in the spatiotemporal domain [56]. Weather satellites, usually working in a constellation, can provide the revisiting time useful for such kinds of applications. Considering that, in the oil spill detection context, these satellites acquire information in the optical band, it is worth noting that the main limiting factor is represented by clouds, which can completely mask the sea surface, hampering any kind of detection. Other issues derive from the usual low level of difference, in terms of radiance reflected or emitted, between the oil-contaminated area and the surrounding oil-free water, which deserves the implementation of very sensitive methods. The results presented in this work seem to indicate that RST can be confidently used for first detecting for certain the presence of a spill, and then mapping its structure. It should be stressed here that a fundamental requirement of a RST near-real-time implementation is the availability of adequate reference fields for a specific area of interest that, as previously mentioned, relies on the availability of a historical dataset of satellite images larger, at least, than 80 records collected for the same month over different years [49; 52].  
To further assess the accuracy of the achieved results, the event image (Figure 4) was analyzed implementing the Floating Algae Index (FAI) following guidelines indicated by [33]. FAI was calculated using R\({}_{\text{rc}}\) at 645, 859, and 1240 nm [57]. These wave bands in the red and infrared regions are less sensitive to atmospheric effect and the presence of colored, dissolved organic matter (CDOM). The corresponding FAI map (Figure 9) generally indicated higher values in the oil-contaminated areas than in adjacent oil-free areas. It is also worth noting that at the edge of the wind-driven feather of oil slicks, the FAI in the oiled water was lower than in the non-oiled water. This phenomenon could be related to the thickness of the oil slicks. Thin oil sheens indicated low FAI values as demonstrated by [33], while thick oil slicks forming mats floated on the surface and indicated high FAI values. R\({}_{\text{rc}}\) spectra (data not shown) over the contaminated areas did not show chlorophyll-a fluorescence peaks in the red region which can be clearly distinguished when algal blooms happen. This corroborates that algal blooms can be ruled out of the causes of high FAI values shown in Figure 9. On the other hand, it can be seen that the FAI over land is much higher than over water with values generally >0.02, and areas with FAI < 0 are due to water such as reservoirs. A general good agreement exists between the FAI map and the results provided by RST, indicating the potential of the latter in giving reliable results. It should also be pointed out that the texture of oil spills is different from that of algal blooms. However, these two phenomena are, to some extent, related. Oil can kill snails and other algae grazers, which is then favorable for a period of unimpeded phytoplankton growth [58]. The algal bloom in 2010 in the Gulf of Mexico after the Deepwater Horizon oil spill is one of the examples across the world [59].