# Improving the RST-OIL Algorithm for Oil Spill Detection under Severe Sun Glint Conditions
## 3 Results
### Standard RST-OIL Results  
The first available no-cloudy Aqua image related to the DWH area is the one from 25 April 2010 at 18:55 GMT (LT = GMT-6) (Figure 4a), while, for the Terra satellite, the first useful image is from 29 April 2010 at 16.55 GMT (Figure 4b). These two images have been used to test the difference between the results achievable with the RST-OIL standard configuration and the advanced one proposed in this work. Figure 4a,b show average \(\theta_{g}\) values lower than 40\({}^{\circ}\) (31\({}^{\circ}\) and 34\({}^{\circ}\), respectively) indicating a contribution to the total signal coming from the high glint effect (Section 2.3). In both images, the area affected by the oil spill (i.e., the bright spatial feature around the DWH location), shows a positive contrast against the surrounding seawater. In order to quantitatively assess such a contrast, the R\({}_{0.8}\) signal variability along the transect Z-Z\({}^{\prime}\) was investigated (Figure 5), and the relative reflectance trend is shown in Figure 5b, where it is possible to note the clear increase (up to 100% of variation) in correspondence of the oil affected area. A very similar behavior was found for the Terra image (Figure 6). In both cases, the R\({}_{0.8}\) measured for the  
Figure 3: Logical flux for reference field computation. clear sea surface was also relatively high (over 6%), which confirmed that a possible contribution from the atmosphere and/or a relation with acquisition geometry has to be taken into account. To this aim, an analysis carried out on \(\mathrm{R_{rc}}\) values in the same NIR spectral band (not shown here) for the Aqua and Terra images (Figure 4), indicated that, even after removing the aerosol contribution, the sea surface signal still showed high values in both images, which confirmed the impact of sun glint on these acquisitions. Figure 4: (a) MODIS-Aqua true color RGB image from 25 April 2010 at 18:55 GMT, (b) MODIS-Terra true color RGB image from 29 April 2010 at 16:55 GMT. The red star indicates the DWH platform position. Figure 5: (a) MODIS Aqua image of 25 April 2010 at 18:55 GMT at 0.8 μm. The red point indicates the DWH platform position. (b) Reflectance trend along the transect Z-Z’. Results achieved by implementing the ALICE index (Equation (1)) on the MODIS 0.8 \(\upmu\)m channel (\(\otimes_{\text{R0.8}}\)) for both previous images are shown in Figures 7 and 8, where land and clouds are masked in black and the red point indicates the DWH platform position. Looking at the maps (Figures 7a and 8a), as well as at the ALICE profiles (Figures 7b and 8b) along the same transects of Figures 5 and 6, several considerations can be made. The first evident result is that, in both images, the entire scene shows values that are statistically significant, according to the RST approach (at least \(\otimes_{\text{R0.8}}>2\)), which also becomes very high (\(\otimes_{\text{R0.8}}>8\)) over a large area. This means that the whole scene is apparently in an anomalous condition, i.e., the reflectance recorded for almost all of the sea surfaces is higher than expected, which does not make any clear detection of the oil presence and position possible. Focusing on the platform area and analyzing ALICE profiles along the two transects (Z-Z' and T-T'), a clear increase is visible for both images. In detail, \(\otimes_{\text{R0.8}}\) reaches values of up to 50 in the oil affected area, likely as a consequence of the higher NIR signal absorption by water, which emphasizes contrast between reflecting bodies (e.g., oil) and the surrounding water. Despite this behavior, such a signal increase cannot be easily isolated in the whole scene without implementing other further processing steps (i.e., image segmentation, image classification, spatial filter, and ROI extraction), which may reduce the accuracy of a fully automatic detection system as well as increasing its elaboration time, especially if the presence of an operator is required.