## 2 Data Preparation
### Data Preprocessing  
All Sentinel-1/2/3, Landsat-8, and ICEYE-X images collected were preprocessed to obtain the required inputs, namely, the Sentinel-1/ICYE-X NRCS, Sentinel-2/Landsat-8 oil index, and Sentinel-3 T865 variable [39].  
To obtain the NRCS used for oil-slick detection, the Level-1 Sentinel-1 and ICEYE-X SAR images were preprocessed, applying the following steps:  
* Border and thermal noise removal [40];
* Radiometric calibration [41];
* Speckle noise filtering [42];
* Land masking;
* Terrain correction.  
The images were filtered using a Boxcar filter with a window size of 5 \(\times\) 5 pixels, which represents a good compromise between preservation of spatial detail and signal-to-noise ratio [42]. Figure 2 illustrates the NRCS elements (in dB) of Sentinel-1 and ICEYE-X images after pre-processing. Oil slicks on the two images are observed as dark objects with low NRCS.  
Regarding optical imagery, Equation (1) delineates the calculation of the optical oil index selected, i.e., the average values of Red (R), Green (G), and Blue (B) bands extracted from Sentinel-2 or Landsat-8 Level-2 images.  
\begin{table}
\begin{tabular}{c c c c c} \hline \multicolumn{2}{c}{**Floating Oil Slick**} & \multicolumn{3}{c}{**Offshore Qatar**} \\ \hline \multirow{3}{*}{\#Q2} & \multirow{3}{*}{5–6 July 2021} & Sentinel-1 & Sentinel-2 & Landsat-8 & S-1/S-2: 4 h \\  & & 02:23:25 & 06:56:21 & 06:58:26 & S-2/L-8: 24 h \\  & & 05/07 & 05/07 & 06/07 & S-1/L-8: 28 h \\ \hline \multirow{3}{*}{\#Q3} & \multirow{3}{*}{3 September 2021} & Sentinel-1 & Sentinel-2 & & \\  & & 02:23:54 & 06:56:21 & & \\ \hline \multicolumn{2}{c}{**Fixed-source oil slick**} & \multicolumn{3}{c}{**Offshore Mauritius**} \\ \hline \multirow{3}{*}{10–11 August 2020} & Sentinel-1 IW & Sentinel-1 EW & \multirow{3}{*}{ICYE-X} & IW/EW: 13 h \\  & & 01:37:55 & 14:36:16 & 11:12:41 11/08 & EW/ICE: 21 h \\ \cline{1-1} \cline{3-5}  & & 10/08 & 10/08 & 11:12:41 11/08 & IW/ICE: /34 h \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 2: Oil slicks with low NRCS (dark objects) observed on the extracted scenes of (**a**) a Sentinel-1 image, 28 March 2021, 14:33:06; and (**b**) an ICEYE-X image, 6 August 2020, 18:33:23.  
We tested different RGB combinations, including those proposed in the referenced studies [20; 24; 25]. We selected the averages of the RGB bands in order to preserve the oil features observed on RGB images, i.e., dark and bright pixels, depending on the oil thickness (Bonn Agreement Oil Appearance Code), and reduce the number of bands to be processed. Figure 3 presents an oil slick, as captured in a Sentinel-2 RGB image (extracted scene) and via the oil index (in dB) calculated from the averages of RGB bands. In the two images, the oil slick consists of dark (low dB) and bright pixels (high dB). The difference in oil pixel values is likely attributable to variations in oil thickness. This topic is not explored here in detail because it falls beyond the paper's scope. Instead, we focus on detecting dark and bright-oil pixels using the average RGB oil index.  
In addition to evaluating Sentinel-2 and Landsat-8, we explored various combinations of Sentinel-3 bands and identified the T865 variable (extracted from Sentinel-3 Level-2 data) as being particularly relevant for this investigation. Figure 4 presents an oil slick observed on a Sentinel-3 OLCI tristimulus image with RGB channels obtained from the combinations of different bands, i.e., bands 1-10 for red, 3-10 for green, and 1-5 for blue. Figure 4 illustrates the T865 variable [39] (in dB), with the oil slick appearing as a dark object (low dB).  
Figure 4: Oil slick observed on the Sentinel-3 image, offshore Qatar, 28 March 2021, 06:34:43: (**a**) Sentinel-3 OLCI tristimulus image (Sentinel-3 User Handbook); (**b**) T865 variable (in dB) from Sentinel-3 Level-2 data.  
Figure 3: Oil slick observed on the extracted scene of a Sentinel-2 image, offshore Qatar, 3 September 2021, 06:56:21: (**a**) RGB image; (**b**) oil index (in dB) calculated from the averages of RGB bands.