## 2 Data Preparation  
### Data Collocation  
This study utilized different satellite images for oil-slick detection and oil-drift observation, including SAR (C-band Sentinel-1 IW/EW and X-band ICEYE-X) and optical (Sentinel-2, Landsat-8, and Sentinel-3) technologies. The SAR images are acquired with vertical co-polarization (VV-pol). The spatial resolution (pixel sizes) and swath widths of these data are described in Table 1. The ICEYE-X images have the highest resolution (3 m) but smallest swath (80 km), while Sentinel-3 offered the lowest resolution (300 m) but the largest swath (1270 km). Sentinel-1 SAR (IW) and Sentinel-2 (RGB) optical images have a similar resolution (10 m) and close swaths (250 km and 290 km).  
The long repeat cycle of most satellites shows that the collocation of different sensors proposed in this study is an effective solution for detecting oil slicks and observing oil drifts for short- and long-term durations. Table 2 presents the collocation of various satellite data for observing the evolution of oil slicks in time and space. For offshore Qatar (Figure 1a), we focused on three cases (#Q1-3) based on the different combinations of Sentinel-1/2/3 and Landsat-8 images. Case #Q1 combines Sentinel-2/3/1 images with time lags of about 24, 8, and 32 h, respectively. Case #Q2 collocates Sentinel-1/2 and Landsat-8 images with differences of approximately 4, 24, and 28 h, respectively. Finally, case #Q3 combines Sentinel-1/-2 images with a time lag of about 4 h. To observe the oil spills offshore Mauritius (Figure 1b), we combined Sentinel-1 IW/EW and ICEYE-X images, with time lags of approximately 13, 21, and 34 h, respectively.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensor** & **Type** & **Mode/Band** & **Pixel Size** & **Swath** &
\begin{tabular}{c} **Repeat** \\ **Cycle** \\ \end{tabular} \\ \hline \multirow{2}{*}{Sentinel-1} & \multirow{2}{*}{SAR (C-band)} & IW (VV-pol) & 10 m & 250 km & \multirow{2}{*}{6 days} \\ \cline{3-3} \cline{5-6}  & & EW (VV-pol) & 30 m & 400 km & \\ \hline ICEYE-X & SAR (X-band) & Strip mode (VV-pol) & 3 m & 80 km & 1–22 days \\ \hline Sentinel-2 & Optical & Red-Green-Blue (RGB) & 10 m & 290 km & 5 days \\ \hline Landsat-8 & Optical & Red-Green-Blue (RGB) & 30 m & 185 km & 16 days \\ \hline Sentinel-3 & Optical & OLCI & 300 m & 1270 km & 27 days \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of multi-source satellite imagery used for oil-slick detection.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Floating Oil Slick**} & \multicolumn{3}{c}{**Offshore Qatar**} \\ \hline Case study & Date & \multicolumn{2}{c}{Sensors and observation time (UTC)} & Time lag (hour) \\ \hline \multirow{3}{*}{\#Q1} & \multirow{3}{*}{27–28 March 2021} & Sentinel-2 & Sentinel-3 & Sentinel-3 & \multirow{3}{*}{\
\begin{tabular}{c} Sentinel-3 \\ 114:33:0628/03 \\ \end{tabular} } & S-2/S-3: 24 h \\ \cline{3-3}  & & 06:56:21 & 06:34:43 & \multirow{3}{*}{
\begin{tabular}{c} Sentinel-3 \\ 114:33:0628/03 \\ \end{tabular} } & S-3/S-1: 8 h \\ \cline{3-3}  & & 27/03 & 28/03 & \\ \cline{3-3}  & & & 27/03 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Datasets of all satellite images for oil-slick observation in areas offshore of Qatar and Mauritius.  
Figure 1: Footprints of multi-sensor and multi-temporal images for observing oil spills. **(a)** Offshore Qatar, as covered by Sentinel-1 (28 March 2021, 14:33:06), Sentinel-2 (27 March 2021, 06:56:21), and Sentinel-3 (28 March 2021, 06:34:43). **(b)** Mauritius Island, as covered by Sentinel-1 IW (10 August 2020, 01:37:55), Sentinel-1 EW (10 August 2020, 14:36:16), and ICEYE-X (11 August 2020, 11:12:41).