## 5 Observation of Oil Spill Drift Off the Shore of Mauritius  
Figure 16 presents the detection of oil spills associated with the MV Wakashio shipwck off the shore of Mauritius in August 2020. This oil spill scenario differs from those observed off the shore of Qatar, which consisted of floating oil slicks. However, we applied a similar methodology (Figure 5) to this case. The oil detected included oil objects and contours as found for the floating oil @Q1-3. Figure 16a-c illustrate the scenes extracted (NRCS) from Sentinel-1 IW, EW, and ICEYE-X images for oil spill detection. The time lags between these images are about 13, 21, and 34 h. The ICEYE-X image has the highest spatial resolution (about 3 m), the Sentinel-1 EW image has the lowest (40 m), and the Sentinel-1 IW spatial resolution is 10 m. Figure 16d-f present the oil detected in the extracted scenes (Figure 16a-c). The oil detection based on the Sentinel-1-EW image is less accurate than  
Figure 15: (**Left–Right**) Surface wind and current speed and direction (indicated by the arrows), corresponding to the extracted Sentinel-1/2 scenes @S1–3, respectively. (**a**) ERA-5 wind vectors for 02:00, 3 September 2021. (**b**) CMEMS current vectors for 02:30, 3 September 2021. (**c**) Mean values of wind and current fields from 02:00 to 07:00, 3 September.  
the others, i.e., more false positives from the classified oil pixels, arguably due to its lower spatial resolution (40 m vs. 3-10 m), which makes the distinguishing between oil pixels and sea background more complicated and less accurate. Between Sentinel-1 IW and EW images (13 h time lag), the detected oil has undergone several changes; however, the oil surface, when comparing Sentinel-1 EW and ICEYE-X images (21 h time lag), is more extended along the coast.  
Figure 17 presents wind and current fields corresponding to the three extracted scenes (Figure 16a-c) and their mean values. Due to the coarse spatial resolution of CMEMS data, we do not have accurate current fields near the coast. From 01:00 to 15:00 on 10 August, the wind was blowing in a northwestern direction with wind velocity relatively stable at about 6-7 m/s, while the current moved southwestward with current speeds of 0.2-0.35 m/s. The wind direction matched the oil-drift direction. Likewise, due to the stability of wind velocity, the oil detected on the Sentinel-1-IW image changed little after 13 h. From 15:00 on 10 August, to 11:00 on 11 August, the wind continued blowing in a northwestern direction but at a lower velocity (2-5 m/s). The oil surface in this period (Figure 16e,f, between Sentinel-1 EW and ICEYE-X images) was more significant than in the previous period (Figure 16d,e, between Sentinel-1 IW/EW images). This result leads to an assumption that the low wind speed may have slowed down the movement and dispersion of the oil spill toward the coast. Additionally, due to low wind velocity, current (unfortunately not available at a high spatial resolution for this tiny area) may be one of the factors causing the oil spill to spread over a large surface. This assumption is aligned with the observations in Section 4 (for the movement of the floating oil). It is more general and adequate than the conclusion in [32], in which only the surface wind speed is mentioned as a primary source of this oil drift.  
Figure 16: Oil-slick observation on Sentinel-1 IW/EW and ICEYE-X images, offshore Mauritius, 10–11 August 2020. (**a–c**) Extracted scenes corresponding to the MV Wakashio oil spill, from Sentinel-1 IW (10 August, 01:37:55); Sentinel-1 EW (10 August, 14:36:16); and ICEYE-X (11 August, 11:12:41), respectively. (**d–f**) Oil spill, as detected by HSBA from the extracted scenes (**a–c**).