## 4 Observation of Floating-Oil-Slick Evolution Off the Shore of Qatar
### Case #Q2: 5-6 July 2021  
Figure 10 illustrates the oil slicks detected for Case #Q2. In Figure 10a, the proposed method can identify and delineate all oil objects with oil contours from the Sentinel-1 image. The oil slicks detected by Sentinel-2 include dark- and bright-oil pixels (Figure 10b). As for the dark-oil parts, they are classified as white (dark-oil object) or pink (dark-oil contours). The HSBA cannot identify some small oil parts and misclassifies some look-alikes. However, we can detect these look-alikes thanks to the lack of oil contours. As in the previous case, HSBA failed to identify bright-oil objects, although we can determine bright-oil contours (classified in red). Some cloud pixels are also misclassified in red since they have index values similar to the bright-oil ones. Figure 10c presents the oil slicks detected by Landsat-8, which only consisted of bright-oil pixels. We make some assumptions to explain the lack of dark-oil parts, including the coarse spatial resolution of Landsat-8 images (30 m vs. 10 m of Sentinel-2), sun glint conditions, and the evaporation of the dark-oil parts. The oil objects detected by Landsat-8 are classified as white (for oil objects) and green (for oil contours). Figure 11 presents oil-drift observation by collocating Sentinel-1/2 and Landsat-8 images. The time lags between Sentinel-1/2, Sentinel-2/Landsat-8, and Sentinel-1/Landsat-8 are about 4, 24, and 28 h, respectively. To facilitate the oil movement observations, we delineated the small oil objects that were not detected by the proposed method. In Figure 11a, compared to the oil observed by Sentinel-1, the slick identified on the Sentinel-2 image (about 4 h later) had moved slightly, in a direction between westward and northwestward, by about 1.5-4.5 km (depending on different oil objects). Some small-scale oil objects disappeared, probably due to surface wind and evaporation, and their shapes evolved more significantly than the moderate-scale ones. Concretely, the long and narrow tail of the oil detected by Sentinel-1 fragmented into many small parts, as shown on the Sentinel-2 image, while the size of its head section was reduced. Figure 11b illustrates the oil movement after one day, from 5 July, 06:56:21 to 6 July, 06:58:26. The oil continued moving northwestward for a distance of about 12.3-12.9 km. Some small-scale oil objects seemed to have completely evaporated, and the moderate-scale ones reduced their size. Figure 11c indicates that the oil tended to move northwestward, with significant changes observed for both small- and moderate-scale oil objects after 28 h.  
Figure 10: Floating-oil-slick detection for Case #Q2, 5–6 July 2021. (**a–c**) Extracted scenes from Sentinel-1 (July 5, 02:23:25), Sentinel-2 (5 July, 06:56:21), and Landsat-8 (6 July, 06:58:26), respectively. (**Left**) Sentinel-1 NRCS, Sentinel-2 RGB, and Landsat-8 RGB, respectively. (**Right**) Oil slicks detected from Sentinel-1/2 and Landsat-8 images (**left**), respectively. Figure 12 shows wind and current fields corresponding to the extracted Sentinel-1/2 and Landsat-8 scenes (Figure 10a-c) and their mean values. From 02:00-07:00 on 5 July, corresponding to the oil detected by Sentinel-1/2 (Figure 11a), winds blew between westward and northwestward, with speeds of 5.5-7.5 m/s. These wind values are quasi-ideal for oil detection from SAR and optical data. The current moves first in a direction between northwestward and northward, at 02:00, and then eastward, at 07:00, with velocities of 0.26-0.31 m/s. The more detailed observations in Figure 12 iii show that the oil generally moved in the same direction as the wind and current, i.e., between west and northwest. From 07:00 on 5 July to the following day, the wind continued blowing between northwest and north, with little change in direction, while wind speed decreased from 8 m/s to 3 m/s and then increased again to 8 m/s.