## 4 Observation of Floating-Oil-Slick Evolution Off the Shore of Qatar
### Case #Q3: 3 September 2021  
Figure 13 presents the oil slicks detected for Case #Q3. The first scene (left) illustrates an oil spill leaked from a rig off the shore of Qatar, and the other two (middle and right) show floating oil slicks close to the first one. The method proposed can detect all oil slicks from the three Sentinel-1 scenes quite accurately, including oil objects and contours, which are classified in light and dark blue, respectively. In the second and third cases, HSBA misclassified the look-alikes (low wind-intensity areas) as oil objects; however, we could identify the oil slicks based on their oil contours. Just as in Cases #Q2 and #Q3, we applied the HSBA and oil contour approach to the Sentinel-2 dark- and bright-oil pixels twice (Figure 13c,d). For the dark-oil parts, HSBA could identify oil objects (in white) but misclassified some look-alikes (Figure 13d, scenes #1 and #3). However, the oil could be distinguished from look-alikes thanks to the oil contours classified in green. For scene #2 (Figure 13d, middle), as the oil is similar to the background, HSBA failed to identify it. Only the oil contours were identified and classified as green. One can observe the same result for the bright-oil pixel observation. As HSBA could not detect oil objects, only oil contours were detected and classified in red. The bright-oil part in scene #2 (Figure 13c, middle) only consists of oil contours. Figure 14 presents the collocation of the Sentinel-1/2 scenes (Figure 13) for observing the oil drift after about 4 h. In the first scene (Figure 14a), it is hard to accurately determine the oil-drift direction based only on the changes in location between Sentinel-1/2 images since this oil is leaked from a rig and is not the same type as found in floating oil slicks. The size of the oil mass detected is generally reduced after about 4 h. Some light (or thin) oil pixels disappeared, probably due to evaporation. In the second scene (Figure 14b), the oil moved southward about 2.95 km. Many oil pixels detected by Sentinel-1 disappeared, probably due to evaporation, after 4 h. Practically, only the oil contours can be observed on the Sentinel-2 image. This observation strengthened the determination of oil contours used to distinguish oil from look-alikes. In the last scene (Figure 14c), the oil slicks, including small- and moderate-scale slicks, are assumed to have moved northeastward by about 3.75-4.8 km. Additionally, compared to scene #2 (Figure 14b) and Case #Q2 (Figure 11a), the shapes and sizes of the small- and moderate-scale oil slicks evolved less significantly in this case. They tended to extend vertically and narrow horizontally in the same direction as their movements. Some small oil parts were still present after 4 h.  
Figure 15a-c presents wind and current fields and their mean values, corresponding to the extracted Sentinel-1/2 scenes (Figure 13). In the first scene (Figure 15-left), the wind blew between the north and northwest with a wind velocity of 2-4 m/s, and the current moved in the same direction. As noted in the comment above, as it is not easy to estimate the oil-drift direction in this case; we cannot accurately evaluate the impact of wind and current on the oil movement. In the second case (Figure 15-middle), the wind blew between the northwest and north with a velocity of 4-6.5 m/s while the current moved in a direction between the southeast and south, and the oil (Figure 14b) moved southward. The latter tended to move in the same southern direction as the current but stagnated due to the wind blowing in the northwest direction. In the last scene (Figure 15-right), the surface wind, current, and oil detected (Figure 14c) moved in the same northeastern direction. The wind velocity was relatively low (2-4 m/s), and this may explain the small changes in the oil shape, while the oil-drift distance is similar to the other scenes because the current speed is almost the same. Figure 13: Floating-oil-slick detection for Case #Q3, 3 September 2021.