Origins and features of oil slicks in the Bohai Sea detected from satellite SAR images
Introduction
Oil slicks, which are formed from all kinds of oil, are one of the most common ocean pollutants. It has been estimated that 0.25% of the world's gross oil eventually enters the global ocean (Lean and Hinrichsen, 1992). The sources of oil slicks may be leaked from oil exploitation pipes, ships, and oil-polluted wastages from rivers and catchments. Lean and Hinrichsen (1992) reported that improper disposal of oil-polluted water from ships is the primary source of oil slicks.
Satellite remote sensing technology greatly facilitates monitoring of oil slicks. The Northern Branch of the State Ocean Administration (NBSOA) of China has already established an operational monitoring system of oil slicks for the Bohai Sea that is based on satellite Synthetic Aperture Radar (SAR) images. Identifying potential origins of oil slicks and their regional and seasonal distributions are the long-term goals of the operational monitoring system.
Fig. 1 shows the bathymetry of the Bohai Sea. It is a semi-closed coastal sea with an average depth of 18 m. The deepest sill is up to 86 m deep, and shallow areas (< 10 m) occupy about 26% of the total area. Circulation in the Bohai Sea consists of a tail of the Yellow Sea Warm Current (YSWC) and local coastal currents (Fig. 2). In winter, the tail of the YSWC can penetrate westward through the central basin, and it bifurcates into southward and northward coastal currents. The northward currents circle clockwise along the coastline in Liaodong Bay, whereas the southward currents circle anti-clockwise along the coastlines of Bohai Bay and Laizhou Bay. The summer circulation is similar to the winter circulation in the south, but the currents circle anti-clockwise in the north (Guan and Chen, 1964, Guan et al., 1977, Guan, 1994).
As a sedimentary basin, the Bohai Sea contains abundant oil resources. Large oil fields in the Bohai Sea include Liaohe, Bohai, Jidong, Dagang, and Shengli. To date, the area contains 26 oil fields, over 3000 oil platforms, about 2000 km of underwater pipes, and over 200 other facilities in total. The Bohai Sea hosts over 60 seaports, including Tianjin, Dalian, Yingkou, and Qinhuangdao, which constitute one of world's most prosperous shipping networks with dozens of ship routes (Li and Han, 2011, Liu et al., 2011). All oil exploitation facilities, ship routes, and seaports are potential origins of oil slicks.
Section snippets
Satellite SAR images
This investigation used 296 SAR images from the following satellites: ENVISAT ASAR, Canadian Radarsat-2, Italian COSMO-SkyMed, and German TerraSAR. The technical parameters are listed in Table 1.
Methods to detect oil slicks
Oil slicks produce extra stress tension on surface water so that capillary waves are greatly muffled, which means that oil slick can affect the microwave back scattering property of surface water. This property makes oil slick visible in SAR images (Fig. 3). The visibility of oil slick in SAR images
Regional features of oil slick distribution
Among the 296 SAR images scanned, 151 of them were identified as containing Oil slick(s) (Table 2), and 897 pieces of oil slick were found in total. Fig. 4a shows the overlaid map of the 897 oil slick, which was used to evaluate the regional features of Oil slick distribution. Most of the oil slick appeared near ship routes connecting four large seaports (Tianjin, Qinhuangdao, Dalian, and Yantai). In fact, they mainly appeared along ship routes to and from Tianjin Seaport (the largest in the
Origins and causes of oil slick
The fact that most oil slick were found much closer to ship routes than to oil platforms and seaports (Fig. 7) indicates that the oil slick more likely originated from ships. Direction of oil slick deviating from ship routes reflects a connection between oil slick drifting and ship movement (ship routes). Angles of oil slick deviating from ships were mostly < 45°, whereas angles of oil slick deviating from winds and currents were nearly homogenous at 0–90° (Fig. 8). These results support the
Summary
Oil slicks were identified by scanning 296 satellite SAR images covering the Bohai Sea in 2010. Oil slicks mostly appeared along ship routes rather than around oil exploitation platforms and seaports. Directions of oil slick movement were much more strongly correlated with directions of ship routes than with directions of winds and currents. These linkages between oil slick and ships together with the strikingly lower occurrence of oil slick near seaports suggest that oil slick in the Bohai Sea
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