ReviewOil spill detection by satellite remote sensing
Introduction
Oil spills on the sea surface are seen relatively often. Observed oil spills correlate very well with the major shipping routes (e.g. in the Southeast Asian Waters (Lu, 2003, Lu et al., 1999), and in the Yellow and East China Sea (Ivanov et al., 2002) and commonly appear in connection with offshore installations (e.g. in the North Sea (Espedal & Johannessen, 2000). Annually, 48% of the oil pollution in the oceans are fuels and 29% are crude oil. Tanker accidents contribute with only 5% of all pollution entering into the sea (Fingas, 2001). After analysing 190 ERS-11 SAR images of the Mediterranean Sea, Pavlakis et al. (1996) found that “deliberate” oil spills appear with considerably higher frequency than oil spills corresponding to reported ship accidents. According to the European Space Agency (1998), 45% of the oil pollution comes from operative discharges from ships. When taking into account how frequent such spillages occur, controlled regular oil spills can be a much greater threat to the marine environment and the ecosystem than larger oil spill accidents like the Prestige tanker (carrying >77,000 ton of fuel oil (Oceanides Web-site, 2004) accident at Galice, northwest coast of Spain in 2002. The impact of not monitoring oil spills is presently unknown, but the main environmental impact is assumed to be seabirds mistakenly landing on them and the damage to the coastal ecology as spills hit the beach (Shepherd, 2004). Simecek-Beatty and Clemente-Colón (2004) describes how oiled birds lead to the use of SAR for locating a sunken vessel leaking oil.
Active microwave sensors like Synthetic Aperture Radar (SAR) capture two-dimensional images. The image brightness is a reflection of the microwave backscattering properties of the surface. SAR deployed on satellites is today an important tool in oil spill monitoring due to its wide area coverage and day and night all-weather capabilities.
Satellite-based oil pollution monitoring capabilities in the Norwegian waters were demonstrated in the early 1990s by using images from the ERS-1 satellite (e.g. Bern et al., 1992b, Skøelv & Wahl, 1993, Wahl et al., 1994b). A demonstrator system based on ERS for the Spanish coast was presented by Martinez and Moreno (1996). Today, RADARSAT-1 and ENVISAT are the two main providers of satellite SAR images for oil spill monitoring.
Access to an increased amount of SAR images means a growing workload on the operators at analysis centres. In addition, recent research shows that even if the operators go through extensive training to learn manual oil spill detection they can detect different slicks and give them different confidence levels (Indregard et al., 2004). Algorithms for automatic detection that can help in screening the images and prioritising the alarms will be of great benefit. Research on this field has been ongoing for more than a decade, and this paper reviews various methods for satellite-based oil spill detection in the marine environment.
As SAR is just one of many remote sensing sensors available an evaluation of the applicability of other satellite sensors for oil spill monitoring is included as well. Most studies done on airborne remote sensing techniques are excluded. For a review of airborne sensor technology for oil spill observation see Goodman (1994). The detectability of oil spills in SAR images are discussed, in terms of wind conditions, sensor characteristics and ambiguities caused by other phenomena than oil spills. Finally, our emphasis is on methodology and algorithms for oil spill detection in spaceborne SAR imagery.
Section snippets
Satellite sensors for oil spill detection
Microwaves are commonly used for ocean pollution monitoring by remote sensing. They are often preferred to optical sensors due to the all-weather and all-day capabilities, and examples of SAR-equipped satellites are presented in Table 1. Mainly spaceborne instruments are covered here, but airborne Side-Looking Airborne Radar (SLAR) is another possibility. SLAR is an older but less expensive technology than SAR, but SAR has greater range and resolution (Fingas & Brown, 1997). Airborne
Detectability of oil spills in SAR images
Oil slicks dampen the Bragg waves (wavelength of a few cm) on the ocean surface and reduce the radar backscatter coefficient.2
Methodology for oil spill detection in SAR images
We distinguish between manual approaches and automatic algorithms for oil spill detection. Detection of oil spills can be divided in (Indregard et al., 2004):
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Detection of suspected slicks.
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Manual verification of the slicks (oil/look-alike) and assignment of confidence levels.
This section addresses issues regarding the design of oil spill detection systems.
Automatic techniques for oil spill detection in SAR images
Several of the published papers on oil spill algorithms for SAR images (e.g. Fiscella et al., 2000, Frate et al., 2000, Solberg et al., 1999) describe a structure comparable with the one in Fig. 4.
The importance of the wind vector was emphasised in Section 3.2, and Salvatori et al. (2003) include two additional steps of wind direction estimation and wind speed calculation. Manual wind estimation was included by Solberg et al. (1999). SAR image calibration, land masking, speckle reduction and
Conclusion and suggestions for further work
Synthetic aperture radar is the most applicable spaceborne sensor for operational oil spill detection, mostly because of its all-weather/all-day detection capabilities and wide coverage. It can operate from light wind to wind speeds up to 12–14 m/s, but the maximum wind speed for oil slick detection depends on oil type and age. Sensors operating in wide swath mode with a spatial resolution of 50–150 m are found to be sufficient and allow covering large ocean areas efficiently.
The largest
Acknowledgements
This work is performed as a part of a PhD study funded by the Norwegian Research Council and the Norwegian Defence Research Establishment. The authors would like to thank the Oceanides project, in particular KSAT, for the SAR scenes used for illustration purposes.
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