The results of a recent 3-year study that looked at all the different technologies for detecting oil slicks (passive buoys, platform radar, maritime patrol aircraft, surface vessels, and satellite imagery) will be summarized. That study concluded that satellite-based detection offers the best ratio of price to operational effect. Large-scale oil spills have been successfully identified in the Arabian Gulf with satellite imagery, such as the massive-scale blowout on an Iranian platform that happened on or about 8 March 2017 and sent nine separate oil slicks drifting toward the UAE. Other examples come from Saudi Arabia, Qatar, and the UAE, including how polluting vessels off the coast of Fujairah, UAE, have been successfully identified. Against the background of these results, the conclusion drawn is that significant benefits would accrue from the countries in the Arabian Gulf adopting a collaborative oil-spill-monitoring program based on effective satellite technologies. This challenge is particularly pressing given that an oil spill at one end of the Arabian Gulf will inevitably threaten beaches and delicate marine ecosystems at the other end.
While standard in Europe since 2007, the use of satellite-based monitoring in the Arabian Gulf is neither widespread nor well-known. This paper aims to address that knowledge gap.
Satellite Technology Basics—How It Can Enhance Oil-Spill Response
By most counts, more than 2,000 manmade satellites are currently orbiting the Earth. However, only a handful of those are suitable for oil-spill detection; these are predominantly Synthetic Aperture RADAR, or SAR, satellites. All of those are polar orbiting and maintain a height of 400–800 km above the surface of the planet. Those satellites make one orbit around the Earth’s north and south polls approximately once every 100 minutes, or 14 times per day, while the earth turns beneath them. The satellite passes converge at the poles and diverge around the equator.
This has two major implications for satellite-based oil-spill detection. First, global near-real-time service is enhanced by data capture from polar satellite stations. Second, oil-spill detection near the equator requires multiple satellites to be properly comprehensive.
In any consequence-management scenario, the most precious commodity is time. The most important contribution that a satellite-based oil-spill-detection program can offer crisis management is ensuring the quickest possible detection of the spill; tracking it’s movement and evolution thereafter; and getting this invaluable information in to the hands of crisis managers and actual oil-spill responders in near real time, meaning fast enough for it to be genuinely actionable.
SAR Satellites: The Optimum Remote Sensor for Oil-Spill Response
SAR is a side-looking radar system that makes a high-resolution image of the Earth’s surface. As an imaging side-looking radar moves along its path, it accumulates data. In this way, continuous strips of the ground surface are illuminated parallel and to one side of the flight direction. From this record of signal data, processing is needed to produce radar images. The across-track dimension is referred to as range. Near range edge is closest to nadir (the points directly below the radar) and far range edge is farthest from the radar. The along-track dimension is referred to as azimuth.
In a radar system, resolution is defined for both the range and azimuth directions. Digital signal processing is used to focus the image and obtain a higher resolution than achieved by conventional radar.
SAR is the ideal remote-sensing platform for oil spill detection because it can provide an image of the surface of the Earth at all times. It does not require daylight, nor is it affected by cloud cover, as optical satellites are. Because the area a satellite can image is large, it is also ideal for monitoring large swaths of the ocean with proficiency and cost-effectiveness.
The use of SAR satellites to detect oil spills was pioneered by a Norwegian commercial company in 1995 and has since demonstrated its effectiveness at detecting spills, and supporting consequence management, at spots around the globe.
In 2013, the industry grouping of every oil and gas operator active in Norway, called the Norwegian Clean Seas Association for Operating Companies (NOFO) commissioned a study to determine which oil spill technologies offer the optimum ratio between price, coverage, and operational effect. Other technologies examined included surveillance by patrol aircraft; use of active and passive buoys; subsurface surveillance; and platform radar. On balance, NOFO decided that satellite-based monitoring offered the best all around solution. On that basis, NOFO decided to engage a Norwegian company, Kongsberg Satellite Services (KSAT), to execute daily satellite-based oil-spill detection monitoring of every single piece of oil and gas infrastructure on the Norwegian Continental Shelf.
How Does SAR Detect Oil Spills?
The SAR sensor emits a radar pulse and measures how much is scattered back from the surface, be it the ocean or the ground. The backscatter depends, among other things, on the surface roughness. The C-band radar backscatter is caused by Bragg scattering by interaction of the incident radar waves with short gravity waves with wave lengths in the range of 5–7 cm.
The capillary waves and short gravity waves are generated by winds blowing over the ocean surface. Under low wind conditions, the energy content in this part of the wave spectrum is low or almost zero, resulting in low radar backscatter and in dark patches in the SAR imagery. These type of waves are mainly wind generated but can also be created, or modulated, by ocean current shears, and will be dampened to varying degrees by different types of surface film or other floating matter.
Surface film of high-viscosity material, such as oil present on the sea surface, will damp the Bragg waves, and give rise to dark signatures. It is these dark features that are the target of oil-spill-detection monitoring and may indicate the presence of an oil slick.
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