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How does SAR detection work?

EMSA’s services are capable of monitoring wide areas at regular intervals. Long range detection is mainly based on radar sensors that measure the roughness of the sea surface. Radars generate electromagnetic pulses that ‘illuminate’ the ocean surface. Radar pulses are reflected by capillary waves which the wind creates at the surface of the sea (sea clutter). Radar systems will therefore detect any phenomena that suppress capillary waves. Some substances, for example oil, smooth the sea surface and reduce the level of the signal returned to the emitter. The signal is processed into an image where a clean sea will appear as a grey background; oil spills will appear as dark areas and vessels and platforms as bright spots. Oil, but also other substances and natural phenomena such as certain current patterns, ice and surface slicks associated with biological activity, will also appear as dark patterns on the radar image.

how sar works

SAR radars are able to detect very thin oil films floating on the sea surface day and night and through the cloud cover. However there are limitations to this process as sea roughness is driven by the local wind speed and direction. Wind speeds below 2-3 m/s mask the dampening effect whereas speeds above 15 m/s also reduce detection capability. For this reason and due to the fact that SAR satellite images cannot provide information on the nature of the spill (mineral oil, fish or vegetable oil, other look-alikes), it is important to note that CleanSeaNet does not detect "oil spills" but "possible oil spills“. Discrimination between oil spills and look-alikes require more information and most often on site verification.

EMSA can also provide optical satellite images to support Member States in their response operations given the detailed view of the accident area this type of data is able to provide.


Spill detected by CleanSeaNet (left).
Same spill overlaid with SafeSeaNet information (right)