Special care has to be taken when interpreting radar images, as radar interacts with surface features in different ways. The detection of oil spills by radar systems is based on the dampening effect oil has on surface waves. An oil slick at sea “smoothes” the water surface and thus reduces the radar backscatter to the sensor. This creates a darker signature in the image which, after automatic processing, experienced analysts can then interpret as a possible oil slick.
A condition for detecting mineral oil spills on the sea surface is that the wind is strong enough for the generation of waves of a minimum size (wavelengths in the centimetre to decimetre range). In addition there is also a maximum wind speed above which oil films become invisible to radar, since the dampening effect disappears against the wind-generated waves. Depending on the type of oil and the radar, the wind threshold usually lies between 2 and 14 metres per second. Many other factors also affect the interpretation of an image and can result in a dark patch appearing on the radar image. False positives can be due to: sea ice; turbulence generated by a ship propeller; rain; naturally occurring algae blooms; reduced wind speeds in wind shadow areas (behind an island or coastal mountains); reduced wind stress due to colder sea surface temperature. It is therefore complicated to unambiguously distinguish between oil slicks and other phenomena on the sea surface based only on an analysis of reduced backscatter in the satellite images. Algorithms have been developed to minimize the risk of false classification, which are based on various parameters including wind currents, bathymetry, area and perimeter of patch, and historical statistics, to mention a few. |
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