Radar Remote Sensing of Oil Spills from Satellites: Possibilities and

Radar Remote Sensing of Oil Spills from Satellites: Possibilities and Limitations
Prof. Dr. Werner Alpers, University of Hamburg, Centre for Marine and Atmospheric
Sciences, Germany
Mineral oil spills floating on the sea surface become visible on radar images as dark black areas
because they damp the short surface waves which are responsible for the radar backscattering. But
dark areas do not always originate from mineral oil spills, but can also originate from:
• natural surface films which are produced by plankton or fish
• low winds which are often encountered in the lee of islands or coastal mountains
• cold upwelling water which changes the stability of the air-sea interface
• divergent flow regimes associated, e.g., with internal waves or tidal flow over sand
banks
• dry fallen sand banks
• turbulent water as encountered in ship wakes
• grease ice.
Often it is possible to identify dark areas visible on radar images of the sea surface as radar signatures
of mineral spills by their shape and location. E.g., a broad dark band trailing a ship canbe attributed
unambiguously to mineral oil spilled from a ship. But in many other cases dark areas are often
misinterpreted and lead to “false alarms”, in particular when they originate from natural surface films.
For oil spill monitoring from satellites using synthetic aperture radar (SAR) often the Wide Swath
(WS) mode is used which has typically has a swath width of 400 km and a resolution 150m. Although
large sea areas can be surveyed in this mode, the revisit time for imaging the same area is still very
long (of the order of a week) which is a severe drawback for operational oil spill monitoring using
one satellite.
Oil spill monitoring must include 1) catastrophic pollution events arising from tanker collisions or
groundings or from off-shore accidents and 2) chronic pollution caused by ships in transit (tanker
cleaning, discharge of sludge oil). According to the International Charter on Space and Major
Disasters, in the case of a catastrophic event, all SARs flying on non-military satellites should be
programmed in such a way to take images whenever they fly over the site of the oil spill accident. In
this case it is - in principle- possible to obtain SAR images from the same oil spill every day or every
second day, which can be used as input in numerical models for predicting the drift and spreading of
the oil spill.
Another important application of spaceborne SAR images is to use them for statistical analyses of the
distribution of oil spills. E.g., hot spots of oil pollution can be identified by using satellite data.