CASI images of the Sea Empress at birth in Milford Havenon February 22 (left) and February 27 (right).
Click on the images for a more detailed view.
(The images are 146KB in size, and may take some time to download.)
ACKNOWLEDGEMENTS: This work is based on airborne optical data collected by the Environment Agency with the Compact Airborne Spectro- graphic Imager (CASI) from field experiments and from the Sea Empress oil spill on the West Coast of Wales in 1996, on laboratory and field experiments undertaken at Southampton Oceanography Centre, with funding from the Natural Environment Research Council (NERC) and Smith System Engineering Ltd., and on data provided by AEA Technology plc. from field experiments for the Marine Pollution Control Unit of the Coast Guard in September, 1997.
Applications for airborne remote sensing of oil include the
surveillance of routine tanker and off-shore operations,
reconnaissance in support of major oil spill response, and the
collection of information needed in order to assess the environmental
impact of pollution incidents. Information required includes:-
o
Postion and extent of surface oil
o
Thickness of surface oil
o
Oil type and physical properties
o
Position and extent of dispersed oil
Optical measurements may contribute in all these areas, complementing other sensors used in oil spill monitoring, such as radar-based, ultra-violet or thermal instruments.
Crude and heavy refined oils have three optical properties which vary slightly from oil to oil, and which make them detectable at sea by optical sensors:-
1. Their refractive index is greater than that of seawater
2. Their coefficient of light absorption is much stronger than that of water, particularly at shorter wavelengths.
3. They fluoresce when subjected to bright natural light.
These properties make it possible, not just to detect oil spilt at sea, but also to determine the thickness of the oil layer, and to classify surface oil into broad categories such as light refined, light crude, heavy crude and heavy refined oil. This information is available because the presence of oil modifies the signal received from the sea-water by increasing surface reflection, reducing the signal from the underlying water and adding a contribution from solar induced fluorescence from and scattering by water-droplets in the oil layer and from oil droplets within the water. As a result the interpretation of optical data from oil spills require careful consideration of the viewing angle and the back-ground conditions, particularly the properties of the under-lying water, the incident light conditions and, in shallow water, depth and the bottom type.
CASI images of the Sea Empress at birth in Milford Haven
on February 22 (left) and February 27 (right).
Click on the images for a more detailed view.
(The images are 146KB in size, and may take some time to download.)
During the Sea Empress oil spill off Milford Haven, in 1996, optical remote sensing provided information about the position, extent and relative thickness of surface oil.
Absolute thickness measurements were not possible, however, and attempts at distinguishing cargo oil from fuel oil were inconclusive, as were attempts at detecting and quantifying dispersed oil.
Research carried out in the past two years has, however, provided improved algorithms for oil classification. A semi-analytical model has also been developed, which will make it possible to model water-leaving radiance from surface and dispersed oil, and which may in future provide algorithms for determining the absolute thickness of surface oil, and for estimating dispersed oil concentrations.
Updated 3. June, 1999
For more information, please contact Val Byfield
Val Byfield's Home Page
School of Ocean and Earth Science
Southampton Oceanography Centre