What
is ENVISYS?
Oil spill as seen in SAR images:
Oil-spill detection by Synthetic Aperture Radar (SAR) is based on the dampening effect oil has on capillary and short ocean surface waves. The microwave backscatter from the ocean surface is reduced in areas where oil is present. The result is that oil slicks turn out as dark areas on a brighter background.
The synthetic aperture radar (SAR) onboard the ERS satellites are the most suitable single instruments for oil spill detection on board present and near-future satellites. This instrument is working on principles quite similar to the SLARs (Side-Looking Airborne Radar) in traditional surveillance aircrafts, producing a greyscale image which represents the radar backscatter from the ocean surface. With a 6 cm wavelength, VV polarisation, and an incidence angle of 23 degrees, the instrument is very sensitive to the presence of short gravity waves on the ocean because of Bragg scattering. Such waves are dampened by oil slicks. Thus, oil slicks can be seen as dark spots in ERS SAR images.
For oil spill detection, low-resolution (LRI) SAR images with a spatial resolution of 100 m * 100 m have been found to be suitable. The oil spill detection capabilities of different radar satellites are as follows:
- ERS-1 and ERS-2: Adequate for oil slick detection at low and moderate winds (2-8 m/s).
- JERS-1 SAR: not suited for oil slicks.
- Radarsat-1 is acceptable for oil spill when operating in Narrow-ScanSAR-Near-Range mode. The first images from Radarsat show a much better performance regarding oil spill detection capabilities than expected due to a better signal-to-noise ratio.
- ENVISAT: The ASAR Wide Swath mode seems to be good for oil spill monitoring.
Satellite coverage:
The radar satellites are in polar orbits. The coverage, or the number of passes per day, thus depends on the latitude. The coverage is good close to the poles and decreases with the distance to the poles. The coverage is twice as good in the Norwegian Sea (65° N) as in the Mediterranean (35° N). The ERS satellites have a swath width of 100 km. This means that while the average number of satellite passes per day will be 0.09 in the Norwegian Sea, the corresponding number for the Mediterranean will be 0.04. Radarsat can detect oil spills when operating in Narrow-ScanSAR-Near-Range mode (swath width 300 km). When operating in this mode, a maximum number of 0.54 satellite passes per day in the Norwegian Sea and 0.27 passes per day in the Mediterranean can be acquired. The planned ENVISAT satellite (scheduled for launch in 1999) will be able to operate in Wide Swath mode (swath width 400 km), resulting in 0.72 satellite passes per day in the Norwegian Sea and 0.36 passes per day in the Mediterranean.
The limited satellite coverage must be viewed in light of the lifetime
of the oil slicks. Small slicks might be dissolved in hours, but larger
slicks will have a lifetime of several days. ENVISAT is thus very promising
in term of detecting and giving an early warning of oil slicks of a certain
size. The development of the detected spill must be monitored using additional
sensors.
Slick visibility:
The SAR can operate well with cloud coverage. However, there are some limitations regarding weather conditions in identifying oil slicks. At high winds, the oil may be washed down into the sea, and no surface effect is observed in the SAR image. At very low winds, no SAR signal is received from the sea, thus, no slicks can be seen. ERS-1 SAR imaging of oil spills can be characterised as follows:
- 0 m/s: No backscatter from the undisturbed sea surface, hence no signature of oil slicks.
- 3 m/s: Excellent perturbation of the slightly roughened sea surface, no impact from the wind on the oil slick. A high probability of oil slick look-alikes due to local wind variations.
- 3 to 7-10 m/s: Better conditions relative to false alarms from local low-wind areas. The oil slick will still be visible and the background more homogeneous.
> 7-10 m/s: Only thick oil will be visible. Thinner slicks will be invisible due to combination of oil slick dispersion. The maximum wind strength for slick detection is a variable depending on oil type and slick age. Thick oil can be visible with wind stronger than 10 m/s.
