Le radar aéroporté à balayage latéral (3/3)(en anglais)

The Side-looking Airborne Radar (3/3)

Oil detection

We have seen that a SLAR is an imaging radar, and its data give a map of the backscatter of the overflown area. The backscatter from the sea surface is dependent on wind and waves. When no wind and waves are present, the sea surface is amost perfectly flat. Then the radar pulses are reflected just like a light beam on a mirror surface: except for pulses emitted vertically downwards (which are not used anyway) the radar pulses are reflect away from the aircraft, and no signal intensity would be measured. When wind is present waves will appear: capillary waves having wavelengths of a few millimetres at wind speeds of slightly more than 50 cm/s, and also gravity waves with larger wavelengths at higher wind speed. It is this surface roughness which gives rise to backscatter signals that are detected by the SLAR antenna even at large incidence angles θ of the radar pulse. The reason is that there are always wave facets with an orientation, such that the normal to the facets points into the direction of aircraft, thus causing reflection towards the SLAR antenna.

The rough sea surface causes diffuse reflection of the radar wave, and a small fraction of the reflected wave is detected by the SLAR antenna. An oil slick dampens the waves on the sea surface, backscattering decreases and the signal gets lost.

A more precise explanation: Bragg scattering ↓

The radar signal from the rough sea surface is due to Bragg scattering of the radar wave which arises from short gravity and capillary waves. We consider water waves with a wavelength d, and a radar wave with wavelength λ hitting the water surface at an incidence angle θ. Resonance reflection into the direction of the incoming wave occurs if the Bragg scattering condition is met:

d sin θ = \frac{λ}{2}

Obviously, the wavelengths of radar and capillary waves are in the same order of magnitude: capillary waves of a few millimetres to centimetres wavelength cause resonant Bragg backscattering of centimetre radar waves. This effect is then modulated by longer gravity waves on the water surface and by currents which affect the sea surface roughness. Therefore these phenomena are also visible in radar images of the sea surface.

Detection of oil on the sea surface is based on the different backscatter efficiency of an oil slick from that of the surrounding unpolluted area. Oil on the water surface dampens capillary waves very efficiently, ripple waves on the sea surface disappear, thereby lowering the radar backscatter towards the SLAR antenna. The damping effect occurs with very thin surface films of a few micrometre thickness, and has been observed even with layers of one molecule thickness (monomolecular films). Therefore, SLAR is very efficient for detecting oil films, but there is no information on the film thickness in the SLAR images.

SLAR image of a ship seen as a white spot because of its high backscatter efficiency. The ship is spilling oil, seen as a dark structure due to the backscatter reduction from damped waves on the sea surface. The aircraft track is parallel to the left edge of the image, which explains the backscatter intensity loss from left to right, i.e., with increasing slant range.
Source: The Archimedes 2 Experiment, Joint Research Centre, Ispra Establishment

The side-looking airborne radar is an excellent instrument for oil spill detection due to its very large swath width of 10 to 30 km. The backscatter signal from water waves disappears with even very thin surface films, and therefore SLAR images cannot provide information on oil film thickness. However, a SLAR is able to work in nearly all weather conditions except for very low wind when capillary waves are not present.