7. Water Analysis Using Lidar

Detection of oil spills (3/4)

Oil spills measured at sea

When operating a laser fluorosensor aboard an aircraft and flying over the sea one measures - as shown on the previous pages - fluorescence from naturally occuring substances and Raman scattering from the water. In the presence of an oil spill the intensity of water Raman scattering decreases, and the “natural” fluorescence signals are overlaid by oil fluorescence.

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Ölfleck
Oil spill following a discharge of 1000 litres of crude oil. The photo had been taken with a nadir-looking camera at 200 m flight altitude. The image size on the ground is approx. 370 m x 250 m. The white line corresponds to the flight track where data were taken with a laser fluorosensor, shown in the right column. The scale in the photo corresponds to the distance scale in the graphs to the right.

This behaviour can be seen in the data from an oil spill (photo above), shown in the right column. Excited with a laser having 308 nm emission wavelength, the graphs display the signals of water Raman scattering and the fluorescence of yellow substance and oil. The film thickness - calculated with the method given at the end of the previous page - is also shown. When water Raman scattering intensity becomes very low a film thickness can no more be evaluated (why?).

The series of images shown to the right includes photographs (above) from a discharge of 200 litres of gas oil and the results of a film thickness analysis with an imaging laser fluorosensor (below). The flight altitude was 300 m. The film thickness given in μm is colour coded. The gas oil is very light when compared with a crude oil, and evaporates rapidly. Hence, about 30 minutes following a discharge only traces of the oil spill can be made out in the photographs and in the data of the laser fluorosensor.



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344 nm, 360 nm, film thickness
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450 nm, 533 nm, 650 nm
Data of a laser fluorosensor taken above an oil spill, see photograph in the left column. The pixel distance on the sea surface is 10 m. The laser wavelength is 308 nm. Signal intensities are given in arbitrary units and calibrated with respect to each other.
discharge 4 min 8 min 14 min 18 min 23 min 33 min 40 min
discharge


A laser fluorosensor allows

  • to detect very thin oil films in the thickness range of about 0.1 to 10 μm
  • to determine the film thickness, provided the oil absorbance is known
  • to classify the oil type; the absorbance of the oil is estimated from the oil type for calculating the oil film thickness.

Therefore, the laser fluorosensor is primarily used for analysing deliberate discharges of oil into the sea. The thickness of large oil volumes, as they occur after accidental discharges, cannot be measured.

As with all optical instruments a laser fluorosensor can only be operated in good visibility conditions. As an active sensor it does not depend on daylight and can also be used at night.