Supplement 2.19: The Laser Fluorosensor (3/5)

Measuring the film thickness of oil spills

Many substances which are present in clean waters exhibit characteristic signals when irradiated with laser light. One of these substances is Chlorophyll a, which is found as pigments in small algae, the so-called phytoplankton. It emits part of the absorbed light as fluorescence at 685 nm wavelength. Proteins bound to bacteria and algae fluoresce in the ultraviolet. Water-soluble humic substances which are produced by degrading plants on land are transported by river water into the sea; they are also produced at sea by degrading algae. Humic substances absorb light in the UV and blue, therefore they appear yellowish in daylight and are called yellow substances in marine science. Their fluorescence covers the entire visible spectrum.

Water molecules scatter light. Besides Rayleigh scattering which causes the blue colour of clean waters in sunlight, there is also another effect called Raman scattering. The Raman effect is characterised by an emission shift to wavelengths that are higher than the wavelength of the illuminating light. Thus, illumination of water with monochromatic laser light yields an emission spectrum showing a characteristic peak of Raman scattering, as shown in the graph below.

Emission spectrum of a water sample from the North Sea illuminated with ultraviolet light at 270 nm wavelength. The narrow peak at 300 nm is Raman scattering of water molecules. Fluorescence of substances in water is spectrally much broader. At 340 nm, fluorescence of proteins bound to algae and bacteria can be observed; at 685 nm, the fluorescence of the algae pigment Chlorophyll a; and a much broader spectrum with a maximum of around 450 nm, the fluorescence of yellow substances which are formed by dissolved organic humic molecules.

Depending on the concentration of algae and humic substances, these signals are observed when illuminating the sea surface with laser light using a laser fluorosensor aboard an aircraft. If oil is present on the water surface and the oil film is not too thick, the intensity of the signals from the water are reduced by partial absorption in the oil layer. The signals vanish if the oil films are so thick that the laser light is completely absorbed in the oil.

As shown in the graph to the left, Raman scattering of water produces a characteristic narrow peak. Measuring its height allows to calculate the film thickness of oil on the water surface. Because of high absorbance of ultraviolet radiation in oil, films with micrometre thickness can be analysed, as shown in the graph below.

Raman spectrum of water covered with oil

Raman scattering of purified (i.e., non-fluorescent) water in the absence of oil (0 μm) upon illumination from above with laser radiation having 308 nm wavelength, as with laser fluorosensing above the sea. The Raman intensity decreases with increasing thickness of an oil film (given in μm) on the water surface. At the same time, fluorescence of the oil increases. No more water Raman signal can be seen with an 8 μm thick oil film; this proves that the laser light no longer penetrates the oil film.

What is the thickness of oil films? ↓

Illuminating oil on the water surface with laser light results in a partial absorption. In the oil, the light intensity decreases as the film thickness d increases, according to an exponential law:

I (d) = I_{o} e^{- a_{l a s e r} d}

I_o stands for the intensity of the laser light at the surface, I(d) for the intensity of the laser light having penetrated the film thickness d, and a_laser for the absorption coefficient of the oil at the laser wavelength. This is Lambert’s absorption law for light in absorbing media.

The absorption coefficient a has a dimension of an inverse length. The film thickness of the oil - as shown in the graph above - is given in μm; the absorption coefficient has the unit 1/μm.

The laser light produces Raman scattering in the water. The Raman scattered light propagates back through the oil film and is again partly absorbed. Denoting the Raman intensity measured from above with I_meas, it follows:

I_{m e a s} = I (d) e^{- a_{R a m a n} d} = I_{o} e^{- (a_{l a s e r} + a_{R a m a n}) d}

a_Raman denotes the absorption coefficient at the Raman scatter wavelength. Assuming that the absorption coefficients at the laser and the Raman wavelength are the same, the equation can be simplified to:

I_{m e a s} = I_{o} e^{- 2 a d}

A disadvantage is the unknown efficiency of Raman scattering in the water. It does not only depend on the intensity I_o of the laser light but also on the absorbance of the water due to phytoplankton, other suspended particles and yellow substance: clear waters give rise to a higher Raman signal than turbid waters. In case the water surrounding an oil spill has a constant absorbance, then the water Raman signal is also constant. Hence, the Raman signal I_water from the oil-free water can be considered as a reference signal which is reduced by oil on the water surface. The Raman signal from above an oil spill is then:

I_{o i l, w a t e r} = I_{w a t e r} e^{- 2 a d}

This relation is actually used for calculating the film thickness of oil spills with airborne laser fluorosensing.

Questions:
The graph above depicts the decrease of water Raman scattering in the presence of an oil film having a given thickness d.

The signal from the oil-free water is I_water=1. We assume an absorption coefficient a=1.0. Please calculate I_oil,water with films having a thickness d=0, 1, 2, 4 and 8 μm.
In the graph above, estimate the decrease of I_oil,water from the measured data. Calculate from these data the absorption coefficient of the oil.

You will learn more about the molecular properties of Raman scattering in Supplement 3 of chapter 7 of the tutorial Laser Remote Sensing.

The theory of lidar signals from layered structures such as oil films on water is given in Supplement 1 of chapter 7 of the tutorial Laser Remote Sensing.

Marine Pollution

Supplement 2.19: The Laser Fluorosensor (3/5)

Measuring the film thickness of oil spills