Oil-Water Contrast for Surface Oil

Oil -water contrast may be defined as the difference in the light reaching a remote sensing instrument from an oil-covered sea surface and from an area free of oil. The contrast varies with wavelength across the optical part of the electromagnetic spectrum, and is dependent on the properties of the oil and the thickness of the oil layer. Light refined oils will usually appear brighter than water at most wavelengths irrespective of thickness. Crude and heavy refined oils, on the other hand, may be either darker or lighter than the surrounding water, dependent on the wavelength and the background conditions.

An examination of the contributions to measured radiance from an oil-covered and a clean oil sea surface makes it easier to understand how the contrast varies with different conditions

1. Contributions to measured radiance from a clean water surface:-

Light from the sun and sky passes through the surface into the water. Not all the radiance is transmitted through the surface, a small percentage is reflected back up. The magnitude of the diffuse transmission coefficient for incident light is dependent on the sun zenith angle, atmospheric conditions, the refractive index of the water and the sea state. Within the water some of the downwelling light is scattered back up by the water and suspended particulates and passes back through the air-water interface where a proportion is reflected back down (again depending on the incidence angle and the seawater refractive index). This water-leaving radiance is dependent on the absorption and scattering properties of the seawater and its dissolved and particulate constituents, and forms part of the signal recorded by and airborne remote sensing instrument above the surface. The recorded signal also contains a contribution from reflected sky radiance. The magnitude of this specular reflectance component is dependent on the viewing angle, the magnitude and polarisation of the reflected sky radiance, the seawater refractive index of seawater and the sea state. Finally there is a path-radiance contribution from atmospheric scattering, which depends on atmospheric conditions, the sun-angle, the viewing angle and the height of the instrument above the sea surface.

2. Contributions to measured radiance from an oil-covered surface:-

When the surface is covered with oil, incident light passes through the air-oil interface into the oil, through the oil layer, and finally through the oil-water interface. Because oil has a higher refractive index than seawater, the diffuse transmittance coefficient for the air-oil interface is lower than for air-water. (NB! Wave-damping by surface oil means that the sea-state will not be the same for both surface types, and the effect of this on oil-water contrast needs further investigation). Light is also absorbed by the oil, to an extent dependent on the oil absorption coefficient and the thickness of the layer. Most crude and heavy refined oils have high absorption coefficients, particularly at shorter wavelengths. A small fraction of light is also lost through upward reflection at the oil-water interface, but this loss is usually negligible compared to losses at the air-oil interface and through light absorption within the oil. Within the water some of the downwelling light is scattered as described in (1) above. If dispersed oil is present, this will change the optical properties of the water, reducing water-leaving radiance at shorter wavelengths and increasing it at longer wavelengths. . Water-leaving radiance is again modified by absorption as it passes through the oil layer and by transmission through the oil-air interface (lower transmission coefficient than for water). The consecutive losses means that the signal contribution from water-leaving radiance is lower for an oil-covered surface than for one which is clean. However, because of the higher oil refractive index, the specular reflection of sky radiance is greater for an oil-covered surface.

The net difference in measured radiance between surface oil and seawater, the oil-water contrast, thus depends on the balance between the specular reflection contribution (positive contrast) and the water-leaving radiance contribution (negative contrast). In regions of the spectrum where water-leaving radiance is low (e.g. the ultra-violet and near-infrared) overall contrast is likely to be positive because of the higher surface reflection from the oil. In regions of the spectrum where seawater reflectance is higher, oil-water contrast depends on the oil thickness and the oil absorption coefficient, and may be positive for thin oil, but negative for thicker oil. However, when seawater reflectance is sufficiently high (as it may be at green to red wavelengths during algal blooms, or when suspended sediment concentrations are high) even very thin oil may appear darker than the surrounding water, due to the reduced transmittance of light through the oil-air interface.

Oil-water contrast is thus dependent not just on the optical properties of oil , but also on the environmental conditions, such as the incident light conditions and the optical properties of seawater , as well as on the viewing configuraton relative to the sun.

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Updated 17. December, 1998
For more information, please contact Val Byfield

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School of Ocean and Earth Science
Southampton Oceanography Centre