Electromagnetic Radiation and Water

Absorption of electromagnetic radiation by water is highly dependent on wavelength. Near-infrared wavelengths hardly penetrate water at all and are completely absorbed in only a few tenths of a metre of water. It is only visible wavelengths that can penetrate the surface and be backscattered out of the water into the atmosphere. Blue wavelengths penetrate deeply in clear water while longer wavelengths penetrate less well with red wavelengths penetrating only a few metres. These features are illustrated in Figure 1 below which shows the absorption spectrum of pure water in the wavelength range 300-750nm.

Figure 1: Absorption spectrum of pure water

Depth penetration is also restricted by the turbidity of the water. Turbidity is the existence of particles in water and increasing turbidity causes a decrease in the depth penetration of light. This fact is illustrated in Figure 2 below which shows the spectral transmittance for waters of different turbidities.

Figure 2: Spectral transmittance for various waters (courtesy of Floyd F. Sabins)

Figure 2 shows that the best penetration is obtained in the optical region and that penetration decreases with increasing turbidity. Therefore, remote sensing for bathymetry is comprised of mostly optical techniques. However, in regions where optical techniques are not appropriate, due, for instance, to high turbidity or a low sun angle, radar remote sensing techniques can provide valuable information on bathymetric features in water.

Last updated and validated 18/06/99
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