Radiometer for determining oil film thickness

#Sea surface oil spills do not spread uniformly, nor without limit. Thick regions having a thickness of a millimeter or more are formed, which contain the majority of the oil. Reliable detection of oil thickness is of major importance. A knowledge of the oil distribution, and the location of those regions containing the heaviest concentration of oil, would enable the most effective confinement, control and clean-up of the oil.

#Multifrequency microwave radiometry has heretofore been employed as a technique for determining oil thicknesses greater than about 1 mm. ("The Determination of Oil Slick Thickness by Means of Multifrequency Passive Microwave Techniques", James P. Hollinger, NRL Memorandum Report 2953, June 1974.) This technique is based upon the fact that the apparent microwave brightness temperature is different in the region of the oil slick than in the adjacent unpolluted sea by an amount which depends upon the slick thickness. The oil slick acts as an electromagnetic matching layer between free space and the sea, modulating the brightness temperature of the sea. As the thickness of the oil slick is increased, the apparent microwave brightness temperature at first increases and then passes through alternating maxima and minima, due to the standing wave pattern set up in the film. The adjacent maxima and minima are separated by integral multiples of a quarter of the electromagnetic wavelength in the oil.

#Heretofore, the use of microwave radiometry for oil slick thickness measurement has been hindered by several drawbacks in the technology. Chief among these difficulties are the inability to achieve thickness resolution below about 1 mm and the inability to unambiguously measure oil slicks containing a range of thicknesses.

#FIG. 1A is an example of a brightness-temperature curve for a uniform oil film of 0.2-cm thickness.

#FIG. 1B is an oil-film power spectrum obtained by Fourier transforming the curve in FIG. 1A from the frequency domain to the thickness domain, in accordance with the invention.

#FIG. 2A is a computer generated brightness-temperature curve for an oil film containing equal-area patches having 0.1-, 0.2- and 0.4-cm thickness.

#The electromagnetic power emitted by a large body of water into the atmosphere above it can be estimated by basic principles of radiative transport theory. In the microwave and millimeter-wavelength spectral regions, the atmosphere is somewhat absorbing because of water vapor and a few other molecular species. Ignoring the atmospheric absorption and emission, we estimate that the power emitted from the water is the same as that from a blackbody having a temperature equal to the physical temperature of the water, T.sub.W, and an emissivity .epsilon..sub.W given by Kirchoff's relation ›2!, .epsilon..sub.W .apprxeq.1-r, where r is the radiation reflectance between the water and air. The emitted power is thus described by the brightness temperature, T.sub.B .apprxeq..epsilon.T.sub.W. In the absence of the oil film, r.apprxeq.(n.sub.W -1).sup.2 /(n.sub.W +1).sup.2, where n.sub.W is the refractive index of the water. Because n.sub.W is much greater than 1 in the microwave region, r is close to unity and T.sub.B is much less than T.sub.W.

#In the presence of an oil film, the emissivity is modulated by the process of multiple-pass interference of radiation in the film. This is a well-known phenomenon in classical optics. Because the refractive index of oil, n.sub.O, in the microwave and millimeter-wave regions is much less than that of water, the emissivity can be substantially increased in certain frequency ranges and decreased in others by a reduction and enhancement, respectively, in the overall reflectance between the water and the air. This is similar to the effect of an intermediate-refractive-index anti-reflection coating on optical components. For radiation propagating perpendicular to the film, the result is an overall reflectance of: ##EQU1## where r.sub.a and r.sub.w are the reflectances at the oil-air and oil-water interfaces, respectively, t.sub.a and t.sub.w are the transmittances at the oil-air and oil-water interfaces, respectively, k is the free-space wave vector of the radiation, h is the oil-film thickness, and the factor of .pi. in the argument of the cosine accounts for the fact that there is nearly a 180.degree. phase shift upon reflection at the oil-water interface (radiation incident from the oil side).

#In the microwave region r.sub.w is roughly 0.5 but r.sub.a is much closer to zero since r.sub.a =(n.sub.O -1).sup.2 /(n.sub.O +1).sup.2 and n.sub.O is approximately 1.5. With this in mind, we can expand Equation 1 with respect to .sqroot.r.sub.a to obtain: ##EQU2## where the identity cos (2n.sub.o kh+.pi.)=-cos (2n.sub.o kh) has been used.