Le radiomètre à micro-ondes (2/2)(en anglais)

The Microwave Radiometer (2/2)

Oil detection

We have seen that the oil film thickness on water must be in the millimetre to centimetre range, to give rise to interference fringes in the microwave spectral range. This is depicted in the graph below.

Brightness temperature of oil given in absolute temperatures (Kelvin scale) as a function of the oil film thickness in millimetres, shown for three frequencies in the microwave range. The angle of incidence of the radiometer field of view on the oil surface is 41° (5 GHz), 54° (17 GHz), and 50° (34 GHz).
Source: The Archimedes 1 Experiment, Joint Research Centre, Ispra Establish- ment, redrawn.

The fringes should make it possible to measure the thickness of oil layers on the water surface with a MWR Scanner! The signal at a given frequency is modulated during overflight of an oil spill, depending on the local layer thickness. Measured intensities are then converted to brightness temperature variations using Planck’s Radiation Law. But a single detection frequency will lead to ambiguous data due to the periodic variations of the brightness temperature with increasing or decreasing oil layer thickness. This, however, can be overcome with instruments that use several frequencies, as shown in the graph above.

The capabilities of measuring oil layer thickness with MWR are clearly seen in the data of two radiometers having 90 and 32 GHz detection frequency. We know that large volumes of oil discharged on water are characterised by a core having a layer thickness in the order of millimetres to centimetres, surrounded by thin layers of oil only a few micrometres thick which are disrupted by the wind into small patches. Obviously, the MWR Scanner does not “see” these thin films. It locates the thick portions of an oil slick, which makes it suitable for guiding recovery vessels during oil spill clean-up operations.

Serious limitations of MWR applications arise when trying to detect weathered oil volumes. Due to wind and waves, water droplets are constantly dispersed into an oil layer, leading to emulsification. The principle of microwave radiometry is the detection of thermal radiation emitted by the water below the oil. This radiation is transmitted through the transparent oil layer (and hereby modulated by interference). Water droplets in oil however emit thermal radiation in the same way as a clean water surface. Due to this, emulsified oils are invisible to microwave radiometry.

Another limitation occurs in cases where the oil layer thickness varies to strongly over short distances which cannot be resolved with the pixel distribution of the MWR Scanner on the ground. As a consequence one ore more minima or maxima of the interference fringes are not detected, and hence not counted when evaluating the oil layer thickness. This can lead to incorrect results of the oil layer thickness distribution derived with MWR Scanners.

Heavy fuel oil slick with a total volume of 17 m³, measured with a 90 GHz (left image) and a 32 GHz (right image) MWR Scanner. The flight direction in the images is vertically oriented, and the scan width on the ground is 282 m. Brightness temperatures are increasing from light blue over dark blue and red to yellow. Maximum brightness temperature variations are 20 K (left image) and 17 K (right image).
Source: The Archimedes 2 Experiment, Joint Research Centre, Ispra Establishment.

The MWR Scanner is a passive microwave radiometer that detects specific frequency bands in the 1 to 100 GHz range, combined with a scanner for imaging detection of oil on water along the flight track of an aircraft. The physical principle is based on the detection of thermal radiation emitted by seawater at microwave frequencies, while an oil layer on top of the water is transparent to this radiation. Interference effects occur in the oil layer, leading to interference fringes with maxima and minima of the signal detected by the the MWR Scanner. These minima and maxima occur with variations of the oil layer thickness that are in the order of the wavelength of the microwave radiation, i.e., in the millimetre to centimetre range. This makes the MWR Scanner suitable for locating the thick layers of an oil spill following (mostly accidental) discharges of large volumes of oil.