Realistic Electromagnetic Modeling of SAR’s Capability for Oil Spill Thickness Measurement

A layer of oil on the sea surface reduces the components of ocean wave spectra corresponding to the capillary and gravity-capillary waves leading to significantly reduced radar backscatter and making slicks appear dark in synthetic aperture radar (SAR) images.  The ratio of the backscatter between clean and slicked ocean surfaces is known as the ‘damping ratio,’ and has been shown to be sensitive to variations within slicks that correlate with the oil layer’s thickness and fractional water volume.  Although the relative thickness relationship is well established and can be used to identify areas of ‘thicker’ oil within a slick, quantifying the thickness from SAR alone remains to be shown.  There is considerable uncertainty regarding the potential capability of SAR to quantitatively determine the oil layer thickness for slicks in open water given the dependence on bulk and interfacial oil layer properties and the variation of the properties typical in this setting and for different types of oil.  Here, we report the results of a study modeling radar backscatter from slicked and unslicked ocean surfaces based on electromagnetic scattering theory and accounting for oil properties and meteorological conditions.  The electromagnetic scattering model is used to evaluate whether the oil thickness can be quantified with reasonable accuracy based upon SAR backscatter intensities alone, and how information about metocean conditions, oil properties and ocean temperature and salinity can be used to calibrate the model to obtain more accurate thickness estimates.

The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.