We propose the use of a polarimetric two-scale surface scattering model to retrieve the dielectric parameters of oil slick from polarimetric synthetic aperture radar. The ocean surface is modeled as an ensemble of randomly orientated, slightly roughened, tilted facets for which the small perturbation model is assumed to be valid under the condition of no tilt. The orientation of the random facets causes a variation in the in-plane and out-of-plane tilt angles. As the original model utilizes both the co-polarization and cross-polarization channels to determine both the dielectric and roughness characteristics simultaneously from a series of look-up tables, the model is adapted from its original form in order to estimate the roughness characteristics of the scattering surface first, before the dielectric properties are inferred. The performance of the altered scattering model is then evaluated by applying it to multiple sets of quad-polarimetric data containing verified oil slicks, acquired from oil-on-water clean-up exercises in the North Sea. Histograms of retrieved values for the modulus of the dielectric constant indicate that the model is able to invert for values similar to the actual value of 2.3, the dielectric constant of pure crude oil at the lower limit, with successively higher values being found up to values of approximately 20 at the edges of the slicks. An error evaluation is also presented and demonstrates that sources of error are related to the alteration of the model to suit co-polarimetric data and the variance of the speckle which is related to the size of the averaging window. While the results are interesting, the approach is limited to the use of only the ratio of the co-polarimetric channels, from which two unknowns are estimated.

In this study we compare the retrieval results for the dielectric properties of verified oil slick, acquired using airborne multifrequency synthetic aperture radar. A polarimetric two-scale model was used to invert the radar imagery by first employing solely the co-polarization channels, and then by employing the co-polarization channels in conjunction with the cross-polarization channels, and thereby employing the full suite of polarization information available. The goal is to show that the inversion results obtained from both methods are consistent. Given that the ocean surface is a highly nondepolarizing surface scatter, the signal return within the cross-polarization channels is usually negligible and of no practical use when trying to invert the returned backscatter into useful quantities such as the dielectric constant. In this paper, we employ F-SAR data, which was acquired in X-, Sand L-bands and has an extremely low noise floor, implying that the cross-polarization ratio can be employed. A signalto-noise analysis showed that only the L-band acquisitions were suitable for analysis in this paper. The retrieval results are comparable for the two methods in the case of low dielectric values.

The damping ratio is a calculated feature that measures the contrast between oil-slicked water and the open ocean in SAR data. To implement the damping ratio, the current literature suggests estimating the open water backscatter by taking strips of undefined width across the range direction, obtaining the damping ratio as a function of incidence angle. We show in this paper that the method proposed in the literature can be improved by instead sampling open water pixels randomly. The method is tested on RADARSAT-2 quad-polarimetric SAR imagery of a verified oil slick acquired during the 2013 NOFO oilon-water exercise conducted in the North Sea. The results suggest that deviations in the derived damping ratio encountered by implementing the method proposed in the literature can be reduced from of order 10 0 -10 -1 to 10 -3 .

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