Inferring the dielectric properties of oil slick using SAR

Quigley, Cornelius; Eltoft, Torbjørn; Brekke, Camilla


Event: seaSAR2018 - advances in oceanography (Frascati (Rome))

Date: 7. May 2018 –10. May 2018

Organiser: ESA-ESRIN

Oil spills and discharges represent a significant ecological risk to the marine environment. As a result, having timely and accurate information on the location and evolving state of slicks can aid in clean-up efforts as well as to inform future preventive measures. For this purpose, Synthetic Aperture Radar (SAR) imagery has proven to be an indispensable tool given its insensitivity to overlying cloud cover. A notable feature of oil in SAR images of the ocean surface is that oil will appear as a dark patch in relation to the surrounding sea surface if wind speed is sufficiently high. This is due to the damping of the wind induced capillary waves which causes the oil infested region to become smoother than the surrounding sea surface. By applying theoretical scattering models to SAR imagery of oil infested waters it is investigated in this paper, if, and under which conditions, variations in the estimated dielectric constant can be detected over an oil slick. The dielectric constant is a quantity that is intrinsically related to the material in question and has a larger value for sea water than mineral oil. By inverting for the dielectric constant, it is believed that specific internal zones within slick will be observed that correspond to regions where emulsions of various oil/water ratios are present. Inverting for the dielectric constant in this way can yield information on the quantity of oil present within each resolution cell.

A series of images of oil spills of various types and emulsions were taken during oil-on-water exercises for the years 2011-2016 off the Norwegian coast using a variety of sensors such as Radarsat-2 (RS-2), TerraSAR-X (TSX) and UAVSAR. These sensors correspond to the X, C and L bands of the electromagnetic spectrum. For the derivation of the dielectric constant the Small Perturbation Model (SPM), the Extended Bragg Model (X-Bragg) and the Composite model are inverted using a look-up table approach. The linear mixture model, which describes the variation in the complex dielectric constant as the oil emulsifies is used to invert for the volumetric content of oil within slick.

Current work done on quad polarimetric RS-2 observations for the oil-on-water exercises for the year 2011 using the SPM for a period of time when the wind speed was within the validity range of this model suggests that an inversion for the dielectric constant may be possible over the oil infested areas but that the dielectric constant may be underestimated over ocean regions. This is likely due to the insensitivity of the model for high values of the dielectric constant. Current work is being conducted to reliably remove the effects of thermal noise which is induced by the operation of the sensor and manifests itself as an additive signal in the form of radar cross section.

Future work involves inverting for the dielectric constant using the other two methods cited above and using acquisitions that were taken for different wind speeds and incidence angles.