Characterization of Low Backscatter Regions in the Marine Environment by Multipolarization C- and X-band Synthetic Aperture Radar Data

The focus of this thesis is the application of multipolarization synthetic aperture radar (SAR) data for characterization of marine oil spills and other low backscatter ocean phenomena. SAR is a valuable tool for detection and monitoring of oil spills. However, one limitation for operational oil spill detection is the number of natural phenomena that can produce similar SAR signatures as oil spills and cause false alarms. In this thesis, a variety of features based on dual-copolarization measurements are investigated for the purpose of discrimination between oil spills and other low backscatter ocean regions. Both C-band data, which have traditionally been used for oil spill observation, and X-band data, which are only more recently applied for this purpose, are investigated. The analysis is performed on a unique data set collected during annual oil spill exercises in the North Sea. Characterization of low backscatter regions can be limited by the proximity of the received signal to the sensor noise floor. For Radarsat-2 fine quad-polarization data, a high degree of noise contamination is here observed in the cross-polarization channels, and these are discarded. Only copolarization channels are used throughout this thesis. A number of dual-copolarization features are compared in terms of their ability to discriminate between low backscatter regions of varying origin. The two most promising features are selected and used as basis for image classification, and the results show that the feature pair can distinguish between a simulated biogenic slick and mineral oil spills. As X-band sensors are being included in operational oil spill detection services, more documentation on the effect of the frequency and the variation between sensors is requested. X-band data from TerraSAR-X and COSMO-SkyMed are here investigated and found useful for oil spill detection in low wind conditions, except at very large incidence angles. TerraSAR-X is found preferable to COSMO-SkyMed when multipolarization techniques are used, due to the preservation of relative phase information in the former. A comparison between Cand X-band data is conducted, including analysis of near coincident acquisitions by Radarsat-2 and TerraSAR-X. No clear difference in the data quality, including signal-tonoise levels and damping ratios, is found between the sensors. Multipolarization features on the other hand, show enhanced slick-sea contrasts and better discrimination between mineral oil spills and other low backscatter regions in Radarsat-2 compared to TerraSARX. The presence of a non-Bragg scattering component in the data is revealed for both sensors. A relatively higher contribution of non-Bragg scattering and a larger deviation from Gaussian statistics are observed in TerraSAR-X data compared to Radarsat-2 data. A larger contribution of non-Bragg scattering to the total backscatter is also observed in slick-covered regions compared to the slick-free sea surface. A potential for using log-cumulants for discrimination between mineral oil spills and other marine low backscatter regions is demonstrated. This is shown for both Radarsat-2 and TerraSAR-X, and for both single-polarization and multipolarization data. The proposed method has a potential for classification of low backscatter ocean regions of unknown origin. The work presented in this thesis adds to the on-going discussion on the use of multipolarization data for oil spill characterization, including the effect of varying sensor parameters.