# Comparison of Oil Spill Classifications Using Fully and Compact Polarimetric SAR Images
## 1 Introduction
However, based on different polarimetric scattering behaviors, hydrocarbons and biogenic slicks can now be better distinguished: for oil-covered areas, Bragg scattering is largely suppressed, and high polarimetric entropy can be documented. In the case of a biogenic slick, Bragg scattering is still dominant, but with a lower intensity. Thus, similar polarimetric behaviors as those of oil-free areas should be expected in the presence of biogenic films [3]. Figure 1: Demonstration of radar scattering from the sea surface. Despite being helpful to oil spill classifications, fully (or quad-) polarimetric SAR is facing the challenges of its system complexity and reduced swath width caused by the doubled pulse repetition frequency (PRF). To overcome this problem, dual polarimetric SAR systems, which transmit a single polarization signal, are often considered [16]. However, traditional dual polarimetric SAR systems transmitting only horizontal or vertical polarized signals have a limitation when acquiring the complete polarimetric behavior of selected targets. Compared with conventional dual-polarimetric SAR modes, compact polarimetric (CP) SAR systems have higher sensitivity to the polarimetric behavior of some ground targets. Similarly, in CP SAR modes, the radar transmits only a linear combination of horizontal and vertical (\(\pi/4\)) or circularly (\(\pi/2\), also called CTLR) polarized signals and linearly receives both horizontal and vertical polarizations. As a result, compact polarimetric SAR modes can be considered as special kinds of dual polarimetric SAR modes, and vice versa. Based on a general formalism of dual and compact polarimetric SAR data, a unified framework was proposed to analyze different CP SAR modes and its feature products [17]. Since the 2000s, CP SAR has become a new research trend [16; 18; 19]. In the years following the development of this technique, most studies focused on the applications of land monitoring, e.g., biomass and soil moisture estimation [20]. Recently, it began to be considered in maritime surveillance applications [21; 22; 23]. In data received via CP SAR modes, Stokes parameters and covariance matrices can be calculated from the measurement vector of SAR data, and further polarimetric analysis can be employed [24]. Some important polarimetric parameters, such as the degree of polarization (DoP), relative phase, entropy, anisotropy and \(\alpha\), can also be derived [22; 25; 26]. It is noted that the processing method and definitions of some parameters for CP SAR data, in the process of calibration, decomposition and classification, can be different. Some previous studies explored the possibility of taking advantage of dual- and compact polarimetric SAR data to classify oil spills and biogenic slicks [27; 28; 29]. However, there are seldom quantitative comparisons of different polarimetric SAR modes, and their performance for actual oil spill classification applications. One important benefit of Pol-SAR paradigm is its robustness, i.e., it successfully works with airborne and spaceborne SARs and for different frequencies. Due to the fine classification capability of polarimetric features, polarimetric SAR-based methods may work on a wider range of sea status (surface wind and currents). However, because of the complexity of sea surface polarimetric scattering mechanisms, it is unrealistic to consider using any single characteristic to distinguish a variety of kinds of oil spills under different conditions. As a result, a synthetic and proper use of the polarimetric characteristics is the key to the accurate detection and successful interpretation of oil slicks. Moreover, the optimum compact polarimetric SAR mode varies with the different scattering behavior of the targets and also depends on specific classification tasks. Hence, in this study, we compare the oil spill detection using quad-, dual- and compact polarimetric features using supervised oil spill classifications.