THE MODULUS OF THE COMPLEX CORRELATION COEFFICIENT BETWEEN COPOLARIZED CHANNELS FOR OIL SPILL OBSERVATION H. KHENOUCHI (1), Y. SMARA (1), M. MIGLIACCIO (2), F. NUNZIATA (2), A. BUONO (2) (1)
Houari Boumediene University of Sciences and Technology (USTHB) Electronics and Computer Science Faculty, Image Processing and Radiation Laboratory (LTIR) PB 32, El-Alia, Bab-Ezzouar, 16111 Algiers, Algeria. Email:
[email protected];
[email protected] Università degli Studi di Napoli “Parthenope” Laboratorio di Telerilevamento e Diagnostica Elettromagnetica, Dipartimento di Ingegneria, Centro Direzionale isola C4, 80143 Napoli, Italy. (2)
ABSTRACT Sea oil pollution is a matter of great concern since it affects both the environment and human health. Recent studies demonstrated that synthetic aperture radar (SAR) polarimetry is able to provide additional information useful for environmental applications, i. e., oil spill observation. In this context, different approaches based on polarimetric SARs were developed. In this study, a dual-polarimetric feature, namely the modulus of the complex correlation coefficient between the co-polarized channels, is used to discriminate between sea oil spill and weak-damping look-alikes. The proposed approach relies on the fact that high correlation between co-polarized channels is expected over sea surface and weak-damping lookalikes due to the dominant Bragg scattering, while significantly lower correlation is expected over strong-damping oil spills since they are characterized by a no-Bragg scattering behaviour. Experimental results show that the modulus of the complex correlation between the co-polarized channels can be successfully exploited for both the observation of sea oil slicks and their discrimination from weak-damping look-alikes. Keywords — SAR, polarimetry, oil spill, discrimination.
1. INTRODUCTION The Synthetic Aperture Radar (SAR) is an active, coherent, band-limited microwave high-resolution remote sensing sensor capable to provide wide-area surveillance and day-night measurements almost independently on atmospheric conditions. The latter are very advisable for oil spill observation purposes. At the physical basis of SAR capability to observe oil spills there is the interaction between sea surface and the transmitted electromagnetic wave that is very sensitive to variations of surface roughness.
Oil and other surfactants over sea surface locally damp its roughness, i. e., the short resonant Bragg waves, thus producing weaker backscattering in single-polarization SAR imagery if compared to the surrounding sea. As a matter of fact, they appear as dark patches within a brighter area. It must be noted anyway that other natural phenomena as biogenic films, low-wind areas, etc. may generate similar dark features in single-polarimetric SAR images. Further, the latter, are also affected by speckle noise which hamper their visual interpretation. Hence, SAR-based oil slick detection and classification still is an open issue. Nowadays, it is commonly accepted that singlepolarization SAR-based oil spill detection procedures can be framed into three main steps: dark patch detection, features extraction and oil spill classification, i. e., discrimination between oil spill and look-alikes. The advent of new dual- and fully-polarimetric (FP) spaceborne and airborne SAR systems, i. e., Alos PalSAR 1 and 2, RadarSAT-2, UAVSAR, TerraSAR-X, etc., operating at different frequencies and polarizations, has boosted several new scientific/operational studies to improve remote sensing capability of monitoring environmental changes. This also promoted a strong development of polarimetric models and analysis tools to observe oil slicks at sea. The modulus of the co-polarized complex correlation coefficient ρco, has been used for sea oil slick monitoring purposes (Velotto et al. 2011, Skrunes et al. 2014a, 2015), in Skrunes, Brekke, and Eltoft (2014), Skrunes et al. (2014), the capability of dual-polarimetric hh-vv features to provide rough information on the spatial variability of the damping properties within a given surfactant is analysed. A polarimetric SAR data set including both fully polarimetric Radarsat-2 and dual-polarimetric hh-vv TerraSAR-X acquisitions was considered. The data set is related to an oil-on-water exercise undertaken by the Norwegian Clean Seas Association for Operating Companies (NOFO) in the North Sea, in June 2011 and 2012. During these controlled experiments, oils characterized by different chemical/physical properties have
been released into the sea. They include plant oil (Radiagreen ebo), emulsion oil (Oseberg blend) and crude oil (evaporated Balder), which differ in terms of viscosity, density, and volume. More detailed information on composition and properties of the released oils can be found in Skrunes, Brekke, and Eltoft (2014); Skrunes et al. (2014). In addition, the considered data set includes some lookalikes mostly due to natural phenomena. Nonetheless, it can be noted that the kind of plant oil considered in these experiments is expected to behave as a thin biogenic film (Gade, Huhnerfuss, and Korenowski 2006). When dealing with oil characterization, i.e. oil/lookalike discrimination and oil slick properties extraction, the hv channel can be severely affected by noise if low backscattering areas call for a signal below the instrument noise floor. This was the case of Radarsat-2 SAR data and, therefore, only the subset composed by the co-polarized hh and vv channels is considered for processing (Skrunes, Brekke, and Eltoft 2014; Skrunes et al. 2014). In Skrunes, Brekke, and Eltoft (2014), a large set of dual-polarimetric hh-vv features was investigated in terms of sea oil slick observation purposes. In (Migliaccio et al. 2015), the most up-to-date polarimetric SAR-based approaches for sea oil slick observation was presented and critically discussed. The topic has a relevant scientific and environmental importance. The monitoring of sea oil slicks is addressed considering a large variety of realistic scenarios, i.e. oil spills from ships, large accidental oil spills, natural oil seepages, etc. Each operational domain is characterized by different needs and purposes: the identification of polluters very often calls for the detection of small-size oil spills due to illicit operations from vessels; while the characterization of the chemical/physical properties of surfactants is desired in the case of remediation activities for minimizing pollution effects. In this framework, the large polarimetric SAR data set nowadays available, characterized by different wavelength, resolution and coverage, offers an unprecedented opportunity to enhance sea oil slick observation. All the polarimetric approaches widely and successfully used allow exploiting the large amount of physical information to perform oil slick detection, discrimination and characterization overcoming the limitations presented by single-polarization techniques. Nevertheless, they need suitable electromagnetic models and analysis tools to be transformed in useful added-value products. All polarimetric approaches share a common physical rationale based on the departure from the slick-free sea surface Bragg scattering, which applies for intermediate incidence angles and under low-to-moderate regime, due to the presence of an oil slick. A large set of polarimetric features is critically reviewed in terms of oil detection, discrimination and characterization capabilities. A discussion on the role played by the SAR data quality (i.e. calibration, noise floor, etc.) within the frame of polarimetric features extraction is also addressed.
In this paper, we study the use of polarimetric SAR data to assist oil spills observation. The polarimetric approach is based on the use of the dual polarimetric SAR feature (the modulus of the complex correlation coefficient between the co-polarized channels) to observe oil spill on sea surfaces.
2. THEORY The feature used in this paper is described according to the radar configuration and is defined as:
rco =
* ShhSvv
Shh
2
Svv
(1) 2
The correlation coefficient is complex and is computed as the average of the product between the complex amplitude of the HH channel and the conjugate of the complex amplitude of the VV channel. It is normalized by the square root of the product of the powers in the HH and VV channels. Calculated values of the modulus of the complex correlation between the co-polarized channels vary between 0 and 1 and we can then distinguish between slick free sea surface and slick sea surface. When the magnitude of the copolarization correlation coefficient equal to unity it indicates that HH and VV returns are perfectly correlated. Lower values of ρco imply depolarization effects. A component of unpolarized backscatter may be due to the presence of complex surfaces, multiple-scattering surface layers, and/or presence of system noise. Similar to the copolarization power ratio, ρco is independent of the damping of gravity– capillary waves by oil in the tilted Bragg model; it is only a function of the dielectric constant, the RMS slope due to ocean waves of long wavelengths, and the incidence angle. Observations reported by Kasilingam et al. 2002 suggest that the magnitude of the copolarization correlation coefficient is not sensitive to changes in short-scale roughness, but can be modulated by variations in dielectric constant. ρco is inversely related to the incidence angle (Gill et al. 2012). It is expected that ρco has values close to 1 over slickfree sea surface due to its dominant Bragg scattering, while significantly lower ρco values, close to 0, are expected for oil-covered sea surface. Note that, ρco is considered as dual-polarimetric feature, it can not be evaluated using standard products provided by conventional dual-polarimetric SAR. In fact, conventional dual-polarized SAR systems measure HH/HV or
VV/VH polarimetric scattering channels combinations. The only exception is TerraSAR-X, the DLR X-band multi-polarization SAR, that provides routinely dualpolarized HH-VV Stripmap and Sotlight measurments. In addition, it must be noted that although dualpolarimetric HH-VV modes are implemented on ENVISAT, ASAR and COSMO-SkyMed, they are incoherent SAR systems and thus they provide no phase information. When dealing with HH-HV or VV-VH dualpolarimetric SAR modes, the cross-polarized channel offers no polarimetric information useful for sea oil slick observation. Hence, for sea oil slick observation purposes such dual-polarized SARs behave as singlepolarization systems since only the information carried on the co-polarized channels is useful. It must be pointed out that, with respect to the quad-polarimetric case, HHVV dual polarimetric measurements do not suffer from calibration issues and HH-VV dual-polarimetric features are not critically affected by the noise floor.
the range 0.7 – 1. This witnesses that polarimetric SARs are able to easily perform the detection of sea oil pollution, i. e., to identify oil spills and to discriminate them from a broad range of other natural phenomena – weakdamping look-alikes. However, it must be pointed out that despite of the FP SAR-based approaches, no dual-polarimetric SARbased technique allows obtaining a logical binary output automatically, i. e., without any external threshold.
3. EXPERIMENTAL RESULTS In this section, the polarimetric SAR dataset considered in this study is presented. It consists of two FP singlelook complex (SLC) spaceborne SAR data collected by RadarSAT-2, where well-known oil slicks and weakdamping look-alikes are present. The VV-polarized intensity image relevant to the first Radarsat-2 SAR acquisition (ID: PDS_02005750) collected on May 8, 2010 in the Gulf of Mexico, is shown as gray-tone image in Fig. 1. Verified oil slicks are clearly visible as a dark area over sea surface. The VV-polarized intensity image relevant to the second Radarsat-2 SAR acquisition (ID: PDS_00886040) collected on December 14, 2009 off the California coast, is shown as gray-tone image in Fig. 2. A certified weak-damping look-alike due to the produced-water discharge of an oil rig. In Fig. 3 the modulus of the complex correlation coefficient between the co-polarized channels (eq. (1)) related to the first SAR scene is shown in false colours. In Fig. 4 the modulus of the complex correlation coefficient between the co-polarized channels (eq. (1)) related to the second SAR scene is shown in false colours.
Figure 1. VV-polarized RADARSAT-2 SAR intensity image (in dB scale) related to the acquisition of 8 May 2010 at 12:01:25 UTC (ID: PDS_02005750).
4. DISCUSSION As shown, the proposed approach based on the dualpolarimetric feature ρco allows emphasizing the presence of the oil slicks covering the sea surface, i. e., they look as bright blue areas (see Fig. 3) due to their very low correlation values (close to 0.1), and de-emphasizing the presence of weak-damping look-alikes, that since they are characterized by high correlation between copolarized channels they call for high ρco values close to 0.85 (light red slicks in Fig. 4). For both SAR scenes, sea surface calls for very high correlation, i. e., ρco is in
Figure 2. VV-polarized RADARSAT-2 SAR intensity image related to the acquisition of 14 December 2009 (ID: PDS_00886040).
films. Experiments undertaken on actual FP Radarsat-2 SAR data acquired on the Gulf of Mexico and off the California coast witness that the proposed approach based on a dual-polarimetric SAR feature is able to easily and successfully perform the observation of oil spill over sea surfaces.
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Figure 3. ρco image of the SAR data ID: PDS_02005750.
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Figure 4. ρco image of the SAR data ID: PDS_00886040.
5. CONCLUSIONS In this study, the extra information provided by polarimetric SARs with respect to traditional singlepolarization SAR systems is exploited for sea oil slick observation purposed. Following a physical model based on sea surface Bragg scattering, the modulus of the complex correlation coefficient between copolarized scattering channels is interpreted to characterize oil spills, i. e., to detect oil spills and discriminate them from weak-damping look-alikes as thin biogenic
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