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and VV signals are highly correlated, a narrow CPD pdf, it is expected. The main effect of an oil slick over the sea surface is to damp the short waves responsible ...
ON THE USE OF DUAL-POLARIZED SAR DATA FOR OIL SPILL OBSERVATION WITH AND WITHOUT SURFACE SLICKS Ferdinando Nunziata, Attilio Gambardella and Maurizio Migliaccio Università degli Studi di Napoli Parthenope, Dipartimento per le Tecnologie Centro Direzionale, Isola C4 - 80143 Napoli – [email protected]

1. INTRODUCTION Airborne and spaceborne remote sensing is unanimously recognized as a key tool for sea oil pollution monitoring. In particular, due to its all-weather day and night capabilities and its high spatial resolution, the Synthetic Aperture Radar (SAR) is the most useful sensor for sea oil pollution detection, under low to moderate wind conditions [1]. SAR oil spill detection is possible since an oil spill, damping the short waves responsible for the backscattered signal to the SAR antenna, generates a low backscattering area, i.e. a dark area in SAR images. However, due to speckle noise which hampers SAR images interpretability and other physical phenomena which generate similar dark areas in SAR images (oil look-alikes), SAR oil spill detection cannot be based on a naïve gray tones image analysis [1]. To enhance the ability to distinguish between oil spills and look-alikes the usefulness of ancillary external data is nowadays recognized [1]. Although deemed useful for environmental application, radar polarimetry is generally considered unsatisfactory for sea oil spill applications [2]. Only in [3] it was shown that fully polarimetric data can successfully assist oil spill observation. However, although a fully polarimetric SAR is to be desidered, there may be hardware and budget considerations that can suggest to design and implement a simpler polarimetric SAR configuration. For instance, the SARs on board of the ENVISAT and the COSMO-SkyMed operate only in single or in dual polarization modes. In this study it is shown that, once properly interpreted by means of a tailored electromagnetic model, partially polarimetric data are more useful than the single polarization ones. In fact, they allow both observing oil spills and distinguishing between oil spills an biogenic look-alikes. 2. THE APPROACH In this study a model which relates the co-polarized phase difference (CPD), i.e. the phase difference between the HH and VV channels, to the sea surface scattering mechanism with and without surface slicks is firstly developed. In particular, the shape of the CPD probability density function (pdf) is explained, under low to moderate wind conditions and for incidence angles far from the grazing angle, by considering some key reference scenarios. In the case of oil-free sea surface, either described by a Bragg or a two-scale scattering model, since the backscattered HH and VV signals are highly correlated, a narrow CPD pdf, it is expected. The main effect of an oil slick over the sea surface is to damp the short waves responsible for the Bragg scattering and a nonBragg scattering occurs. A low backscattered signal and a low polarimetric entropy is experienced [3]. Since the entropy measures the randomness of the complex polarimetric scattering processes [4], a low correlation between the HH and VV backscattered signals occurs, i.e. a broadening of the CPD pdf is expected for the oil-covered area. In other words, oil slicks, which are generally characterised by strong damping properties [2], are expected to be welldistinguishable from the surrounding sea since their different scattering mechanisms. The presence of a biogenic slick over the sea surface, due to its weak damping property [2], still calls for a Bragg scattering mechanism, i.e. a Bragg scattering with a lower backscattered signal. Therefore, similar CPD pdfs for the biogenic-free and biogenic-covered sea surface are to be expected.

3. EXPERIMENTAL ASSESMENTS Following the model, which predicts under low to moderate wind conditions a different sensitivity with respect to oil spills and biogenic look-alikes, a novel and very effective filtering technique has been implemented and successfully tested on multi-look complex (MLC) C-band SAR data. In detail, 11 C-Band MLC SAR images have been processed (7 acquisitions include scenes in which biogenic look-alikes are present and 4 acquisitions are relevant to oil spills). The data set was acquired by the sensor SIR-C/X-SAR during the missions STS-59 and STS-68. Experiments results (see Figs. 1-2) show that CPD acts as an emphasis filter for oil spills and as a de-emphasis filter for biogenic look-alikes. The new filter could improve classical SAR based oil spill detection techniques, since able to distinguish between oil spills and biogenic look-alikes.

(a) VV power image

(b) ı image

Figure 1. C-Band SAR data relevant to the acquisition of 01-10-1994 at 08:14 UTC (p.n. 44327). Fig.1(a) shows an excerpt of the C-band VV power SAR image in which an oil spill is clearly visible. The estimated ı image is shown in grey tones in Figs.1(b).

(a) VV power image

(b) ı image

Figure 2. C-Band SAR data relevant to the acquisition of 1-10-1994 at 5:33 UTC (p.n. 41370). Fig.2(a) shows an excerpt of the C-band VV power SAR image in which an OLA slick is clearly visible. The estimated ı image is shown in grey tones in Figs.2(b).

4. REFERENCES [1] C. Brekke, “Oil Spill Detection by Satellite Remote Sensing,” Remote Sens. Env., pp. 1-13, March 2005. [3] M. Gade, W. Alpers, H. Huhnerfuss, H. Masuko, and T. Kobayashi, “Imaging of Biogenic and Anthropogenic Ocean Surface Films by the Multifrequency/Multipolarization SIR-C/X-SAR,” J. of Geoph. Res., pp. 18851-18866, August 1998. [4] M. Migliaccio, A. Gambardella, and M. Tranfaglia, “SAR Polarimetry to Observe Oil Spills,” IEEE Trans. Geosci. Remote Sens., pp. 506-511, February 2007. [5] R.S. Cloude, and E. Pottier, “A Review of Target Decomposition Theorems in Radar Polarimetry,” IEEE Trans. Geosci. Remote Sens., pp. 498-518, March 1996.