ALOS-PALSAR polarimetric SAR data to observe
sea oil slicks
M. Migliaccio, A. Gambardella and F. Nunziata
M. Shimada and O. Isoguchi
Università degli Studi di Napoli Parthenope
Dipartimento per le Tecnologie
Centro Direzionale, isola C4 - 80143 Napoli
Email: ferdinando.nunziata at uniparthenope.it
Earth Observation Research and Application Center
Japan Aerospace Exploration Agency (JAXA).
Tsukuba, Ibaraki, Japan, 305-8505
shimada.masanobu@jaxa.jp
Abstract—In this study an electromagnetic approach is proposed for exploiting polarimetric information for sea oil slick
observation in L-band ALOS PALSAR full polarimetric SAR
data. The problem is tackled form an electromagnetic viewpoint
by describing the sea surface scattering mechanism with and
without oil slicks.Following this rationale, a filtering technique,
based on the Mueller scattering matrix, is applied to detect oil
slicks in full polarimetric SAR data. Successively, the filtering
results are verified by the analysis of the slick-free and slickcovered pedestal height and polarimetric entropy (H).
Experiments, accomplished on a meaningful set of Level 1.1 LBand ALOS PALSAR full polarimetric data, demonstrate the
effectiveness of the proposed approach.
I. I NTRODUCTION
A fully polarimetric SAR transmits and receives two orthogonally polarized fields and, as result, gets the scattering matrix
S for each resolution cell. Hence, this measurement process,
taking into account the vectorial nature of the scattered field,
allow retaining all the information in the scattered wave
and describing the polarimetric properties of the observed
scene. In January 2006, the Advanced Land Observing Satellite (ALOS) was launched by Japan Aerospace Exploration
Agency (JAXA). It carries on board the Phased Array type Lband Synthetic Aperture Radar (PALSAR) which is the first
space-borne full-polarimetric radar utilizing horizontally (h)
and vertically (v) polarized microwaves both in transmission
and reception. As a matter of fact, PALSAR measurements,
acquired in the polarimetric mode, can serve as useful data
source to provide additional information for environmental
remote sensing applications. Within this context, in this study
an electromagnetic approach is proposed for exploiting polarimetric information for sea oil slick observation in Lband PALSAR data. Sea oil pollution is a matter of great
concern since it affects both the environment and human
health. Oil slick detection is fundamental to effectively plane
countermeasures and to minimize pollution effects and the
SAR is unanimously recognized, under low to moderate wind
conditions, as the most important imaging sensor for a synoptic and effective oil slick observation since its all-weather day
and night capabilities [2]. In fact, the presence of a surface
slick, reducing the signal backscattered to the radar antenna,
generates a low-backscattering area which, in SAR images,
appears as a dark area [3]. Following this rationale image
978-1-4244-3395-7/09/$25.00 ©2009 IEEE
processing techniques are commonly employed on singlepolarimetric SAR data to detect dark areas [3]. However, other
physical phenomena, such as biogenic films, low winds...,
can generate low backscattering areas (look-alikes) in SAR
images. Thus, oil slick detection and classification techniques
are still an open issue. First investigations on sea oil slick
observation by means of polarimetric SAR data were generally
unsatisfactory [4], only in recent times the usefulness of
fully and partially polarized SAR measurements has been
demonstrated for the C-band [5], [6], [7], [8]. The approach
here proposed is based on the different sea surface scattering mechanism expected with and without surface slicks. It
has been demonstrated in [6], [7], [8] that though the sea
surface scattering follow a Bragg or tilted Bragg scattering
mechanism, the presence of a surface slick, depending on its
damping properties, may lead to a completely different and
non-Bragg scattering mechanism. In this study, following this
theoretical rationale, polarimetric information are exploited for
sea oil slick observation. In detail, provided S, the Mueller
scattering matrix (M) is constructed, and a physically based
filtering technique, dubbed Mueller filtering [6], is applied in
order to detect oil slicks. Successively, the filtering results are
verified by the analysis of the slick-free and slick-covered copolarized signature and polarimetric entropy (H), performed
by using the Kennaugh matrix (K) and the coherence matrix
(T), respectively, following the guidelines developed in [5],
[6], [7], [8]. Experiments, accomplished on a meaningful set
of Level 1.1 L-Band PALSAR full polarimetric data, confirm
the effectiveness of the proposed polarimetric approach and
allow underlining the importance of fully polarimetric L-band
SAR data for oil slick observation.
II. P OLARIMETRIC APPROACH
Polarimetric surface scattering can be described by using
M, a 4 × 4 real matrix, never symmetric [9], which relates the
scattered Stokes vector ss , to the incident one si :
ss =
1
Msi
(kr)−2
.
(1)
Note that the scattering Stokes vector and the Mueller matrix
are meant in the mean sense, since they are related to random
processes.
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In the case of sea surface scattering, as detailed in [6], it
is possible to distinguish the scattering mechanism occurring
with and without surface oil slicks looking at the terms of
M related to the co-polarized and cross- polarized scattering
amplitudes. In a nutshell, in case of oil-free sea surface the
co-polarized term is expected to be greater that the crosspolarized one, while the opposite behavior is expected in case
of oil-covered sea surface. This capability has been exploited
to develop a filter (a.k.a. Mueller filter) capable to both observe
oil slicks and to distinguish them from biogenic look-alikes.
It is important to remark that, from an operational viewpoint,
the filter is very important since its output is a binary image
which is very much advisable for segmentation purposes.
Two polarimetric analysis, accomplished on the slick-free and
the slick-covered sea surface, are then applied to verify the
filter output. The first one is based on the use of the copolarized polarimetric signature. Polarimetric SAR signatures,
have been successfully employed to classify a wide range of
terrain types, according to their different scattering properties
[10] and, recently its usefulness in the context of sea oil slick
observation has been demonstrated [7]. In fact, in [7] it was
shown that, for low to moderate wind conditions, oil-free sea
surfaces are responsible for a low unpolarized backscattered
signal which calls for a co-polarized signature characterized by
a small pedestal height. Conversely, in the case of oil-covered
sea surface, a large unpolarized backscattered signal occurs
which makes the pedestal height larger.
The second polarimetric analysis was firstly proposed for oil
slick observation in [5] and it is based on the use of the
polarimetric entropy derived by the Cloude-Pottier decomposition theorem [11]. As a matter of fact, in the case of oil-free
sea surface, a Bragg scattering mechanism is in place which
is characterized by a low polarimetric entropy [5], [6], [8].
Conversely, in the case of oil covered sea surface, a non-Bragg
scattering mechanism is in place which is characterized by a
low backscattered return an by multiple scattering mechanism
of comparable strength, i.e. a high polarimetric entropy [5],
[6], [8].
III. E XPERIMENTS
In this section results obtained applying the Mueller filter
and performing the polarimetric analysis on a meaningful set
of Level 1.1 L-Band PALSAR polarimetric data are shown
and discussed. The nominal slant (ground) resolution is 9.4
(26.0) meters in range and 4.5 meters in azimuth. The Level
1.1 data, although affected by speckle, are characterized by
the finest spatial resolution and, thus, have to preferred for oil
slick observation purposes.
The first case considered is related to the acquisition of
27 August 2006, 14:22 UTC (PALSAR, ALPSRP031440190,
ascending pass) relevant to a well-known oil spill accident
widely documented [12]. Fig.1 shows the module of the SLC
ground projected VV SAR image relevant to a sub-image of
the PALSAR data in which the oil slick is clearly visible.
The filtering output is a black and white image, which
clearly shows features related to the oil slick, Fig.2. This result
Fig. 1. Modulus of the SLC SAR data relevant to the acquisition of August
27, 2006
Fig. 2.
Filter output
confirms the completely different scattering mechanism which
is expected in case of oil-free and oil-covered sea surface.
Moreover, it must be explicitly noted that for the first time, the
usefulness of L-band polarimetric SAR data is demonstrated.
The analysis accomplished by considering the co-polarized
signatures evaluated on the oil-free and oil-covered sea surface
(not shown to save space) show that the presence of the oil
slick increases the pedestal height. As a matter of fact, a
tailored filtering technique to estimate the pedestal height has
been developed. The estimated pedestal relevant to the first
data set, is shown in gray tones in Fig.3. The result confirms
the theoretical model which predicts an higher unpolarized energy (high pedestal height) within the oil-covered sea surface.
A second confirm of the capability of fully polarimetric Lband SAR data for oil slick observation is provided by the
analysis of the estimated entropy (see Fig.4). It can be noted
that the H values, within the oil-covered area are higher than
the surrounding sea.
The second case considered belongs to the acquisition of 10
March 2007, in which a look-alike is present. In this case the
ground truth was not available and the first guess was provided
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Fig. 3.
Pedestal height
Fig. 6.
Fig. 4.
Filter output
Polarimetric entropy
Fig. 7.
by expert SAR image analysts. The module of the SLC ground
projected VV PALSAR sub-image in which the look-alike is
clearly visible, is shown in Fig.5. The filtering output (Fig.6)
does not show any remarkable feature related to the dark area
of Fig.5. This result allows classifying the dark area as a lookalike. To further confirm filter result, the pedestal height and
the polarimetric entropy have been estimated, see Figs.7-8.
Pedestal height
IV. C ONCLUSION
In this study ALOS-PALSAR polarimetric SAR data has
been firstly exploited to observe sea oil slicks. The working
hypothesis was that the extra-information provided by the
polarimetric measurements can be exploited to distinguish
sea surface scattering mechanism with and without surface
slicks. The data set, which concerns both oil slicks - due to
a tank accident - and oil look-alikes, has been processed and
Fig. 5. Modulus of the SLC SAR data relevant to the acquisition of March
10, 2007
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Fig. 8.
Polarimetric entropy
analyzed. Experimental results show the effectiveness and the
usefulness of polarimetric L-band SAR data for sea oil slick
observation.
ACKNOWLEDGMENT
Authors kindly acknowledge the Earth Observation Research and Application Center (EORC) of the Japan Aerospace
Exploration Agency (JAXA) for providing the data used in this
study.
R EFERENCES
[1] J. J. van Zyl, “Unsupervised classification of scattering behavior using
radar polarimetry data,” IEEE Trans. Geosci. Remote Sens., vol. 27, n.
1, pp. 36-45, 1989.
[2] C. E. Brown and M. F. Fingas, “Synthetic aperture radar sensors: Viable
for marine oil spill response ?,” Proc. 26th AMOP, Ottawa, ON, Canada,
Jun. 10-12, 2003, pp. 299-310, 2003.
[3] M.F. Fingas and C.E. Brown, “Review of oil spill remote sensing,” Spill
Sci. Technology Bull., vol. 4, no. 4, pp. 199-208, 1997.
[4] 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. Geophys. Res., vol.
103, no. C9, pp. 18851-18866, 1998.
[5] M. Migliaccio, A. Gambardella, M. Tranfaglia, “SAR Polarimetry to
Observe Oil Spills,” IEEE Trans. Geosci. Remote Sens., vol.45 , no.2 ,
pp. 506-511, 2007.
[6] F. Nunziata, A. Gambardella and M. Migliaccio, “On the Mueller
Scattering Matrix for SAR Sea Oil Slick Observation,” IEEE Geosci.
and Remote Sensing Letters, vol. 5, n. 4, pp. 691-695, 2008.
[7] M. Migliaccio, F. Nunziata, A. Gambardella, “Polarimetric Signature for
Oil Spill Observation,” Proc. of US/EU-Baltic Int. Symposium, Tallin,
Lithuania, May 27-29, 2008.
[8] M. Migliaccio, F. Nunziata, A. Gambardella, “On The Copolarised Phase
Difference for Oil Spill Observation,” Int. Journal of Remote Sensing,
vol. 30, n. 6, pp. 1587-1602, 2009.
[9] A. Guissard, “Mueller and Kennaugh matrices in radar polarimetry,”
IEEE Trans. Geosci. Remote Sens., vol. 32, no. 3, pp. 590-597, 1994.
[10] D. L. Evans, T. G. Farr, J. J. van Zyl, and H. A. Zebker, “Radar polarimetry: analysis tools and applications,” IEEE Trans. Geosci. Remote
Sens., vol.26, no.6, pp 774-789, 1988.
[11] S. R. Cloude and E. Pottier, “A review of target decomposition theorems
in radar polarimetry,” IEEE Trans. Geosci. Remote Sens., vol. 34, no. 2,
pp. 498-518, 1996.
[12] EORC/JAXA
website,
“Detection
of
oil
spill
caused
by a sunken tanker by using PALSAR,” available at:
http://www.eorc.jaxa.jp/ALOS/img_up/.
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