Oil from the wrecked tanker off the northwest coast of
Spain had already reached the Spanish coast when ESA’s
Envisat satellite acquired this radar image of the oil
slick, stretching more than 150 km, on Sunday, 17 November,
at 10.45 UTC.

The 26-year-old tanker, Prestige, can be seen as a bright
white point located about 100 km off the coast. Support
vessels are identifiable as smaller white points
surrounding the ship. The huge oil slick is clearly
visible as a dark plume emanating from the stricken ship
and stretching to the northwest coast of Spain.

The image was captured by the Advanced Synthetic Aperture
Radar (ASAR) onboard the Envisat spacecraft, launched by
ESA last March. The ASAR was operating in its wide-swath
mode covering an area approximately 400 km by 400 km.

Strong winds in the area resulted in lighter oil
components being blown off while heavier constituents
have been moved around on the sea surface, leaving
characteristic dark feathery trails. These winds also
caused some degree of mixing between the oil and the
surrounding sea, reducing the level of oil at the
surface and making it more difficult to detect. If a
smaller volume of oil had been spilt, or if the spill
had occurred further from the coast, it is possible that
these strong winds could have ensured the oil dispersed
harmlessly, with no adverse impact on the coastal
environment.

Press reports indicate that oil has already fouled a
200-km stretch of Spanish coastline.

The presence of oil on the sea surface damps down smaller
wind generated waves. It is these waves that reflect the
radar signal back in the direction of the source. When
they are damped, the reflected power measured by the radar
is reduced, causing oil slicks to be seen as dark areas
on an otherwise brighter sea.

The slick cuts across a major shipping lane. Shipping
traffic in this sea-lane shows up as the numerous white
points. The wakes of ships passing through the
oil-infested area are seen as light lines in the slick.

ASAR offers all-weather, nighttime observations

ASAR, like other space-based radar systems, essentially
provide its own source of illumination and operates at
longer wavelengths than optical sensors. This enables it
to observe the earth surface at night and through thick
cloud cover. This unique capability allowed Envisat’s
ASAR to “see” the oil leaking from the tanker despite
heavy rain and cloud cover in the region that hindered
the coverage of optical instruments.

The ASAR acquisition was scheduled on an emergency basis
for Sunday 17 November. Envisat, along with ESA’s ERS-2
spacecraft, will continue to acquire images for the next
few weeks over the Galician coast.

The tanker was first damaged last Wednesday and is
estimated to have spilled about 10,000 tonnes of its
70,000-tonne cargo. An additional 10,000 tonnes of oil
is thought to have leaked when the ship split in two and
sank yesterday. According to media reports, approximately
75 percent of the oil shipment went down with it.

Optimistically, the remaining cargo will either stay in
the tanker’s holds, or congeal on the ocean floor. If the
remaining oil leaks to the surface, the resulting spill
would be twice as large as the 1989 Exxon Valdez spill
in Alaska, according to some estimates. Envisat will
monitor the area regularly along the coast in the coming
weeks to trace any oil spills appearing on the sea
surface from the sunken tanker.

For the expected international clean-up efforts, future
ERS-2 and Envisat imagery will be made available as part
of the International Charter on Space and Major Disasters.
The Charter is an international collaboration to put
satellite remote-sensing technology into service for
civil protection agencies and others in response to
natural and man-made disasters.

The Charter was activated on 14 November by the European
Commission’s Civil Protection and Environmental Accidents
Unit to support local authorities in Spain in monitoring
the oil spill. The satellites of other agencies within
the Charter, including the Canadian Space Agency and the
French Space Agency, CNES, are tasked over the area as
well.

Related articles

* What puts the “Advanced” in Envisat’s Advanced Synthetic
Aperture Radar?
http://www.esa.int/export/esaCP/ESAZGOG18ZC_FeatureWeek_0.html
* Oil spills
http://www.esa.int/export/esaSA/ESA1XRVTYWC_earth_0.html
* Workshop on the International Charter on Space and Major
Disasters meets at ESRIN
http://www.esa.int/export/esaCP/QOCVCKSC_index_0.html
* Après le déluge: ERS and Envisat imagery contribute to
European flood relief
http://www.esa.int/export/esaCP/ESAZODZPD4D_index_0.html

Related links

* Envisat
http://envisat.esa.int/
* Earth Watching
http://earth.esa.int/ew/
* Envisat Results
http://www.esa.int/envisat/
* Envisat Instruments
http://envisat.esa.int/instruments/
* ERS
http://earth.esa.int/ers/
* About Earth Observation
http://www.esa.int/export/esaSA/GGGIMV7RVDC_earth_0.html
* Envisat Images
http://asimov.esrin.esa.it:8766/queryIG.html?searchType=general&col=mmg&rf=3&ql=a&showcase=Envisat%20results&tipo=All&lk=8&nh=12&op2=%2B&tx2=ESAJCTG18ZC&fl2=showcase%3A&ty2=w&op0=&tx0=&fl0=&ty0=&op1=&tx1=&fl1=&ty1=

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/ESAGFD7708D_index_1.html]
Envisat’s ASAR image shows tanker, Prestige, 100 km off
Spanish coast. Credits: ESA

[Image 2:
http://www.esa.int/export/esaCP/ESAGFD7708D_index_1.html#subhead1]
ASAR Wide Swath image, acquired 17 Nov. 2002, of the
Spanish oil spill. Credits: ESA

[Image 3:
http://www.esa.int/export/esaCP/ESAGFD7708D_index_1.html#subhead2]
ASAR image shows tanker, Prestige, 100 km off the Spanish
coast. Credits: ESA

[Image 4:
http://www.esa.int/export/esaCP/ESAGFD7708D_index_1.html#subhead4]
Part of an oil slick of several kilometers follows the
stricken Bahamas-flagged Prestige oil tanker, second top
right, that is loaded with 77,000 metric tons, 85,000
US tons, of fuel oil, as it is towed away from the
northwestern coast of Spain near Muxia by two tugboats to
avoid a environmental catastrophe Friday Nov. 15, 2002,
after the ship sprang a leak in a storm Wednesday. Credits:
Associated Press – Torrecilla/Pool

[Image 5:
http://www.esa.int/export/esaCP/ESAGFD7708D_index_1.html#subhead5]
Two people walk accross the Barranan beach covered with oil
in Arteixo, northwestern Spain, Sunday Nov. 17, 2002 after
oil spilled from the Bahamas-flagged tanker ‘Prestige’
washed ashore Saturday. The tanker, that is in danger of
splitting in two, has been towed out to sea to avoid an
environmental catastrophe. Credits: Associated Press –
Carmelo Alen