Galicia, Spain Oil Slick

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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 instrument 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. 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.

Technical Information:

• Satellite: Envisat
• Instrument: ASAR
• Acquisition: 17-Nov-2002
• Orbit nr: 03741
• Center coordinates: lat. 42.74, lon. -7.81

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