Envisat ASAR image earlier the same day (ascending overpass) shows the oil as much darker than the surrounding water due to oil damping of cm scale wind ripples. The arrows point to features of thicker and thinner oil which could be seen in the optical image, but are not distinguishable in the SAR image. On the other hand, the dark area (oil) is larger than in the optical image, because SAR is sensitive to surface oil films that are too thin to be observed in optical images. Credit: ESA LearnEO

the spill a month later as the oil had entered the Gulf of Mexico Loop Current. In this image the oil is found in the sunglint zone, and therefore appears brighter than the surrounding water because it reflects more of the direct sunlight. A transition zone marks where the oil-water contrast shifts from positive (brighter oil) to negative (darker oil).

Annotated Envisat MERIS image of the oil spill just over a week after the explosion on 20 April 2010. Thick oil is visible as brownish red stripes, surrounded by an area of oil with intermediate thickness, and thinner oil, which is darker than the surrounding water

SAR image with coloured lines marking the location of oil slicks observed in a time series of SAR images. These mark the locations of thin oil slicks leaking from the fuel tanks of the Volgoneft, which broke up and sank the previous November. As the oil from the tank resurfaced, the oil spread in a downwind direction. Because the wind direction varied over the 2 month period, the lines marking the oil location in each image form a star centered on the location of the wreck. Credit: Space Research Institute of Russian Academy of Sciences (IKI-RAS).