On November 7, 2007, a freighter leaving a port in the Oakland Channel struck one of the piers supporting the San Francisco-Oakland Bay Bridge. The damaged ship leaked nearly 53,000 gallons of oil into San Francisco Bay. The oil first spread southward, but the bay is influenced by tides, and when the tides changed, the oil spread north and west, blackening beaches throughout the bay. Oil passed beneath the Golden Gate Bridge and out into the Pacific Ocean, polluting shorelines from Point Reyes National Seashore in the north to Pacifica Beach in the south.
This image from Canada’s Radarsat satellite shows the spread of the oil on November 12, five days after the spill. Oil spills on open water can be very difficult to see in photo-like satellite imagery because the oil can form into thin ribbons or numerous small blobs that imperceptibly darken the water’s already dark surface. In radar imagery, however, the oil becomes more obvious. South of the Bay Bridge, the oil is stretched into long ribbons, while north of the bridge, the oil is spread more uniformly over the surface, encircling Angel Island.
In radar imagery, the rougher and more textured a surface is, the brighter it looks. Land surfaces are generally brighter than water, urban land surfaces are generally brighter than natural ones, and rough water is brighter than calm water. The oil-covered water takes on the look of calmer, inland waters, such as the wetlands that occupy the Don Edwards San Francisco Bay National Wildlife Refuge (lower right).
RADARSAT-1 data ©2007 Canadian Space Agency, distributed by MDA Geospatial Services Inc. Image processing and analysis by SkyTruth. Images courtesy of SkyTruth, Ocean Conservancy, Defenders of Wildlife, and San Francisco Baykeeper.