US: By analysing satellite images, oceanographers have found that small oil spills in the heavily drilled northern Gulf of Mexico are often much larger than reported. The researchers presented their results at the Gulf of Mexico Oil Spill and Ecosystem Science Conference in New Orleans, Louisiana.
Small oil spills — ranging from oil-drilling mishaps to ships discharging fuel — occur with surprising regularity, and tend to escape the public”s attention that follows big spills. When someone spills petroleum or derived products in US waters, the accident must be reported to the US Coast Guard’s National Response Center in Washington DC. Those who report such spills are required to provide their own estimates of the area affected.
The oceanographers, based at Florida State University (FSU) in Tallahassee, worked with SkyTruth in Shepherdstown, West Virginia, a non-profit organisation that tracks oil spills and other events using mainly publicly available satellite images. The goal was to take a closer look at the chronic small spills by exploiting an FSU database of much higher resolution satellite images, obtained using a type of radar called synthetic aperture radar (SAR), which had been used to study slicks formed by natural oil seeps.
After identifying images that showed accidental slicks, FSU graduate student Samira Daneshgar Asl analysed the images with a programme that uses telltale differences in water surface texture where oil is present to calculate slick areas. She found that the slicks with known human causes were typically about 13 times larger than the estimates reported to the National Response Center.
“There is very consistent underreporting of the magnitude of [oil] releases,” said FSU team leader Ian MacDonald. “Sometimes it’s quite laughable.” On the positive side, he said that his team did find that the slicks had consistently been reported.
“It is not surprising that there are discrepancies” between the radar images and the assessments reported to the Coast Guard, said Emily Kennedy, a policy analyst at the American Petroleum Institute, an industry group in Washington DC. “Remote-sensing applications can be challenging, since they often provide false positives because of natural phenomenon like sea kelp.” She notes that such images require a lot of “ground truthing” — confirming that the image shows a real slick by visiting the site, for example.