A thousand feet down, the ocean floor shakes. Bubbles of oil and gas burp out. In a wafting plume, the oil heads for the ocean surface where it forms a film less than a millimeter thick and many cover hundreds of square kilometers.
These natural oil leaks are leading geologists to lucrative drilling sites. In fact, 80% of offshore oil exploration starts by searching for seeps, according to Roger Mitchell, program development vice president at Rockville, Md.-based Earth Satellite Crop. (EarthSat).
The oil’s viscosity retards wave formation, causing a “calm spot” on the ocean surface, and that allows synthetic aperture radar (SAR) to image and analyze the seeps for potential oil reserves. “This is cost-effective exploration,” says John Hornsby, worldwide sales director for RADARSAT International (RSI), Richmond, British Columbia, Canada.
Satellites have been used for this purpose for the last 10 years. Initially, Landsat Thematic Mapper optical imagery was used, but that imagery –which works only in ideal weather conditions-must “see” oil that’s one-half to one millimeter thick, according to Mitchell. Radar satellites are sensitive to changes in surface roughness and aren’t affected by weather conditions over target. That’s important because many of the best places to look for undersea oil (Asia, Canadian Maritimes and South America) have frequent, heavy cloud cover. “RADARSAT really detects changes in the surface tension caused by oil only a few microns thick,” says Mitchell. And RADARSAT can detect as little as one-half to one liter of oil escaping per day.

In the Beginning
Natural oil seeps in the Gulf of Mexico aren’t a new phenomenon. In fact, their presence has been documented for some time. Pre-Columbian cultures probably used seep material to make caulk, and Spanish shipping logs from the 1500s refer to floating oil.
More recently, astronauts abroad the Shuttle Atlantis in May 1989 spotted many bright, long, rope-like streaks. A photograph showed at least 124 of these features spread over 15,000 square kilometers. Follow-up investigations, including a subsequent shuttle flight, Landsat images and onsite inspection, proved the streaks were bubbles of oil and gas that escaped naturally from underwater oil fields, floated to the surface and spread out.

How it works
SAR detects the backscattered radar energy from target. The seep appears smoother than the surrounding ocean because it reflects less energy back to the satellite, which shows up in the image as dark spots against a lighter background. But extreme ocean conditions hinder clear detection. For example, rough seas create more backscatter, so the seep blends into the background. Likewise, in calm seas little backscatter bounces back, and the ocean surface appears dark and blends with seeps.

“As long as the waves are less than three meters high, satellite images can show the calming effects of oil on the surface,” says Hornsby.

Geologists use weather satellites to determine the meteorological conditions at the time the images were acquired. Generally, users find the best results are obtained if wind speeds are between three and 12 meters per second at the time of acquisition.

Natural oil slicks are detectable as black patches on this Radarsat ScanSAR Narrow subscene (100km x 100km) of Green Canyon, Gulf of Mexico acquired June 12, 1997. Wind variations associated with a frontal system are seen as two vertical dark lines in the center of the image.
 
A 3-D view of the sea floor, probable sea floor seep location and sea surface oil slicks (purple) was created with bathymetric and RADARSAT data (June 12, 1997). Topgraphic information was derived from National Geophysical Data Center 100-meter multibeam bathymetrry data.
RADARSAT image enhancement and interpretation by Earth Satellite Corp. 1997

By Eng. Hazem Badaw

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