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Lasers map slicks from the air ... and estimate their thickness.

Determining which ecosystems an oil slick will threaten and when hinges upon mapping any floating oil. A group of Italian researchers has integrated a pair of remote-sensing techniques to acquire this information through aerial reconnaissance.

Because thermal anomalies reveal surface pollution, the team fitted a helicopter with a flir (forward-looking infrared) system to passively map sea-surface temperatures. The presence of anomalies, however, tells nothing about the type of pollution, notes project leader Alberto L. Geraci of the University of Catania in Sicily. For that, his team scans anomalous-temperature zones using a lidar (light detection and ranging) system, which irradiates the surface with an ultraviolet-light-emitting laser. A computer analyzes the returning light signature of the sampled zone against a library of known spectra to identify the polluting agent. By correlating these data with maps or positioning signals from satellites, spill-response teams can map oil slicks and project their spread.

Crews field tested the experimental system off Sicily last year at speeds of up to 20 kilometers per hour. Geraci says the region, whose coastal waters are extensively polluted, contains industrial plants, refineries, and offshore oil wells.

The flir/lidar system is especially effective when measurements are taken at a height of about 15 meters, and it not only can detect petroleum, but also can identify its type and roughly gauge how much is present, Geraci reports. Moreover, he adds, the system can distinguish a host of other pollutants as well -- from residues of waste sewage to sulfonate of lignin, a highly polluting by-product of woodworking.

Geraci says his team is already at work on the prototype for a commercial flir/lidar system.

... and estimate their thickness

Though aerial photos can determine the surface dimensions of an oil slick at sea, they lack information about depth that is needed to estimate the volume of oil present. Marc Choquet oi the National Research Council of Canada in Boucherville, Quebec, now reports progress on a new remote-sensing tool to gauge a slick's thickness from instruments aboard a low-flying aircraft.

The technique - borrowed from industrial quality-control experts who conduct nondestructive inspections of materials -- involves directing a short pulse of laser light at the top of the slick to generate ultrasonic waves in the oil. A second laser, coupled to an optical interferometer, analyzes the return signal to measure those ultrasonic waves. If one knows the acoustic properties of the sampled oil, then the time it takes for an ultrasound wave to reflect between the top of the slick and the oil-water interface will provide a "direct and unambiguous determination of the oil layer's thickness," Choquet says.

The two-laser technique proved adequate for measuring oil atop a motionless pool of water - not a condition typical of slicks at sea. To deal with waves, his team added a third laser. It monitors the tilt of the water-borne slick's surface. Only when the angle of a wave or ripple under the first laser is likely to result in a useful return signal will this third laser activate the two-laser ultrasound probe.

Choquet reports that waves do not reduce the system's accuracy for slicks between 200 microns and 15 centimeters thick, only the rate at which individual sampling measurements can be made. Preliminary airborne tests showed that height also reduces sampling rate - from a maximum of three times per second under quiet seas at 91 meters to perhaps only once every 10 seconds or more at sampling heights of 150 meters.

Choquet says a test of the complete system on board a DC-3 aircraft is scheduled for the first week in May,
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Title Annotation:oil spills
Author:Raloff, Janet
Publication:Science News
Article Type:Brief Article
Date:Apr 24, 1993
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