In the Thick of the Slick

When a marine oil spill occurs, some of the oil on the outer edge of the spill disperses, and a certain percentage breaks down and mixes with the water. Depending on the type and quantity of the oil, however, large areas can remain thick and gooey for weeks—posing a threat to wildlife and to nearby shorelines.

Knowing the exact thickness of a slick helps emergency responders direct their energies to the most hazardous areas of a spill, and enables them to determine the most effective means of cleaning it up. Until now, the only way of obtaining this information for thin slicks was to judge by their colour--a method that is accurate only to within 200 per cent of the actual value. For thicker slicks a variety of sensors, including infrared and microwave techniques, have been tested with little success beyond relative thickness information.

Now, after more than 10 years of research and development, scientists are flight-testing a prototype sensor that can measure the thickness of an oil slick on water from the air. The development of the new technology, known as the Laser Ultrasonic Remote Sensing Oil Thickness (LURSOT) sensor, is a joint project involving Environment Canada Imperial Oil Resources Limited, the U.S. Minerals Management Service, and the National Research Council.

To determine the thickness of an oil slick, LURSOT measures the time it takes for an acoustic wave to move through the oil. Three different lasers are involved. The first examines the surface of the slick to determine when the target is flat enough for a measurement to be taken. The second fires an infrared beam at the slick, causing the oil near the surface to heat and expand rapidly, and sending an acoustic pulse downward. When the acoustic pulse reaches the water, a portion is reflected back upward, causing another detectable displacement in the surface of the slick. A third laser measures the Doppler shift that occurs when the surface rises--first from the initial expansion, and again when the echo comes back. Since this technique is accurate to within 0.1 per cent and the speed of sound in different oils varies by less than 10 per cent, even the thickness of an unknown oil can be determined with much greater accuracy than before.

In addition to pointing responders to the thickest area of a slick, so it can be remediated promptly and environmental damage minimized, the new sensor will help evaluate the effectiveness of a variety of response techniques--particularly the application of dispersant, which is used to help oil enter the water column and, thereby, reduce impacts on nearby shorelines. It will also ensure that the most effective methods are used for existing conditions. For example, in situ burning requires a minimum oil thickness of 2.5-3 mm in order to sustain itself. Most importantly, LURSOT will provide valuable information on oil slick dynamics, so responders will have a better understanding of how these slicks spread.

Since its first airborne tests were conducted seven years ago, the sensor has undergone several improvements, and has been extensively tested in large-scale laboratory experiments. Most of these improvements were aimed at reducing the impact of aircraft vibrations on laser alignment, and included the development of a more sturdy housing structure and the addition of components that are less sensitive to and can actively compensate for vertical movement of the aircraft. If the latest set of flight tests, scheduled to take place by this fall, run smoothly, scientists are hopeful that LURSOT could be operational within a couple of years.

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