REMOTE SENSING

A laser-based system for examining the surface of the ocean has been developed jointly by B¥Exploration (London, England) and World Geoscience Corp. (Perth, Australia). The airborne laser fluorosensor (ALF) could make finding offshore oil fields much simpler and more cost-effective than it is with current methods.

REMOTE SENSING

Airborne fluorosensor may find underwater treasure

Paul Mortensen

A laser-based system for examining the surface of the ocean has been developed jointly by B¥Exploration (London, England) and World Geoscience Corp. (Perth, Australia). The airborne laser fluorosensor (ALF) could make finding offshore oil fields much simpler and more cost-effective than it is with current methods.

About 75% of the world`s known on-shore oil basins leak small quantities of petroleum to the earth`s surface. It seems likely, therefore, that most off-shore basins also see¥oil and that detection of the seepage would indicate the presence of petroleum accumulations. In most petroliferous basins, however, seeped oil forms a very thin film that is hard to detect with the naked eye and other passive airborne or satellite-mounted detectors. The ALF system, though, has detected oil and condensate films as thin as 0.01 µm--an order of magnitude thinner than the films detectable by passive airborne and satellite systems.

The system, which is owned exclusively by World Geoscience Corp., uses a solid-state laser to generate ultraviolet light at 266 nm. The light is pulsed at a pulse-repetition frequency of 50 H¥onto the ocean surface from an aircraft--a Fokker Friendshi¥F-27--flying 100 m above sea level (see figure on p. 48). The laser light induces fluorescence in any fresh petroleum film it encounters. The fluorescence is captured by a telescope and separated into its constituent colors by a spectrom eter operating in the wavelength range between 200 and 700 nm. The spectrom eter output is fed to a 500-channel diode array detector. This information is then recorded in 176 channels and stored together with navigation, aeromagnetic, and environmental information.

A video camera records a simultaneous image of the ocean surface where the laser hits. This image is used later to assist in classifying ALF anomalies detected in processing the data. Pollution and foreign matter that can also cause fluorescence are identified and removed from the data, and the remaining petroleum film can be classified as condensate, normal, or heavy oil. The system has been tested in a number of known oil basins and has identified the oil films expected to be present.

Oil fields are not the only targets for the ALF system. It is also providing an opportunity to investigate the mysteries surrounding Australia`s worst maritime disasters. During World War II, for example, two Australian warships--HMAS Sydney and HMAS Kormoran--were lost at sea after being hit by enemy fire; they were apparently unable to return to the West Australian coast. Minute amounts of oil may still be seeping from these sunken wrecks, making the ALF system a prime candidate for finding them.

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