Airborne laser sensors for oil spill remote sensing.

Remote sensing, using a variety of types of airborne laser sensors, may provide oil spill response personnel with strategic information assisting in the prevention of damage to marine environments.

Remote sensing is rapidly becoming an important tool for the effective direction of oil spill countermeasures. Spill response personnel have recognized that remote sensing can increase spill cleanup efficiency. The general public expects that the government and/or the spiller know the location and the extent of the oil contamination. In addition to providing operational oil spill remote sensing, the Emergencies Science Division (ESD) of Environment Canada is engaged in the development of airborne oil spill remote sensors, including the Scanning Laser Environmental Airborne Fluorosensor (SLEAF) and the Laser Ultrasonic Remote Sensing of Oil Thickness (LURSOT) sensor.

It has long been recognized that there is no one sensor which is capable of detecting oil in all environments and spill scenarios. There are sensors which possess a wide field-of-view (FOV) and can, therefore, be used to map the overall extent of the spill. These sensors, however, lack the specificity required to positively identify oil and related products. This is even more problematic along beach and shoreline environments where several substrates are present. The laser-based sensors under development by Environment Canada are designed to respond to specific roles in oil spill response. The SLEAF is being developed to unambiguously detect and map oil in complicated shoreline environments where other non-specific sensors experience difficulty. The role of the SLEAF would be to confirm or reject suspected oil contamination sites that have been targeted by the non-specific sensors. This confirmation will release response crews from the time consuming task of physically inspecting each site, and direct crews to sites that require remediation. The LURSOT sensor will provide an absolute measurement of oil thickness from an airborne platform. This information is necessary for the effective direction of spill countermeasures such as dispersant application and in-situ burning.

Laser-based Oil Spill Remote Sensors

Laser Fluorosensors

Laser fluorosensors are active sensors (i.e. provide their own source of excitation or illumination) which take advantage of the fact that certain compounds (aromatic compounds in particular) in petroleum oils absorb ultraviolet light and become electronically excited. This excitation is rapidly removed through the process of fluorescence emission, primarily in the visible region of the spectrum. Since very few other compounds show this tendency, fluorescence is a strong indication of the presence of oil. Natural fluorescing substances such as chlorophyll, fluoresce at sufficiently different wavelengths to avoid confusion. Since different classes of oil yield slightly different fluorescence spectral signatures and intensities, it is possible to differentiate between classes of oil under ideal conditions.

Most laser fluorosensors used for oil spill detection employ a laser operating in the ultraviolet region between 300 and 355 nm. Crude oil fluorescence return is in the region between 4-00 to 550 nm with peak centres in the 480 nm region. Laser fluorosensors are thought to have significant potential for the future because they may be the only means to discriminate between oiled and un-oiled weeds and detecting oil on a variety of beach types. Tests on shorelines show that this technique has been very successful. Additionally, the sensor offers the only means of reliable detection of oil in certain ice and snow situations.

The Scanning Laser Environmental Airborne Fluorosensor

Following the successful development, testing and operation of the nadir-looking Laser Environmental Airborne Fluorosensor (LEAF), the ESD along with it's co-funders is developing the next generation system. The new system, SLEAF, will incorporate two variable angle scanner heads to provide the cross-track coverage required for investigating shorelines.

The SLEAF will be comprised of three units: (1). an operator's console with data recording and control unit, (2). the laser scanner/transmitter unit, and (3). a spectrometric receiver. The data recording and control unit will include a colour monitor, keyboard/mouse, dual Exabyte tape drives for storage of raw and processed data, a hard copy printer, fax modem, and a down-looking ground track colour video display. The laser scanner/transmitter unit consists of a pulsed excimer laser (XeCl 308 nm, 400 Hz, 100 mJ/pulse), and a variable angle scanner. The scanner will allow for conical scanning of the surface beneath the aircraft with swath widths of one-sixth or one-third of the altitude of the aircraft (300 -600 m). The swath width will be varied by the interchange of two scanner heads. The spectrometric receiver will include a 20-cm diameter, f/3 off-axis parabola, with a 1 x 3 mrad field-of-view and a gated intensified diode array detector. The detector will only be gated on for a time sufficient to collect the laser-induced fluorescence, while rejecting most of the background solar irradiation. The gate width will be operator selectable and can be offset in time to permit interrogation of the water column to depths of up to a couple of metres, depending on water clarity.

Fluorescence spectra (64 spectral channels from 330 to 610 nm), will be analyzed in real-time using a principle component analysis to determine whether or not oil is present in the field-of-view. Pixels containing oil contamination will be classified as light refined, crude or heavy refined oils. This classification, along with the strength of the fluorescence signal will be used to estimate the percentage of the field-of-view which is contaminated with oil. The extent of oil coverage will be categorized as clean, light, moderate or heavy. For display on the operator's console and production of faxable maps, an average coverage over a sampled area of about 50 metres long by the width of the swath will be displayed. The oil classification and extent of coverage will be determined for the swaths on either side of the aircraft. The operator's monitor will graphically display the oil contamination in the form of a coloured geo-referenced map. Oil classification will be identified by different colours, while the extent of coverage will be depicted by a Fine perpendicular to the flight path of the aircraft.

The timely production of useable oil contamination intelligence information is essential for viable response efforts. The geo-referenced map, annotated with oil contamination locations will be available on demand, either as a hard copy or available electronically for transmission via facsimile to response crews on the ground or at sea.

It is the ability of the laser fluorosensor to discriminate between oiled and unoiled surfaces that is of prime importance in a spill response. The location(s) of "suspected" oil contamination can be confirmed or rejected, based on the real-time analysis provided by the SLEAF. This confirmation will allow for the rapid deployment of response crews and equipment to the locations requiring remediation and/or protection, and free the crews from the time-wasting task of manually checking "suspected" sites.

Slick Thickness Sensors

There are presently no reliable methods, either in the laboratory or in the field to provide an accurate measure of oil-on-water slick thickness. Knowledge of slick thickness would allow for the effective direction of spill countermeasures including dispersant application and in-situ burning. The ability to measure oil slick thickness would provide significant advances to the basic understanding of the dynamics of oil spreading and behaviour. In addition, there is a need to calibrate some of the more readily available pieces of remote sensing equipment. Several of these commercially available sensors, including infrared and ultraviolet cameras, provide relative, i.e. thick or thin, indications of slick thickness. Calibration of these wide field-of-view sensors would provide a reliable method of estimating the volume of oil slicks. Current methods of airborne surveillance of slicks often results in erroneous oil quantity estimates.

Laser Ultrasonic Remote Sensing of Oil Thickness

A promising technique for the measurement of oil slick thickness involves the use of laser acoustics. The Industrial Materials Institute of the National Research Council of Canada has recently developed a novel measurement technique called Laser-Ultrasonics for the non-destructive evaluation of materials. This technique has shown considerable promise as a method to provide an accurate measurement of oil slick thickness on the surface of water from an aircraft. The prototype airborne system is known as the Laser Ultrasonic Remote Sensing of Oil Thickness (LURSOT) sensor. A consortium of agencies including Environment Canada, Imperial Oil Resources Limited and the United States Minerals Management Service is pursuing the technology. The LURSOT sensor is a three laser system with one of the lasers coupled to an interferometer to accurately measure oil thickness. The sensing process starts with a thermal pulse created in the oil layer by the absorption of a powerful C[O.sub.2] laser pulse. Rapid thermal expansion of the oil occurs near the surface where the laser beam was absorbed. This causes a step-like rise of the sample surface as well as an acoustic pulse of high frequency. The acoustic pulse travels down through the oil until it reaches the oil-water interface where it is partially transmitted and partially reflected back towards the oil-air interface where it produces a slight displacement of the oil surface. The displacement of the surface is measured by a second laser probe beam (Nd:YAG) aimed at the surface. Motion of the surface induces a phase or frequency shift (Doppler shift) in the reflected probe beam. This frequency modulation of the probe beam is then demodulated with an optical interferometer. The thickness can be determined from the time of propagation of the acoustic wave between the upper and lower surfaces of the oil slick and knowledge of the acoustic velocity in oil. A third laser (cw HeNe) is employed to interrogate the water surface and generate a trigger pulse when the proper geometry for measurement exists. In the laboratory, the LURSOT system has measured a range of oil thicknesses from 250 pm to 35 mm over distances up to 91 metres. The LURSOT system will be mounted on the ESD's DC-3 remote sensing aircraft for a test flight in the fall of 1997.

Benefits to the Oil Spill Response Community

It is our hope that the information provided by these laser-based sensors will aid oil spill response personnel in their quest to mitigate the potentially disastrous effects of an oil spill on sensitive marine and coastal environments.

Acknowledgements

The SLEAF is being developed with support from the Emergencies Science Division of Environment Canada, the United States Minerals Management Service, the United States Coast Guard, the Technology Development Centre of Transport Canada, the Atlantic, Ontario and Pacific Regions of Environment Canada, the Panel on Energy Research and Development, Environment Canada, Barringer Research Limited and Optech Incorporated. The LURSOT system is being developed with support from the Emergencies Science Division of Environment Canada, Imperial Oil Resources Limited, the United States Minerals Management Service and the Industrial Materials Institute, National Research Council of Canada.

Carl E. Brown, MCIC, received his PhD in Physical Chemistry from McMaster University in Hamilton in 1988 and has spent the last five years as a Research Scientist at the Emergencies Science Division, Environment Canada, Ottawa, ON. His specialties include airborne oil spill remote sensor development and the application of laser technologies to environmental problems.