Accurate knowledge of the spatial extents and thickness distributions of an at-sea oil spill is of utmost importance for efficient response. The need to know the locations of the spilled oil is obvious. Just as important, however, is knowledge of the oil film’s thickness variations. This is because most petroleum products spread rapidly on the water surface when released into the ocean, with the majority of the affected area becoming covered by very thin sheens. Although the sheens may ultimately affect very large regions, the total amount of oil they contain is small compared to areas covered by thicker oil accumulations. In an efficient spill response, available recovery resources such as booms and skimmers must be directed to the thicker portions of the oil slick.
In most parts of the world, the necessary recognizance currently relies on visual observations by trained observers aboard low-flying helicopter or aircraft, and does not employ sophisticated imaging technologies. The thickness estimations (sometimes supplemented by drawings or oblique digital photographs) rely on well-established relationships between the thickness of an oil film and its color appearance.
The methodology suffers from three main complications. First, any verbal, graphic or oblique photographic documentation is usually based only on approximate geo-location information obtained through the aircraft’s Global Positioning System. Even if it is later reformatted as input into a computerized Geographical Information System (GIS), the data can contain a great degree of positional error. Second, visual estimation of oil film thickness distribution is highly subjective and, if not done by specially trained and experienced personnel, tends to be inaccurate. Third, comprehensive visual assessments are impossible at night.
Significant advances have been made (primarily in Europe and Canada) in oil spill detection capabilities. Side-looking airborne radar and ultraviolet/infrared (UV/IR) detectors are being used operationally in Europe (Zielinski 2003, Trieschmann et al. 2003). Europe’s oil pollution recognizance programs are nationally or multi-nationally funded, with adequate resources to equip an entire fleet of dedicated aircraft with specialized oil-sensing instruments.
No such program of similar magnitude presently exists in the United States. The availability of one or even several specially equipped aircraft within North America thus does not present a practical monitoring solution for rapid response in most oil spill incidents. What is needed is an economical, portable and easyto- operate aerial system, which could be regionally owned and operated to detect and accurately map the thickness and distribution of an oil slick in coastal and offshore waters in real-time, and could disseminate the information down to the Unified Command Center.
Since 2004 Ocean Imaging Corporation (OI) has conducted research to develop such a sensor system, first through a pilot project funded by the California Office of Oil Spill Prevention and Response (OSPR); then through initial oil spill thickness detection work funded by the US Minerals Management Service (MMS).
Large-scale tank experiments and field testing over natural oil seeps
Successful remote sensing research of oil spills requires the studyof oil-on-water signatures in ‘real-world’ conditions, including variable background watercolor, wave effects and sun angle differences. Actual spills occur relatively infrequently, and so do not provide enough opportunity for experimentation. On the other hand, intentionally releasing petroleum products in U.S. waters is illegal. For the past number of years a largescale experimental oil release has been conducted annually several hundred kilometers off the Norwegian coast, but no such releases have been permitted around North America.
Natural oil seeps occur in several regions near US shores and offer a relatively steady supply of crude oil slicks for study. The initial development work for the OSPR and MMS sponsored projects was done over natural seeps that exist in the Santa Barbara Channel in California. This sensor was flown over the natural seeps, while ship-based thickness measurements were simultaneously gathered throughout each oil slick target. Continuous reflectance spectra were also obtained over films of various thicknesses and provided data for choosing imaging wavelength combinations that maximize the color reflectance changes related to increasing oil thickness.