Satellite information is helping petroleum geologists determine the potential of new oil reserves world wide. One area of interest is the detection of natural oil seeps in the oceans. In offshore exploration, the finding of deep sea oil reserves is based on detective work starting from the location of oil at the surface, to deducing its origin.

Finding the surface oil is a first but critical step. A number of offshore exploration companies, research organizations, data providers and value added companies have cooperated together to find alternatives and methodologies for slick detection using remote sensing technology. The Gulf Offshore Satellite Applications Project (GOSAP) has been evaluating the use of radar remote sensing technologies to detect and monitor the natural oil seeps.

The GOSAP consortium has verified the utility of radar satellites for oil slick detection in a number of sea-truthing exercises conducted in the Gulf of Mexico. They have found that radar technology is a cost-effective method, which is less sensitive to weather conditions than traditional optical satellite data. RADARSAT-1 has proven to be especially beneficial for a number of reasons. The ability to obtain data worldwide with use of the on-board tape recorder is important since many areas of off-shore interest are not accessible to satellite ground receiving stations.

RADARSAT with its flexibility in viewing geometry (incidence angle) provides an image product which is favourable for the detection of oil slicks. The steeper incidence angles, from 20-30 degrees are optimal for wave detection on oceans.

RADARSAT also provides the temporal coverage which is necessary for determining the persistence of the slick. RADARSAT can image most geographic regions from one to five days depending on latitude. The various beam modes contribute an additional level of flexibility in the development of a monitoring program within a region of interest. SCANSAR imagery is a cost effective alternative to repeat coverage of large areas, particularly areas which are unknown for their oil bearing potential. Although the resolution may not be adequate to accurately map the slick characteristics, it does provide a means of detecting slick presence. The use of Wide, Standard and Fine beam modes can then be used to target regions of interest for a more detailed look. Wide beam mode has proven to be of interest because of its coverage size (165X165 km) and resolution (25 m). The increased coverage of this product at resolutions similar to standard beam mode reduces the number of images needed which in turn reduces the processing, enhancement and mosaiking costs.

An example of SCANSAR, Standard and Fine mode for the same geographic area are shown in this profile. Although taken at different dates, the level of information available on slicks within each beam mode is viewable.

The wind condition is one of the factors which will affect the interpretability of the RADARSAT image. Wind speeds must be great enough to permit discrimination between the wind-roughened ocean surface (bright) and the oil-dampened surface (dark areas). If the wind speed is too low, there is little backscatter difference between the relatively calm ocean surface, and the oil-dampened surface, but wind speeds can not be so great as to create turbulence within the slick areas. It is generally accepted that slicks will be most detectable when wind speeds are between 3 m/s and 14 m/s.

To support the effective interpretation of RADARSAT imagery in the marine environment, it is therefore important to have information on the wind, weather conditions and sea state which occur at the time of image acquisition. For example, NOAA-AVHRR imagery can provide information on the weather patterns which, in turn, may be used to help interpret the ocean features evident on coincident SAR imagery. In addition, it is important to know the wind speed and the resulting sea state during image acquisition because these elements will directly influence the ability of the SAR to detect an oil slick on the ocean surface. Although, these types of data may be obtained from various weather sources, Enfotec Technical Services of Ottawa, Canada, provides a specialized environmental information service which includes the collection and integration of a range of weather, ocean current and sea state information worldwide to support RADARSAT acquisitions and interpretation for oil industry clients. One such product includes the modelled wind speed and direction as seen in the image above. The vectors are then overlaid upon the RADARSAT image to assist clients in interpreting features. Above all, auxiliary information on environmental conditions helps in achieving the ultimate objective which is the interpretation of ocean features.