Oil spill monitoring

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The development of remote sensing techniques allows the detection and monitoring of oil spills. This article describes the possibilities and requirements to detect oil spills with remote sensing techniques. It is concluded that radar techniques are generally more suitable for oil spill monitoring than satellite or airborne techniques.

Introduction

The ability to remotely detect and monitor oil spills at sea is becoming increasingly important due to the constant threat posesd to marine wildlife and the ecosystem. As the demand for oil based products increases, shipping routes will consequently become much busier and the likelihood of slicks occurring will also increase. If applied correctly, remote sensing can act as a beneficial monitoring tool. It can allow for early detection of slicks, provide size estimates, and help predict the movement of the slick and possibly the nature of the oil. This information will be valuable in aiding clean-up operations, and will not only help save wildlife and maintain the balance of the local ecosystem, but will also provide damage assessment and help to identify the polluters.

Remote sensing allows for the detection and the monitoring of oil spills. Typical platforms are satellites and aircrafts. In respect to the type of energy resources one must differentiate between two techniques:

  • Passive systems: Passive systems make use of sensors that detect the reflected or emitted electro-magnetic radiation from natural sources (Visible spectrum, reflective Infrared and Thermal Infrared).
  • Active systems: Active systems detect reflected responses from objects that are irradiated from artificially-generated sources, such as radar or laser systems.

Constraints and possibilities to monitor oil spills

Technical requirements

Figure 1. ASAR image at 7:51 GMT 6 Aug 2006. Courtesy of ESA, INGV and JRC.

Due to the nature of an oil slick a satellite remote sensing platform is required to have the following:

  • High temporal resolution, due to the changing nature of the oil and its immediate threat to the ecosystem
  • The ability to image a given area regardless of cloud cover and prevailing weather conditions (even time of day)
  • High spatial resolution, to identify individual small oil patches (windrows)
  • Wide spectral resolution, as the position and width of the spectral band is important in distinguishing the oil from the adjacent water

Presently, no existing remote sensing platform, in space or airborne, can meet all of the above requirements. For more information about available techniques, see also the website of cereac

There are certain times when visual techniques and optical satellite image are not suitable for the mapping of an oil spill; it is in these cases when radar remote sensing is required. These situations include spills covering vast areas of the marine environment, or when the oil cannot be seen or differentiated from the surrounding water. The distinction of oil in these circumstances presents several unique problems. For example, the remotely sensed data collected in these situations often provide complex signatures, which must be deciphered in order to locate the spilled oil.

Monitoring with Synthetic Aperture Radar (SAR)

SAR (Synthetic Aperture Radar) seems to be one of the most effective instruments for the detection of slicks since slicks damp strongly short waves measured by SAR and oil spills appear as a dark patch on the SAR image. SAR observations do not depend on weather (clouds and sunshine), which permits the showing of illegal discharges that most frequently appear during night. SAR can also survey storms areas, where accident risks are increased. An example of an image is shown in figure 1 (Courtesy by ESA, JRC and INGV). The image refers to the Lebanese oil spill accident of July 2006. The most suitable SAR radar configuration for oil pollution study is C-band radar frequency with VV polarization, with a 20 to 45° incident angle. This is the case for ERS , RADARSAT and Envisat http://envisat.esa.int/satellites. ERS SAR imagery is also useful for oil spill monitoring because of its ability to recover the surface wind characteristics which are very important for oil weathering. Wind direction can be retrieved from an orientation of the organized structures in the atmospheric boundary layer (convective rolls) that clearly manifest themselves in the SAR images, as well as can be estimated from backscatter variation near the islands and capes.

The spatial coverage of the SAR imagery is adapted to pollution survey (100 x 100 km for ERS-1 and ERS-2; 300 x 300 km for RADARSAT, and from 100x 100 km to 405 x 405 km (wide swath) for ASAR Envisat). A problem is the satellite coverage frequency (35 days for ERS), but now RADARSAT SAR and Envisat ASAR allow covering every 2-3 days for accident cases.

Table 1. Spaceborn synthetic aperture radars (SAR) (from http://cearac.poi.dvo.ru/en/background/satellites/).
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Satellite monitoring

Remote-Sensing Satellites are characterized by their altitude, orbit and sensor. They cover vast areas and have a specific repetition time (ranging from several to 16 days). A list of earth observation satellites are given at Wikipedia. Figure 2 sketches the resolution and swath of different satellites.

Figure 2: Resolution and swath of different satellites
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Airborne monitoring

Airborne oil spill monitoring has become a global concern over the last three decades. Currently there are a multitude of specialized airborne remote sensing systems all around the world, which are operated for this purpose, especially for the deterrence of potential polluters and the support of oil spill clean-up activities. In the 1970s and 1980s the main effort has been directed toward developing sensors with enhanced spill monitoring capabilities which explains the large number of existing well-established oil spill remote sensors like infrared/ultraviolet line scanners, microwave radiometers, laser fluorosensors, and X-band radar systems. Recently, more attention has been given to the automated processing of remotely sensed oil spill data acquired by airborne multi-sensor platforms in terms of data analysis and fusion. (Robbe N. and T. Hengstermann, 2006[1]). Using satellite platforms to monitor oil spills is more cost effective than applying airborne monitoring techniques but operation of aircraft is still the only possible way to perform a spatio-temporally flexible surveillance, so airborne monitoring can be seen as complementary to satellite monitoring. Existing users advocate a combined satellite and airborne monitoring service.

See also

Internal links

Use of radar techniques:

External links

References

  1. Robbe N. and T. Hengstermann. Remote sensing of marine oil spills from airborne platforms using multi-sensor systems. WIT Transactions on Ecology and the Environment, Vol 95, © 2006 WIT Press. Water Pollution VIII: Modelling, Monitoring and Management. pp 347 doi:10.2495/WP060351


The main author of this article is Renata Archetti
Please note that others may also have edited the contents of this article.