What is the importance of oil spill monitoring?
Oil spills occur when any form or amount of fossil fuel is released into the ocean. Since the start of offshore oil drilling and the transportation of fossil fuels via ships and pipelines, the number and severity of anthropogenic oil spills has surged. Even though safety measures have increased significantly, the overall amount of oil released into the ocean has risen due to the increased aforementioned activity. Oil spills can have significant impacts for aquatic and terrestrial ecosystems. The composition, quantity and quality of the oil released as well as the environment in which the oil released heavily influence the degree and level of environmental, social and economic impact. The primary environmental effects include, but are not limited to, the destruction of ecosystems, loss of animal and human habitat as well as damage to local fishing and other water based industries. These impacts are most often non-permanent. However, cleaning efforts are often high in cost and are often not "completed" due to the spreading of oil in water. Thick oil slicks from a fresh spill diffuse rapidly in water, making clean-up operations extremely difficult. For this reason it is necessary to gather information on the oil thickness and its distribution as quickly as possible (SEOS). Space-based information have previously been used to map and monitor oil spills including their location, extent and "movement" of the released oil. According to Fingas and Brown (2017), oil spill mapping is especially important for:
- Oil spill mapping for both tactical and strategic countermeasures
- Slick detection and surveillance
- Gathering of legal information
- Law enforcement such as regarding ship discharge
- Direct support for oil spill countermeasures
- Slick trajectory determination
For disaster management, points 1,2,5 and 6 are of major importance to reduce the earlier discussed environmental and subsequent effects of an oil spill.
Oil spill preparation and response
Before oil spills occur, sensitivity maps of vulnerable areas can be created from space-based information. The International Convention on Oil Pollution Preparedness, Response and Cooperation OPRC Convention which was established in 1990, recommends a procedure to include the topography, the ecosystem composition and socio-economic features, such as built up infrastructure in the region. The flowchart below presents the OPCR approach:
The environmental impact is strongly dependent on the characteristics of the surrounding ecosystem. Habitats and the species affected are strongly determined by the shoreline terrain. Shoreline types can be identified from existing topographic or thematic maps or from aerial photos and high resolution satellite imagery. The digital elevation maps can be determined from overlapping satellite images. Each shore type can be ascribed an environmental sensitivity index based on the:
- Shoreline type (slope, terrain) which affects how oil penetrates or accumulates on the shore
- Exposure to currents and tides (wave energy) which determines how long oil remains on the shore
- General sensitivity of the environment (e.g. protected areas which can be found at WDPA) This pre-spill or baseline data can be used as inputs to models to forecast potential spills, to determine spill timelines, to identify resources at risk and establish an emergency response plan IPIECA. Once a spill has been detected, further satellite monitoring can be applied with the following methods.
How are oil spills monitored from space?
Two factors are especially important when trying to limit the impact of an oil spill: The amount of oil released and the direction in which the oil film is moving. Since this information is vital, a number of geospatial data based methods have been developed to remotely assess the impact of an oil spill. These can be based on two different sensor types, a.) optical sensors which receive and measure natural emissions or reflections and are thus sensitive to cloud cover, and b.) radar sensors which measure a signal transmitted by the sensor and reflected from earth and which can pass through clouds NASA. The properties of oil which are captured by various wavelengths are illustrated below.
Optical satellite imagery can provide decision makers with an idea of the extent and the severity of an oil spill event. To do so, the color and reflectance differences between oil and water are exploited (SEOS). Nevertheless, the appearance of sun glint, as it has similar properties to the silver reflection properties of oil, as well as potential cloud cover can influence the accuracy of images.
Another method that is often employed uses spaceborne radar imagery (SAR) in order to monitor oil spill developments. Since oil slicks on the sea surface dampen the waves thereby reducing the surface roughness, oil will appear as dark spots or areas in the image (Skrunes, 2012). However, a limitation is that some natural phenomena like biogenic slicks look similar and can thus potentially give false alarms. Well adapted algorithms to reduce the potential noise of the radar images are thus needed. Since radar imagery can be used even under cloud cover and there is well-established research on the SAR's advantages and limitations for oil spill mapping, it is an often used method for this application (KSAT).