Difference between revisions of "Oil spill monitoring"

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The development of [[remote sensing]] techniques allows the detection and monitoring of [http://en.wikipedia.org/wiki/Oil_spill 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 spills|oil spill]] monitoring than satellite or airborne techniques.  
+
 
 +
The development of [[remote sensing]] techniques allows the detection and monitoring of [[oil spills]]. This article describes the possibilities, techniques and requirements to detect oil spills using remote sensing.  
  
 
==Introduction==
 
==Introduction==
−
The ability to remotely detect and monitor [http://en.wikipedia.org/wiki/Oil_spill oil spills] at sea is becoming increasingly important due to the threat that such [[pollution]] poses to marine wildlife and the [[ecosystem]]. As the demand for oil based products increases, shipping routes will consequently become much busier, the likelihood of slicks occurring is increasing. If applied correctly, [[remote sensing]] can act as an important monitoring tool. It can provide 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 invaluable in aiding clean up operations, and consequently help save wildlife and the balance of the local ecosystem, provide damage assessment and help to identify the polluters.
+
The ability to remotely detect and monitor oil spills at sea is important due to the constant threat posed to marine wildlife and the ecosystem. Remote sensing 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 to detect and monitor [[oil spills]]. Typical platforms
+
Remote sensing can use two types of sensor systems:  
−
are satellites and aircrafts. In respect to the type of energy resources
+
* '''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).
−
it must be differentiated between two techniques:  
+
* '''Active systems''': Active systems detect reflected responses from objects that are irradiated from artificially-generated sources, such as radar or laser systems.
−
* '''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==
+
Remote sensing platforms for oil spill monitoring are satellites, aircrafts and drones.
−
===Technical requirements===
+
 
−
[[image:uno2.gif|thumb|300px|right|Figure 1. ASAR image at 7:51 GMT 6 Aug 2006. Courtesy of ESA, INGV and JRC.]]  
+
===Satellite monitoring===
−
Due to the nature of an oil spill a satellite remote sensing platform is required to have the following:
+
Remote-Sensing Satellites are characterized by their altitude, orbit and sensor. They cover vast areas and have a repetition time ranging from several to 16 days. In the past, times from the tasking of the satellite to image delivery were as long as 12 h, currently, times of 4 h are possible. A further consideration is overpass time. Several satellites cover an area once per day. Satellite constellations such as Cosmo (Constellation of Small Satellites for Mediterranean basin Observation) give revisit times of a few hours compared to the present one-day.
−
* High temporal resolution, due to the changing nature of the oil and its immediate threat to [[ecosystem]],
+
Satellite-carried radars with their frequent overpass, high spatial resolution and their day–night and all-weather sensors are essential for detecting large spills and monitoring ship and platform oil releases.
−
* 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),
+
However, satellite observation of oil spills is not yet operational for the global oceans. For example, the most extensive oil spill ever recorded in tropical oceans worldwide (5-12 million liters) that hit the Brazilian coast in 2019 over a length of almost 3000 km, escaped satellite observation. The origin of this 'orphan' oil spill has never been determined<ref>Soares, M.O. and Rabelo, E.F. 2023. Severe ecological impacts caused by one of the worst orphan oil spills worldwide. Marine Environmental Research 187, 105936</ref>
−
* Wide spectral resolution, as the position and width of the spectral band is important in distinguishing the oil from the adjacent water.
+
 
 +
===Airborne monitoring===
 +
 
 +
[[File:OilSlick.jpg|thumb|left|300px|Fig. 1.  Aerial image of an oil slick. Photo credit The Norwegian Coastal Administration/NOFO/Sundt Air.]]
 +
 
 +
Airborne oil spill monitoring has become a global practice over the last three decades.
 +
In the 1970s and 1980s a major effort has been directed toward developing sensors with enhanced oil spill monitoring capabilities based on various techniques like infrared/ultraviolet line scanners, microwave radiometers, laser fluorosensors, and X-band radar systems.
 +
Currently a multitude of specialized airborne remote sensing systems are operated, especially for the deterrence of potential polluters and support to oil spill clean-up activities.
 +
 
 +
Using satellite platforms to monitor oil spills is more cost effective than using 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. Users advocate a combined satellite and airborne monitoring service.
 +
<br clear=all>
 +
 
 +
==Oil monitoring sensors and techniques==
 +
 
 +
===Monitoring requirements===
 +
 
 +
[[image:uno2.gif|thumb|300px|right|Figure 2. The Lebanese oil spill accident of July 2006. SAR image at 7:51 GMT 6 Aug 2006. Courtesy of ESA, INGV and JRC.]]  
 +
 
 +
Requirements for oil slick monitoring are:
 +
* 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)
 +
* Capability to distinguish the oil from the adjacent water
 
   
 
   
−
At the present, 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 [http://cearac.poi.dvo.ru/en/background/techniques/ website of cereac]
+
===Monitoring with radar===
 +
 
 +
There are two configurations of active side-looking radar systems: [https://en.wikipedia.org/wiki/Synthetic-aperture_radar Synthetic Aperture Radar (SAR)] and side-looking airborne radar (SLAR). SLAR is cheaper and uses a horizontal antenna to produce imagery along the flight path. Synthetic aperture  radar uses the forward motion of the platform to attain spatial resolution. SAR resolution is not dependent on range, but uses extensive electronic processing to produce high resolution images. SAR has larger range and resolution than SLAR and is used in all radar satellites. SLAR is mostly used for airborne oil spill remote sensing, as it is cheaper. The imaging geometry of SAR and SLAR is an oblique projection type. The working wavelength, incidence angle, polarization mode of the radar sensor, surface roughness, and dielectric constant of the ground object all affect the backscattering of the signal. Primary pre-processing of SAR images involves radiometric calibration, geocoding, and land masking.
 +
 
 +
Winds above 1.5 m/s generate capillary waves at the water surface that appear as so-called 'sea clutter' on the radar image due to Bragg scattering<ref>Phillips, O.M. 1988. Radar Returns from the Sea Surface—Bragg Scattering and Breaking Waves. J. Phys. Ocean. 18: 1065-1074</ref>. As these capillary waves are damped by the oil slick, the slick appears as a dark patch on the SAR image where sea clutter is suppressed. However, other features may produce slick look-alikes, such as fresh water slicks, internal waves, wave shadows behind structures, floating macro-algae such as sargassum and kelp, etc. Several automatic or semi-automatic Radar Image Processing techniques have been developed that can interpret a radar image for oil slicks in a matter of minutes, especially using deep learning algorithms (e.g. [[Random Forest Regression|Decision Tree Forest]] <ref>Topouzelis, K. and Psyllos, A. 2012. Oil spill feature selection and classification using decision tree forest on SAR image data. ISPRS J. Photogramm. Remote. Sens. 68: 135–143</ref>, [[Support Vector Regression|Support Vector Machine classification]] <ref>Baek, W.-K. and Jung, H.-S. 2021. Performance Comparison of Oil Spill and Ship Classification from X-Band Dual- and Single-Polarized SAR Image Using Support Vector Machine, Random Forest, and Deep Neural Network. Remote Sens. 13, 3203</ref>, [[Artificial Neural Networks and coastal applications|convolutional neural network]] <ref>Fan, Y., Rui, X.; Zhang, G., Yu, T.; Xu, X. and  Poslad, S. 2021. Feature Merged Network for Oil Spill Detection Using SAR Images. Remote Sens. 13, 3174</ref>) for feature extraction.
 +
SAR observations do not depend on weather (clouds and sunshine), which enables the detection of illegal discharges that most frequently appear during night. SAR can also survey areas during storms, where accident risks are increased. An example of an image is shown in Fig. 2.   
  
−
There are certain times when visual techniques and optical satellite image are unsuitable for mapping of oil spill; it is in these cases where radar remote sensing is required. These situations include spills covering vast areas of the marine environment, and when the oil cannot be seen or discriminated against the background. The discrimination of oil in these circumstances presents several unique problems. The remotely sensed data collected in these situations often provide complex signatures, which must be deciphered in order to locate the spilled oil.
+
{| class="wikitable" style="float:right; text-align: center; font-size:80%; margin-right: 2px"
 +
|-
 +
! Band !! Frequency [GHz]  !! Wavelength [cm]
 +
|-
 +
| L|| 1-2 || 15-30
 +
|-
 +
| C|| 4-8 || 3.75-7.5
 +
|-
 +
| X || 8-12 || 2.5-3.75
 +
|}
  
−
===Monitoring with Synthetic Aperture Radar (SAR)===
 
−
SAR (Synthetic Aperture Radar) seems to be one of the most suitable instruments to 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 allows showing illegal discharges that most frequently appear during night. SAR can also survey storms areas, where accident risks are increased. An example of 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  [http://it.wikipedia.org/wiki/ERS ERS] ,  [http://en.wikipedia.org/wiki/RADARSAT-1 RADARSAT] and  [http://en.wikipedia.org/wiki/Envisat Envisat] http://envisat.esa.int/satellites. ERS SAR imagery are additionally useful for oil spill monitoring due to ability to recover the surface wind characteristics which are very important for oil weathering. Wind direction can be retrieval 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.
+
The lengthy availability of oil during the Deepwater Horizon spill provided several researchers with the opportunity to study radar remote sensing as well as band relationships. Overall, X-band is superior to other bands, but C-band radar and even L-band radar, to a degree, can provide useful oil spill data<ref name=F18>Fingas, M. and Brown, C.E. 2018. A Review of Oil Spill Remote Sensing. Sensors 18, 91</ref>. The contrast between oil and water is highest in X-band, moderate in C-band, and lowest in L-band. Signal polarizations using vertical (V) and horizontal (H) electromagnetic wave propagation can be used to provide further information. Polarimetric SAR yields information to aid in the discrimination between slicks and look-alikes<ref>Chen, Y. and Wang, Z. 2022. Marine Oil Spill Detection from SAR Images Based on Attention U-Net Model Using Polarimetric and Wind Speed Information. Int. J. Environ. Res. Public Health 19, 12315</ref>.
 +
A 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 [https://en.wikipedia.org/wiki/European_Remote-Sensing_Satellite ERS] , [http://en.wikipedia.org/wiki/RADARSAT-1 RADARSAT], [http://en.wikipedia.org/wiki/Envisat Envisat] and [https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-1-sar/acquisition-modes Sentinel-1]. ERS and Sentinel SAR imagery provide the option to recover the surface wind characteristics which important for oil weathering (see [[Oil spill pollution impact and recovery]]). 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<ref>Wang, C., Vandemark, D., Mouche, A., Chapron, B., Li, H. and Foster, R.C. 2020. An assessment of marine atmospheric boundary layer roll detection using Sentinel-1 SAR data. Remote Sensing of Environment 250, 112031</ref>, as well as can be estimated from backscatter variation near islands and capes.  
  
−
[[image:table.gif|thumb|1100px|centre|Table 1. Spaceborn synthetic aperture radars (SAR) (from http://cearac.poi.dvo.ru/en/background/satellites/).]].
+
===Visible light sensors===
 +
The visible part of the electromagnetic spectrum ranges from 400 to 700 nm. Thin layers of oil or sheen appear silvery to the human eye and reflect light over a broad spectrum range – up to blue. Thick oil layers appear to be the same color as bulk oil, typically brown or black (Fig. 1). However, the spectral information reflected by oil and by the water on which the oil floats is fairly similar<ref name=F18/>. Three other obstacles hinder the use of visible light: darkness, cloud cover and sun glitter. Sun glitter, which is often confused with oil sheen, is problematic in visible remote sensing. Sun glitter can be attenuated through signal processing techniques. Although inspection of different spectral regions does not yield strong discrimination, multispectral observations of optical images can help to distinguish actual oil spills from other features such as algal blooms. Infrared oil spill detection uses thermal infrared with wavelengths of 8–14 µm. Oil with a thickness greater than about 10 µm absorbs light in the visible region and re-radiates some of it in the infrared spectrum. However, this does not provide information about the thickness of the oil layer. In addition, natural objects can resemble oil, such as seaweed, sediment, organic matter, coastlines and oceanic fronts<ref name=F18/>. On the other hand, infrared sensors are cheap and easily available.
  
−
===Satellite monitoring===
+
===Laser fluorosensors===  
−
Remote-Sensing Satellites are characterised by their altitude, orbit and  
+
Laser fluorosensors employ a UV laser operating between 308 and 355 nm. Laser fluorosensors take advantage of the phenomenon that aromatic oil compounds interact with ultraviolet light, absorb light energy and release the extra energy as visible light. The absorption and emission wavelengths are unique to oil. Crude oil fluoresces from 400 to 650 nm with the 308 nm excitation. Distinguishing different oil classes is possible because different types of oil have different fluorescent intensities and spectral properties<ref name=F18/>. The laser fluorosensor is best at distinguishing between light, medium and heavy oils. In some fluorescent sensors, the detector is activated at the exact time the light returns from the target surface. This technique is called 'gating'. This gating technique enhances the differentiation of the oil from other interfering phenomena. Some fluorosensors can also gate their detectors to target areas below the sea surface. This enables oil detection in the water column<ref>Brown, C.E. 2017. Laser fluorosensors. In Oil Spill Science and Technology, 2nd ed. (Fingas, M., Ed.) Gulf Publishing Company, Cambridge, MA, USA. Chapter 7: 402–418</ref>. Laser fluorosensors can also be useful for detecting oil pollution because they provide a method to detect oil on coastlines and to distinguish between, for example, oiled and unoiled seaweed<ref name=F18/>.
−
sensor. They cover vast areas and have a specific repetition time (usually
 
−
several days up to 16 days). A list of earth observation satellite is given at [http://en.wikipedia.org/wiki/List_of_Earth_observation_satellites Wikipedia]. Figure 2 sketches the resolution and swath of different satellites.  
 
−
[[image:satelliti.jpeg|thumb|500px|centre|Figure 2: Resolution and swath of different satellites]].
 
  
−
===Airborne monitoring===
 
−
Airborne oil spill monitoring has become a global concern during the last three decades.
 
−
Currently there are a multitude of specialized airborne remote sensing systems all over 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 towards 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 paid 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<ref>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</ref>).
 
−
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 complementary to satellite.  Existing users are positive to a combined satellite and airborne monitoring service.
 
  
−
==See also==
+
==Related articles==
−
===Internal links===
+
* [[Remote sensing]]
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* [[remote sensing]]
+
* [[Oil spills]]
 +
* [[Oil spill pollution impact and recovery]]
 
* [[Index of vulnerability of littorals to oil pollution]]
 
* [[Index of vulnerability of littorals to oil pollution]]
 
* [[Overview of oil spills events from 1970 to 2000]]
 
* [[Overview of oil spills events from 1970 to 2000]]
 
* [[North Sea pollution from shipping: legal framework]]
 
* [[North Sea pollution from shipping: legal framework]]
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Use of radar techniques:
 
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* [[Use of ground based radar in hydrography]]
 
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* [[Waves and currents by X-band radar]]
 
  
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===External links===
 
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* [http://earth1.esrin.esa.it/ew/ EESA: About earth watching]
 
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* [http://esapub.esa.int/eoq/eoq44/lichten.htm Using ERS-1 SAR images for oil spill surveillance]
 
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* [http://www.itopf.com/stats.html The International Tankers Owners Pollution Federation]
 
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* [http://response.restoration.noaa.gov/ Ocean Serve Office of Response and Restoration]
 
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* [http://www.swan.ac.uk/empress/index.htm Example of Oil spill]
 
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* [http://www.swan.ac.uk/empress/oil/oil.htm Oil spill]
 
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* [http://www-cenerg.cma.fr/eng/tele/cseas/welcome.html Centre for Energy and Processes (French)]
 
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* [http://www.geog.ucl.ac.uk/~salmond/essay.html The Remote Sensing Of Oil Slicks From Satellite Platforms]
 
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* [http://oils.gpa.unep.org/ Global Marine Oil Pollution Information Gateway]
 
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* [http://www.memac-rsa.org/ Marine Emergency Aid Centre]
 
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* [http://www.satobsys.co.uk/CSeas/ Clean Seas project]
 
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* [http://www.unep-wcmc.org/latenews/emergency/ UN Environment Programme: Emergencies]
 
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* [http://marsais.ucc.ie/ About SAR]
 
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* [http://www.eia.doe.gov/emeu/cabs/pgulf.html Oil and gas in the Persian Gulf]
 
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* [http://www.gisdevelopment.net/application/miscellaneous/misc027.htm Oil Spill Detection and Monitoring from Satellite Image]
 
  
 
==References==
 
==References==
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[[Category:Coastal and marine observation and monitoring]]
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[[Category:Techniques and methods in coastal management]]
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[[Category:Oil spills]]
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Latest revision as of 22:13, 10 August 2024

The development of remote sensing techniques allows the detection and monitoring of oil spills. This article describes the possibilities, techniques and requirements to detect oil spills using remote sensing.

Introduction

The ability to remotely detect and monitor oil spills at sea is important due to the constant threat posed to marine wildlife and the ecosystem. Remote sensing 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 can use two types of sensor systems:

  • 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.

Remote sensing platforms for oil spill monitoring are satellites, aircrafts and drones.

Satellite monitoring

Remote-Sensing Satellites are characterized by their altitude, orbit and sensor. They cover vast areas and have a repetition time ranging from several to 16 days. In the past, times from the tasking of the satellite to image delivery were as long as 12 h, currently, times of 4 h are possible. A further consideration is overpass time. Several satellites cover an area once per day. Satellite constellations such as Cosmo (Constellation of Small Satellites for Mediterranean basin Observation) give revisit times of a few hours compared to the present one-day. Satellite-carried radars with their frequent overpass, high spatial resolution and their day–night and all-weather sensors are essential for detecting large spills and monitoring ship and platform oil releases.

However, satellite observation of oil spills is not yet operational for the global oceans. For example, the most extensive oil spill ever recorded in tropical oceans worldwide (5-12 million liters) that hit the Brazilian coast in 2019 over a length of almost 3000 km, escaped satellite observation. The origin of this 'orphan' oil spill has never been determined[1]

Airborne monitoring

Fig. 1. Aerial image of an oil slick. Photo credit The Norwegian Coastal Administration/NOFO/Sundt Air.

Airborne oil spill monitoring has become a global practice over the last three decades. In the 1970s and 1980s a major effort has been directed toward developing sensors with enhanced oil spill monitoring capabilities based on various techniques like infrared/ultraviolet line scanners, microwave radiometers, laser fluorosensors, and X-band radar systems. Currently a multitude of specialized airborne remote sensing systems are operated, especially for the deterrence of potential polluters and support to oil spill clean-up activities.

Using satellite platforms to monitor oil spills is more cost effective than using 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. Users advocate a combined satellite and airborne monitoring service.

Oil monitoring sensors and techniques

Monitoring requirements

Figure 2. The Lebanese oil spill accident of July 2006. SAR image at 7:51 GMT 6 Aug 2006. Courtesy of ESA, INGV and JRC.

Requirements for oil slick monitoring are:

  • 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)
  • Capability to distinguish the oil from the adjacent water

Monitoring with radar

There are two configurations of active side-looking radar systems: Synthetic Aperture Radar (SAR) and side-looking airborne radar (SLAR). SLAR is cheaper and uses a horizontal antenna to produce imagery along the flight path. Synthetic aperture radar uses the forward motion of the platform to attain spatial resolution. SAR resolution is not dependent on range, but uses extensive electronic processing to produce high resolution images. SAR has larger range and resolution than SLAR and is used in all radar satellites. SLAR is mostly used for airborne oil spill remote sensing, as it is cheaper. The imaging geometry of SAR and SLAR is an oblique projection type. The working wavelength, incidence angle, polarization mode of the radar sensor, surface roughness, and dielectric constant of the ground object all affect the backscattering of the signal. Primary pre-processing of SAR images involves radiometric calibration, geocoding, and land masking.

Winds above 1.5 m/s generate capillary waves at the water surface that appear as so-called 'sea clutter' on the radar image due to Bragg scattering[2]. As these capillary waves are damped by the oil slick, the slick appears as a dark patch on the SAR image where sea clutter is suppressed. However, other features may produce slick look-alikes, such as fresh water slicks, internal waves, wave shadows behind structures, floating macro-algae such as sargassum and kelp, etc. Several automatic or semi-automatic Radar Image Processing techniques have been developed that can interpret a radar image for oil slicks in a matter of minutes, especially using deep learning algorithms (e.g. Decision Tree Forest [3], Support Vector Machine classification [4], convolutional neural network [5]) for feature extraction. SAR observations do not depend on weather (clouds and sunshine), which enables the detection of illegal discharges that most frequently appear during night. SAR can also survey areas during storms, where accident risks are increased. An example of an image is shown in Fig. 2.

Band Frequency [GHz] Wavelength [cm]
L 1-2 15-30
C 4-8 3.75-7.5
X 8-12 2.5-3.75


The lengthy availability of oil during the Deepwater Horizon spill provided several researchers with the opportunity to study radar remote sensing as well as band relationships. Overall, X-band is superior to other bands, but C-band radar and even L-band radar, to a degree, can provide useful oil spill data[6]. The contrast between oil and water is highest in X-band, moderate in C-band, and lowest in L-band. Signal polarizations using vertical (V) and horizontal (H) electromagnetic wave propagation can be used to provide further information. Polarimetric SAR yields information to aid in the discrimination between slicks and look-alikes[7]. A 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, Envisat and Sentinel-1. ERS and Sentinel SAR imagery provide the option to recover the surface wind characteristics which important for oil weathering (see Oil spill pollution impact and recovery). 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[8], as well as can be estimated from backscatter variation near islands and capes.

Visible light sensors

The visible part of the electromagnetic spectrum ranges from 400 to 700 nm. Thin layers of oil or sheen appear silvery to the human eye and reflect light over a broad spectrum range – up to blue. Thick oil layers appear to be the same color as bulk oil, typically brown or black (Fig. 1). However, the spectral information reflected by oil and by the water on which the oil floats is fairly similar[6]. Three other obstacles hinder the use of visible light: darkness, cloud cover and sun glitter. Sun glitter, which is often confused with oil sheen, is problematic in visible remote sensing. Sun glitter can be attenuated through signal processing techniques. Although inspection of different spectral regions does not yield strong discrimination, multispectral observations of optical images can help to distinguish actual oil spills from other features such as algal blooms. Infrared oil spill detection uses thermal infrared with wavelengths of 8–14 µm. Oil with a thickness greater than about 10 µm absorbs light in the visible region and re-radiates some of it in the infrared spectrum. However, this does not provide information about the thickness of the oil layer. In addition, natural objects can resemble oil, such as seaweed, sediment, organic matter, coastlines and oceanic fronts[6]. On the other hand, infrared sensors are cheap and easily available.

Laser fluorosensors

Laser fluorosensors employ a UV laser operating between 308 and 355 nm. Laser fluorosensors take advantage of the phenomenon that aromatic oil compounds interact with ultraviolet light, absorb light energy and release the extra energy as visible light. The absorption and emission wavelengths are unique to oil. Crude oil fluoresces from 400 to 650 nm with the 308 nm excitation. Distinguishing different oil classes is possible because different types of oil have different fluorescent intensities and spectral properties[6]. The laser fluorosensor is best at distinguishing between light, medium and heavy oils. In some fluorescent sensors, the detector is activated at the exact time the light returns from the target surface. This technique is called 'gating'. This gating technique enhances the differentiation of the oil from other interfering phenomena. Some fluorosensors can also gate their detectors to target areas below the sea surface. This enables oil detection in the water column[9]. Laser fluorosensors can also be useful for detecting oil pollution because they provide a method to detect oil on coastlines and to distinguish between, for example, oiled and unoiled seaweed[6].


Related articles


References

  1. ↑ Soares, M.O. and Rabelo, E.F. 2023. Severe ecological impacts caused by one of the worst orphan oil spills worldwide. Marine Environmental Research 187, 105936
  2. ↑ Phillips, O.M. 1988. Radar Returns from the Sea Surface—Bragg Scattering and Breaking Waves. J. Phys. Ocean. 18: 1065-1074
  3. ↑ Topouzelis, K. and Psyllos, A. 2012. Oil spill feature selection and classification using decision tree forest on SAR image data. ISPRS J. Photogramm. Remote. Sens. 68: 135–143
  4. ↑ Baek, W.-K. and Jung, H.-S. 2021. Performance Comparison of Oil Spill and Ship Classification from X-Band Dual- and Single-Polarized SAR Image Using Support Vector Machine, Random Forest, and Deep Neural Network. Remote Sens. 13, 3203
  5. ↑ Fan, Y., Rui, X.; Zhang, G., Yu, T.; Xu, X. and Poslad, S. 2021. Feature Merged Network for Oil Spill Detection Using SAR Images. Remote Sens. 13, 3174
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Fingas, M. and Brown, C.E. 2018. A Review of Oil Spill Remote Sensing. Sensors 18, 91
  7. ↑ Chen, Y. and Wang, Z. 2022. Marine Oil Spill Detection from SAR Images Based on Attention U-Net Model Using Polarimetric and Wind Speed Information. Int. J. Environ. Res. Public Health 19, 12315
  8. ↑ Wang, C., Vandemark, D., Mouche, A., Chapron, B., Li, H. and Foster, R.C. 2020. An assessment of marine atmospheric boundary layer roll detection using Sentinel-1 SAR data. Remote Sensing of Environment 250, 112031
  9. ↑ Brown, C.E. 2017. Laser fluorosensors. In Oil Spill Science and Technology, 2nd ed. (Fingas, M., Ed.) Gulf Publishing Company, Cambridge, MA, USA. Chapter 7: 402–418


The main authors of this article are Renata Archetti and Job Dronkers
Please note that others may also have edited the contents of this article.
Citation: Renata Archetti; Job Dronkers; (2024): Oil spill monitoring. Available from http://www.coastalwiki.org/wiki/Oil_spill_monitoring [accessed on 25-11-2024]
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