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Detecting illegal discharge

Visual aerial detection. Remote sensing. Tools of the future.


In order for a case of illegal discharge of hydrocarbons at sea to lead to legal action, the offender must be clearly identified. Legal action can be taken by the coastal State if the offence takes place in its territorial waters, its exclusive economic zone, or indeed in its marine protected area. For offences in international waters, it is the responsibility of the flag State to undertake proceedings. The coastal State will transfer the pollution report and other relevant elements to the flag State.  Offences are then dealt with by the justice system of the flag State according to its judicial practices and its attitude towards this kind of case.

To establish the liability of a vessel suspected of pollution, it must be proven that the discharge came from the vessel in question, and a much evidence as possible must be gathered on the nature and extent of the pollution. Incidences of pollution are recorded in the form of polreps (lien), pollution reports, generally during patrols by spotter planes.  These polreps are then sent to the Maritime Rescue Coordination Centres (MRCC). The MRCC centralise the information, monitor the evolution of the pollution reported at sea and coordinate the first actions in terms of research, identifying the vessel and recording the offence.


Detection means

Visual aerial detection

Aerial observation is the main tool used to detect operational pollution by ships. It enables the pollution to be identified, located, accurately described and, where possible, allows the polluter to be identified. A pollution observation report is drawn up to enable legal action to be taken.

Visual observation of slicks in the wake of ships from aircraft, together with colour photographs, constitutes the first element used to establish the existence of discharged pollutant. In practice, a series of photographs will be taken, showing the ship and its polluted wake, the extent of the wake (without discontinuity), the ship’s name and the surroundings. Cedre runs a training course on maritime surveillance of pollution and has produced an operational guide on aerial observation of oil pollution at sea. The aircraft crew will use an appearance code in their report to express the quantities of oil spilt based on visual aerial observation. The code used was established through scientific studies conducted by the Bonn Agreement for the protection of the North Sea.


Layer thickness (µm)

Quantity (litres/km²)

1. Sheen (silvery/grey)

0,04 - 0,3

40 to 300

2. Rainbow

0,3 - 5,0

300 to 5,000

3. Metallic

5 - 50

5,000 to 50,000

4. Discontinuous True Colour

50 - 200

50,000 to 200,000

5. Continuous True Colour

> 200

> 200,000

Bonn Agreement appearance code
Bonn Agreement appearance code
Remote sensing

Remote sensing is a complementary method of observation, in addition to observation by the human eye. French Customs, for instance, have two specialised planes (Polmar II and III) equipped with specific equipment: Side-Looking Airborne Radar (SLAR), infrared and ultraviolet scanners, microwave radiometers, very low light level camera. Very low light level cameras are able to identify vessels at night. Radar, scanners and microwave radiometer are used to characterise the pollutant.

SLAR is an active sensor which detects anomalies on the water surface. The image displayed on the screen by the radar is in fact already behind the plane. The image must therefore be analysed and then the plane must turn around to back track and observe what appears to be pollution, the ship and its wake. The radar equipment onboard will depend on the missions of the aircraft on which its is intalled. SLAR (Side Looking Airborne Radar) is more specialised in detection at sea thanks to its RAR version (Real Aperture Radar), which is able to detect slicks according to the reflection differences on the water surface.  Search and surveillance radar systems are less effective but this is partly compensated for by the higher intensity signal they receive.

Infrared, ultraviolet and microwave are passive sensors. They do not send waves, but only receive them. Infrared shows the differences in surface radiation, which reflect the differences in water temperature. Ultraviolet shows the ultraviolet component of the sun’s rays reflected by the oil. The ultraviolet sensor only functions in daylight and detects the outline of slicks while the infrared sensor also determines slick thickness both during the day and at night as wave emission depends on slick thickness. The microwave radiometer indicates the quantity of oil through a sensor which measures the differences in radiation at several frequencies according to the thickness.

Spotter planes will typically: 1. use SLAR detection at high altitude, 2. turn around and move in towards the ship to take photos or videos before it changes direction or stops polluting, 3. gain altitude and use infrared, 4. make radio contact with the ship to attempt to establish acknowledgement of the discharge. In practice, the judge’s decision will be based on the combination of the pollution report, the photos and the declaration of the agent in court. A ship inspection report established by a qualified inspector will add the final touch to the evidence if it indicates an unauthorised modification to the oil discharge monitoring equipment.

Discharge detected by airborne SLAR
Discharge detected by airborne SLAR
Infrared image of a ship discharging oil
Infrared image of a ship discharging oil
Tools of the future

One of the new means of detection currently under development is detection by satellite, and in particular satellites equipped with Synthetic Aperture Radar (SAR). This can be used to detect oil on the sea surface, but are however unable to identify the polluting ship and therefore cannot be used as evidence without cross-checking with other information.

They could now be of great interest in collecting elements from entire seas. They can also be used to monitor pollution from permanent sites (such as oil rigs or harbours) or affecting areas classed as sensitive.
In 2007, the European Maritime Safety Agency launched a satellite based monitoring service which provides satellite images, Cleanseanet.
The forthcoming launch of a completely automated data exchange system, AIS or Automatic Identification System, will be beneficial both for users onboard ships and maritime surveillance bodies. This system continuously provides information such as the identity, position or route of a ship. The combination of AIS and satellite images will be able to prove that a given ship is responsible for an instance of illicit discharge.
Oil and/or fuel marking using chemical or biochemical processes was also considered and presented to IMO (the International Maritime Organization) in 1998 by the United Kingdom after conclusive experiments were undertaken. This technique allows the identification of the origin of the cargo, however it appears to be a difficult process to carry out due to the large number of cargos transported.
In Sweden, sampling buoys have been developed which can be launched directly from an aircraft onto a slick, where they absorb a sample of pollutant for subsequent analysis. So far, only one case has been taken to court in which samples taken by sampling buoys played a decisive role in obtaining a guilty verdict. In Denmark, quick test kits are used to analyse the nature of the pollutant.

Last update: 01/02/2007