Centre for Remote Imaging, Sensing and Prcessing (CRISP)	CRISP's Research
Contact: crisp@nus.edu.sg	Copyright © CRISP, 2001

Satellite Remote Sensing for Oil Spill Monitoring

Ocean oil pollution is a major environmental concern that affects many countries in the world. It has been reported that operational tanker oil discharges (i.e. dumping of oil during tanker cleaning operations) form about 45% of the total ocean oil pollution in the world while ship accidents and oil platform accidents contribute only 5% and 2% respectively. Hence, deliberate oil emissions from ships impose a much greater long-term threat to the ocean environment than those from big ship accidents. Monitoring illegal ship discharges is thus an important component in ensuring compliance with the marine protection legislation and the general protection of the coastal environment.

Space-borne remote sensing is an important tool in monitoring marine oil spills due to its wide spatial coverage and regular revisit capability.

Imaging of Oil Films with ERS and SPOT

Oil films tend to make the water surface smooth, i.e. they dampen water waves.The smoothness or roughness of the water surface, in turn, can have a great effect on how the water surface is imaged by the synthetic aperture radar (SAR) carried by the ERS or by the HRV optical sensor carried by the SPOT satellites.

Oil films and ERS SAR

A synthetic aperture radar (SAR)like the one carried by the ERS satellites emits microwaves and records how strongly they are backscattered by the "target"(land, sea). The more short-scale waterwaves there are, the more microwaves are backscattered to the SAR, and the brighter the resulting SAR image looks. Specifically, the SAR of ERS is sensitive to surface waves with a length ofabout 6 to 8 cm; they are called "Bragg waves".

Thus, oil films appear as dark patches on ERS SAR images.

Oil films and SPOT HRV

The HRV optical sensor carried by the SPOT satellites measures the radiance received from the "target" (land, sea) that is lit by the sun.The radiance from the sea consists of two contributions,

sunlight reflected and scattered by the bottom and the water itself, and attenuated by the water
sunlight reflected by the sea surface, called sun glitter radiance

Whereas the first contribution depends on the water depth, the bottom type, and the water turbidity, the second contribution - the sun glitter radiance -is very sensitive to the position of the sun and the sensor and to the presence of waves that tilt the surface and change the direction of reflection.

We can consider two situations: First, the look direction of the satellite sensor is close to the direction of direct sunlight reflection by a flat water surface. In this case, the water surface appears brighter, the smoother it is. An oil film (causing smooth water surface) would thus appear bright in the image.

Second, When the look direction of the satellite sensor deviates by some 10° from the direction of direct sunlight reflection by a flat water surface.In that case, water waves are needed to tilt the water surface so that it reflects sun light directly into the sensor, and a rough water surface (i.e. with waves) will appear brighter than a smooth water surface. Thus, an oil film will appear dark on the image.

When the look direction of the satellite sensor deviates widely from the direction of direct sunlight reflection by a flat water surface, the sun glitter radiance is negligible, and the sensor basically looks through the surface without detecting oil films or surface slicks.

Oil Spill Statistics for Regional Seas

Based on the analysis of more than 5000 ERS SAR images acquired and processed at CRISP,an oil pollution map of Southeast Asian seas was compiled. The ERS images are from the period September 1995 to September 1998.

Spatial distribution of map of oil pollution in Southeast Asian waters, derived from ERS SAR images (Sept. 1995 to Sept. 1998)

The above map shows that all major shipping routes exhibit various degrees of oil pollution. The most polluted areas are found in the Gulf of Thailand and the South China Sea off the southern Vietnamese coast. In these areas, more than 25 oil slicks can be detected within a typical ERS scene (100 km by 100 km). Most of the oil slicks are due to deliberate discharge from ships. Such ship pollution slicks often show up as dark lines with sharp edges, with more than 10% of them extending over 10 km in length. Some other slicks are due to leakage from offshore oil rigs or natural seepage from coastal oil reserves. Some typical images of ocean oil slicks are shown below.


Two sub-scenes from an ERS SAR image acquired on 15 April 1996. Numerous dark oil slicks.

Sub-scene from an ERS SAR image acquired on 4 April 1997. Numerous dark oil slicks.

Some Oil Accidents near Singapore

Song San Oil Spill, Singapore Straits, August 1996

In August 1996, a tanker dumped oil into the Singapore Straits. The oil drifted towards Singapore and polluted the beaches of the East Coast and Southern Islands including Sentosa. About S$ 1 million was spent to clean up the affected beaches.

An ERS image acquired by CRISP showed clearly the vessel discharging a 5 kmlong plume of oil. With the ERS image and other evidence, the culprits were convicted in court and fined a total of S$ 1.25 million.This is the first ever remote sensing image accepted as court evidence for oil pollution investigation in the world.


Oil Stain on Body of Vessel (courtesy of MPA)	Oil slicks floating in the Sea (courtesy of MPA)

Tanker Collision, Singapore Straits, October 1997

Oil tanker after collision with the VLCC (Straits Times 17 October 1997)

In October 1997, a very large crude carrier (VLCC) and an oil tanker collided and spilled about 28,000 tonnes of oil, comparable to the amount in the Exxon Valdez spill, into the sea. Clean-up operations were very successfully carried out, with nearly all the spill enclosed and cleaned up within a few days.

ERS and RADARSAT images acquired 5 days and 10 days after the collision showed some residual oil slicks drifting northwestward along the Malacca Straits.

© CSA 1997
RADARSAT SAR image acquired on 26 October 1997. Some residual oil slicks have drifted about 200 km up the Malacca Straits.

© ESA 1997
ERS1 image acquired on 21 October 1997. Dark patches to the southwest of Singapore are residual oil slicks, some drifting into the Malacca Straits.

Natuna Sea Oil Spill, Singapore Straits, October 2000

On 3 October, 2000, at 6 a.m., the oil tanker "Natuna Sea" ran aground in the Singapore Strait. The Panama-registered tanker was on the way from Syria to China. The accident happened south of the main strait, in Indonesian waters, just off the Batu Berhanti Beacon (about 8 km south-east of Sentosa). Of the ship's over 40,000 tonnes of crude oil, about 7000 tonnes leaked in to the sea. With the varying wind and currents, oil slicks drifted into several directions during the following days.

© CNES 2000
© CNES 2000	© CNES 2000
SPOT panchromatic image acquired on 11 October, 2000, at 11:45 local time, 8 days after the accident. Possible oil slicks are visible across the whole Strait (see the zoom images on the right). The bright lines and filaments are very likely caused by oil slicks. The reason is that in this SPOT scene, the sensor look direction is close to the direction of sunlight reflection by a smooth water surface. Thus, a smooth water surface (as caused by oil) looks bright because of "sun glitter".

ERS SAR, 11 Oct 2000, Singapore Strait oil spills

ERS SAR, 11 Oct 2000, Zoom, Singapore Strait oil spills

ERS SAR image, 11 October, 2000, at 23:44 local time, almost 8 days after the accident. Possible oil slicks can be seen as dark patches and streaks across the whole Strait. Note that dark zones directly at the coast line (see, e.g., East Coast) are caused by wind shadow, not oil. The other dark lines and filaments on the open water, however, are very likely caused by oil slicks. The very bright dots are ships.

Conclusion

Satellite images, particularly SAR images from the satellites ERS and RADARSAT, are suitable for compiling statistics of oil pollution on the sea on a large scale. This can provide information that is essential for any effort to tackle the oil pollution problem.

Furthermore, satellite images, such as ERS SAR, RADARSAT SAR and SPOT HRV images, are useful for monitoring specific oil pollution events such as tanker accidents. They can help to assess the extent of the pollution and can sometimes even help to pinpoint the source.