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Remote Sensing of Oil Spills

Werner AlpersInstitute of Oceanography, University of Hamburg, Germany

Abstract

Mineral oil spills floating on the sea surface are detectable by imaging radars, becausethey damp the short surface waves that are responsible for the radar backscattering. The oil

spills appear as dark patches on radar images. However, natural surface films that are oftenencountered in coastal regions where the biological activity is high also damp the shortsurface waves and thus also give rise to dark patches on radar images. However, oil spills canoften be identified by their shape. Examples of synthetic aperture radar images acquired overcoastal waters by the European Remote Sensing satellites ERS 1 and ERS 2 showing bothtypes of surface films are presented.

Oil pollution surveillance planes that are operated by several nations bordering the Northand Baltic Sea carry real aperture radars as the prime sensor. Other sensors flown on thesesurveillance planes include: (1) microwave radiometers which allow the determination of theoil thickness, (2) ultraviolet and infrared scanners, which allow the detection of very thin and

very thick oil films, respectively, and (3) laser fluorescence sensors, which allow thedetermination of the oil type. Statistical results obtained from the surveillance of the Northand Baltic Sea by the 2 German oil surveillance planes are presented.

Finally, it is noted that remote sensing data can be used for initializing and validatingmodels that describe the drift and dispersion of oil spills.

Keywords: Oil Spills, Oil Pollution, Biogenic Slicks, Aerial Surveillance System, Radar,Microwave Radiometer, Laser Fluorosensor, UV/IR Scanner

1 Introduction

Pollution of the sea by mineral oil-spills is one of the major environmental problems.Increased public pressure has forced national and international organisations to set upeffective legislative protection of the marine and coastal environment over the last 15-20years. As a result, many countries have signed the MARPOL 73/78 convention which setsstandards for ship discharges, allowing them only beyond certain limits from the nearest coastand only at very small amounts. Several sea areas have been declared as Special Areas, whereship discharges are prohibited almost completely. Such Special Areas are the MediterraneanSea, the Baltic Sea, and the North Sea. However, despite of the MARPOL convention, largequantities of mineral oil are still discharged in these Special Areas from ships.


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Synthetic aperture radar images acquired by the European Remote Sensing satellites ERS-1 and ERS-2 have been extensively used for obtaining statistical information on oil pollution(see, e.g., Gade and Alpers, 1999). In particular satellite images are very useful in locating thepreferred areas ("hot spots") where tankers are washed and/or engine room effluents aredischarged.

However, satellite imagery is not very useful in identifying ships from which have spilledmineral oil. This is because satellite images of a given sea area can only be obtained atrelatively large time intervals and furthermore, the acquisition time is also predictable.Therefore oil pollution surveillance planes are operated by many countries for fighting illegaloil pollution in their territorial waters.

2 Mineral Oil Films

Mineral oil floating on the sea surface often originates from ships, but they also canoriginate from land-based sources, like refineries and industrial plants, and from sea-based

sources like natural oil seeps and oil platforms. The main contribution of oil pollutionoriginating from transportation activities comes not from ship accidents, but from routine shipoperations like tank washing and engine effluent discharges (mostly sludge).

Mostly the ships discharge their oily effluents en route, leaving back linear spills. In theideal case of discharging in a current-free and calm sea, the resulting overall spill geometrywill follow the route of the ship. Automatic oil detection techniques using radar images oftenuse this linearity for identifying oil spills generated by moving ships. However, when the shipis maneuvering or in the presence of a non-uniform surface current, the shape of the spill candeviate significantly from linearity. When oil is discharged from a ship it will spread alsolaterally and will form an elongated V-shaped trail. Typical spreading rates are 0.6 m2/sec.However, only freshly spilled oil attains this shape. When the spreading comes to the end, theshape of the spill will be an elongated parallelogram . During their fate in the sea, mineral oilfilms are subject to the action of the wind, evaporation of their lighter chemical components,chemical transformation like photo-oxidation processes and decomposition, as well as tomixing with sea water (emulsification). Thus, after some time, oil slicks become undetectableby remote sensing techniques. This time depends on the type of the oil, its quantity andthickness, and on the environmental conditions. It usually varies between a few days andseveral weeks. Winds strongly affect the shape of the oil trail. High winds usually give rise toa "feathered" structure of the trail. The heavy constituents of oil accumulate always at thedownwind side.

3 Detection of Oil Spills by Radar

Mineral oil and/or other surface-active material floating on the sea surface become visibleon radar images because they damp the short gravity-capillary waves which are responsiblefor the radar backscattering. Real aperture radars (RARs) as well as synthetic aperture radars(SARs) used for monitoring coastal waters for oil pollution usually operate at incidenceangles between 20 and 80 degrees, i.e., at incidence angles where the radar backscattering canbe described by Bragg scattering theory (see, e.g., Valenzuela, 1978). According to thistheory the backscattered radar power is proportional to the amplitude squared of those surfacewaves which propagate towards or away from the look direction of the radar antenna and

which have the wavelength sin2/0=B , where 0 is the radar wavelength and is theincidence angle. These waves are called Bragg waves and have wavelengths in the centimeter


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to decimeter range. A precondition for detecting mineral oil spills on the sea surface is thatthe wind is strong enough for generating Bragg waves. For X- and C-band radars having radarwavelengths around 3 and 5 cm, respectively, the threshold for generating Bragg waves liesbetween 2 and 3 cm/sec. On the other hand, there is also maximum wind speed above whichoil films become undetectable by radar. At sufficiently high wind speeds, the oil disappears

from the sea surface because it is washed down by breaking waves. Depending on the type ofthe oil and the thickness of the oil film, this happens at wind speeds between 10 m/sec and 14m/sec.

Examples of synthetic aperture radar images of the sea surface acquired by the EuropeanRemote Sensing satellites ERS 1 and ERS 2 showing oil spills are shown in Figures 1-3.

4 Look-Alikes of Oil Spills on Radar Images

Not only mineral oil floating on the sea surface damp the short gravity-capillary waves,but also surface-active substances of natural origin which are called biogenic slicks. Theydamp short gravity-capillary waves with a similar strength than mineral oil films. This makes

it often difficult to decide whether the dark patches, or more accurately, the areas of reducedbackscattered radar power relative to their surroundings, are see areas covered with mineraloil or biogenic slicks.

Biogenic slicks are natural surface films that consist of surface-active compounds that aresecreted by marine plants or animals. In general, the biogenic surface slicks are only onemolecular layer thick (approximately 3 nanometers). This implies that it needs only few litersof surface-active material to cover an area of 1 km2. At times when the biological productivityis high, i.e., during plankton blooms, the probability of encountering natural biogenic surfacefilms is strongly enhanced. In some cases the sea area covered by natural surface films can beseveral thousand square kilometers.

Examples of large sea areas covered with natural surface films are shown in Figures 4 and5. Because the surface films tend to accumulate along convergence zones of current systems,they render surface current patterns visible on radar images as evident in these images.

What makes things even worse is that dark patches on radar images of the sea surface canalso be caused, e.g., by turbulence generated in the water by the propeller of a ship or by rainimpinging on the sea surface, or by the presence of grease ice which also damps short gravity-capillary waves. Furthermore, dark patches can also result from reduced wind speed asencountered, e.g., in the wind shadow behind islands or coastal mountains. Or they can result

from reduced wind stress due to colder sea surface temperatures, as encountered, e.g., inupwelling regions or in cold plumes from river outflows. The colder water often changes thestability of the air-sea interface such that the wind speed cannot generate short gravity-capillary waves.

Thus we conclude that it is often not possible to detect oil films on radar images of the seasurface unambiguously by the reduction of the radar backscattered radar power which givesrise to reduction of the radar image intensity relative to the background.

However, the discrimination of mineral oil patches from natural surface films is in mostcases possible by their shape.


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5 The German Airborne Surveillance System

Aircraft equipped with a remote sensing instrument for detecting oil pollution in coastalwaters are flown in various countries since the early 1980s (see, e.g., Lodge, 1989). Theobjective of these airborne surveillance systems is 1) to detect oil spills over long distances

and register their position and dimension, 2) to measure the thickness distribution of oil spillsand hence the quantity of oil spilled, 3) to identity the ships from which the oil has beenspilled, 4) to support and coordinate cleaning operations following accidental discharges oflarge quantities of oil (Gruener et al., 1991).

Since 1983 the Federal Republic of Germany operates an airborne surveillance system foridentifying oil pollution in its territorial waters in the North and Baltic Sea. At present itconsists of two DO 128 aircraft carrying the following sensors:

1) a real aperture radar or side-looking airborne radar (SLAR), 2) a microwaveradiometer(MWR), 3) a laser fluorosensor (LFS), 4) an infrared/ultraviolet line scanner

(IR/UV scanner), and 5) a camera system.

The side looking airborne radar (Gruener et al.,1991) it is the prime sensor on theseaircraft. It can be operated day and night and under all-weather conditions. This sensor is a so-called far-range sensor which can detect oil spills up to a distance of 30 km in cross-flightdirection. It operates at a frequency of 9.4 GHz (X-band) and has a peak power of 20 kW.

With the microwave radiometer it is possible to measure the thickness of oil spills. Incontrast to the side-looking airborne radar the microwave radiometer is a passive, near-rangesensor which measures the natural (thermal) multi-spectral electromagnetic radiation ofmatter in the cm to mm electromagnetic wavelength regime. This is expressed in terms ofbrightness temperature. The thickness of the oil film is measured by an interference effect:The sky radiation reflected at the upper boundary of the oil layer interferes with the radiationreflected from the lower boundary. When the oil thickness is an odd multiple of a quarter of awavelength, then the brightness temperature signal is strongest, and when it is an evenmultiple, the brightness temperature signal is weakest. Thus the brightness temperature is anoscillatory function of the thickness of the oil layer. Its amplitude is of the order of 20 K to 70K for a 90 GHz radiometer and up to 110 K for a 32 GHz radiometer depending on the seastate, oil type and oil water mixture (dielectricity constant), atmospheric influences, etc..However, due to the periodicity of the brightness temperature as a function of the layerthickness, there is an ambiguity in determining the layer thickness. The ambiguity problem

can be overcome by using a multi-frequency system. The German system operates at 3frequencies: 18 GHz, 36 GHz, and 89 GHz. This allows to determine the oil layer thicknessunambiguously in the range between 0.05 mm to 3 mm.

The basis of the detection of oil pollution by means of a laser fluorosensor(Hengstermannand Reuter, 1990) is the spectroscopic property of oil. Crude oil and also fuelused in ship engines consist of a mixture of different hydrocarbons. These oils ,like most ofcomplex organic compounds, have the property to fluoresce when exposed to light radiation.The emitted fluorescent light, compared to the excitation light, is shifted to longerwavelengths. Information on the type of oil can be obtained from the analysis of the spectralintensity distribution of the fluorescent light. This instrument is also capable of measuring the

thickness of thin oil films in the range between 0.1 to 20 m


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Ultraviolet (UV) channel of the UV/IR scannerregisters sunlight reflected from the seasurface in the ultraviolet wavelength regime. Therefore the sensor can only operate during theday and when the visibility to the ground is good. Detection of oil is based on the effect thatthe reflectivity of the sea surface increases when an oil film is present. An increase of thereflectivity is observed even when the film has a thickness of less than 0.1 m. The upper

limit of the dynamical range is about 10 m. However, quantitive estimates of the filmthickness are not possible with this sensor.

The infrared (IR))channel of theUV/IR scanner measures the brightness temperature ofthe sea surface in the thermal infrared. Temperature differences between oil and water areinduced by the higher absorption of daylight in oil compared with water. This yields atemperature increase over oil spills by a few degrees compared with the surrounding waterdepending on illumination, wind and other factors. Oil films that have thicknesses of about0.1 mm up to a few millimeters can be detected by the infrared of this scanner. Temperaturedifferences have been observed also in the dark where the oil appears to be colder, an effectwhich is attributed to the lower emissivity of oil in the thermal infrared. However,

quantitative information about the film thickness cannot be obtained from infrared images.

The camera systemis used to identify ships which have illegally discharged oil.

In Table 1 results from the German aerial surveillance system obtained in the years 1986-2000 are graphically displayed. It shows that, on the average, during every second to thirdflight an illegal oil spill was observed, but that relatively seldom (in 2000 in about 20% of thecases) the ship that has caused the pollution could be identified.

6 Oil Drift and Dispersion Models

Numerical models are being used to predict the drift and dispersion of oil following anaccident. The German Federal Maritime and Hydrographic Agency (Bundesamt frSeeschiffahrt und Hydrographie - BSH) has developed such a model (Dick and Soetje, 1990,Dick and Mueller- Navara, 2002). These models need remote sensing data as input.Furthermore, remote sensing data are used for model validation.

The BSH model is a 3-dimensional model that takes into account the meteorologicalconditions in the sea area, tidal currents and external surges. An oil spill is represented by aparticle cloud that is transported by winds and currents and undergoes changes in its shapeand its physiochemical properties due to oil weathering processes. In this model the wind-

induced drift of the oil spill in direction of the wind is assumed to be 2.3 % of the windvelocity. In the past year the BSH model has been used successfully to assist the coastguard infighting oil pollution after ship accidents.

Acknowledgements

I would like to acknowledge the input to this report given by U.Bustorff([email protected]) of the German Federal Marine Pollution Control Unit,Cuxhaven (www.wsv.de/Schifffahrt), T. Hengstermann ([email protected]) ofOptimare GmbH (www.optimare.de), and of S. Mueller-Navarra ([email protected]) of the German Federal Maritime and Hydrographic

Agency (BSH), Hamburg (www.bsh.de).


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0

500

1000

15002000

Table 1. Aerial Surveillance Results 1983 - 2001, North- and Baltic - Sea

Identified Polluter Number of Flights Pollutions observed Flight hours Covered Ar

Identified Polluter 11 12 16 13 6 10 10 13 11 23 18 18 15 16 27

Number of Flights 58 230 201 197 251 275 262 226 189 348 255 340 360 427 406

Pollutions observed 104 186 190 119 138 104 160 152 90 267 142 213 210 187 176

Flight hours 72 307 284 493 530 614 576 600 514 942 689 920 1038 1097 1073

Covered Area [km] 241 244 294 273 50 87 150 76 180 443 382 401 934 445 420

Est. Quantities [m] 287 287 287 287 103 77 102 166 60 243 189 218 1795 166 343

83 84 85 86 87 88 89 90 91 92 93 94 95 96 97


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ESA

Figure 1: A ship travelling northward (bright spot at the front of the black line) dischargingoil. The oil disperses with time causing the oil trail to widen. This oil trail is morethan 80 km long. (This ERS-1 SAR image was acquired on 20 May 1994 at 14:20UTC over the Pacific Ocean east of Taiwan; orbit: 14874, frame: 2364, framecenter: 23 01'N, 121 41'E, images area: 100 km x 100 km)


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ESAFigure 2: Oil polluted sea area off the east coast of Malaysia (near Kuantan) which is a busy

shipping lane. The wind blows from an easterly direction causing the feathering of

oil trails. (ThisERS-2 SAR image was acquired on 4 April 1997 at 3:25 UTC overthe South China Sea; orbit: 10221, frame: 3519, frame center: 4 20'N, 103 59'E,imaged area: 100 km x 100 km)


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ESA

Figure 3: Oil polluted area in the Mediterranean Sea northwest of Port Said (near theentrance to the Suez Canal). Visible are several oil spills of different size and agewhich apparently originate from tanker cleanings. The analysis of a large numberof ERS SAR images has revealed that this area is a "hot spot" for tanker cleaning.(This ERS-1 SAR image was acquired on 1 June 1992 at 8:31 UTC; orbit: 4589,frame: 2961, frame center: 31 49'N, 31 49'E, imaged area: 100 km x 50 km)


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ESAFigure 4: Area of the Baltic Sea with the German island of Fehmarn (in the center) and the

Danish island of Lolland (in the upper right-hand section). The sea area is partlycovered with biogenic slicks which are abundant at this time of the year in thisarea because it is the time of spring plankton bloom in the Baltic Sea. The slicksact as tracers for the surface currents associated with eddies and thus render themvisible on the SAR image. (This ERS-2 SAR image was acquired on 10 May 1998at 21:15 UTC over the Baltic Sea; orbit: 15972, frame: 1089, frame center: 5428'N, 11 13'E, imaged area: 100 km x 100 km)


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ESAFigure 5: Eddies in the Caspian Sea south of the Volga estuary. This river carries a heavy

load of pollutants originating from fertilizers washed out from agricultural fieldsand from industrial and municipal plants. They serve as nutrients for the marineorganisms which experience a rapid growth and then generate biogenic surfaceslicks. The oceanic eddies, which become visible on the radar images because thesurface slicks follow the surface currents, are very likely wind-induced. The mostremarkable feature on this image is the mushroom-like feature consisting of twocounter-rotating eddies. (This ERS-2 image was acquired on 12 October 1993 at18:54 UTC; orbit: 11724, frame: 0891, frame center: 44 45'N, 49 03'E, imagedarea: 100 km x 100 km)


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Dick, S., Mueller-Navarra, S.H., 2002. An operational oil dispersion model for the North Seaand the Baltic Sea. Third International Research and Development Forum on High-DensityOil Spill Response, Brest, ID No.55.

Gade, M., Alpers;W., 1999. Using ERS-2 SAR for routine observation of marine pollution inEuropean coastal waters. The Science of the Total Environment 237/238, pp. 441-448

Grner, K., Reuter, R., Smid, H., 1991. A new sensor system for airborne measuremensts ofmaritime pollution and of hydrographic parameters. GeoJournal 24.1, 103-117.

Hengstermann, T., Reuter, R., 1990. Lidar fluorosensing of mineral oil spills on the sea

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