detection, analysis, and remote sensing of oil...

CHAPTER 5

Detection, Analysis, and RemoteSensing of Oil Spills

Special instruments are sometimes required to detect an oil spill, especially ifthe slick is very thin or not clearly visible. For example, if a spill occurs at night,in ice, or among weeds, the oil slick must be detected and tracked using instrumentsonboard aircraft, satellites, or spacecraft. This technology is known as remote sens-ing. There are also surface technologies available to detect and track oil slicks. Inaddition, samples of the oil must often be obtained and analyzed to determine theoil’s properties, its degree of weathering, its source, or its potential impact on theenvironment. This analysis, as well as tracking and remote sensing technologies, arediscussed in this chapter.

THE IMPORTANCE OF ANALYTICAL AND DETECTION TECHNOLOGIES

In the past, when an oil spill occurred, the location and extent of the spill, thepotential behaviour of the oil, and its impact on the environment were often notimmediately known. Today, technology is available to provide much of thisinformation.

Laboratory analysis can provide information to help identify an oil if its sourceis unknown and a sample is available. With a sample of the source oil, the degreeof weathering and the amount of evaporation or biodegradation can be determinedfor the spilled oil. Through laboratory analysis, the more-toxic compounds in theoil can be measured and the relative toxicity of the oil at various stages of thespill can be determined. This is valuable information to have as the spillprogresses.

©2000 by CRC Press LLC

SAMPLING AND LABORATORY ANALYSIS

Taking a sample of oil and then transporting it to a laboratory for subsequentanalysis is common practice. While there are many procedures for taking oil samples,it is always important to ensure that the oil is not tainted from contact with othermaterials and that the sample bottles are pre-cleaned with solvents, such as hexane,that are suitable for the oil.

The simplest and most common form of analysis is to measure how much oil isin a water, soil, or sediment sample. Such analysis results in a value known as totalpetroleum hydrocarbons (TPH). The TPH measurement can be obtained in manyways, including extracting the soil, or evaporating a solvent such as hexane andmeasuring the weight of the residue that is presumed to be oil.

The oil can also be extracted from water using an oil-absorbing and water-repelling solid. The oil is then analyzed from this substrate by a variety of means,including measuring the amount of light absorbed in certain selected narrow bands.Still another method is to use enzymes that are selectively affected by some of theoil’s components. A test kit that uses colour to indicate the effect of the oil on theenzymes is available.

A more sophisticated form of analysis is to use a gas chromatograph (GC). Asmall sample of the oil extract, often in hexane, and a carrier gas, usually helium,are passed through a small glass capillary. The glass column is coated with absorbing

Photo 39 Oil must often be analyzed in the laboratory to determine its origin and charac-teristics. (Environment Canada)


materials and, as the various components of the oil have varying rates of adhesion,the oil separates as these components are absorbed at different rates onto the columnwalls. The gases then pass through a sensitive detector. The system is calibrated bypassing known amounts of standard materials through the unit. The amount of manyindividual components in the oil is thereby measured. The components that passthrough the detector can also be totalled and a TPH value determined. While it ishighly accurate, this value does not include resins, asphaltenes, and some othercomponents of the oil with higher molecular weight that do not pass through thecolumn.

A typical chromatogram of a light crude oil with some of the more prominentcomponents of the oil identified is shown in Figure 11.

One type of detector used on a gas chromatogram is a mass spectrometer (MS).The method is usually called GC-MS and can be used to quantify and identify manycomponents in oil. The mass spectrometer provides information about the structureof the substance so that each peak in the chromatogram can be more positivelyidentified. This information can then be used to predict how long the oil has beenin the environment and what percentage of it has evaporated or biodegraded. Thisis possible because some of the components in oils, particularly crude oils, are veryresistant to biodegradation, while others are resistant to evaporation. This differencein the distribution of components then allows the degree of weathering of the oil tobe measured. The same technique can be used to “fingerprint” an oil and positivelyidentify its source. Certain compounds are consistently distributed in oil, regardlessof weathering, and these are used to identify the specific type of oil.

Photo 40 Taking a sample from a thin sheen can be difficult and results in more water thanoil. (National Oceanic and Atmospheric Administration)


FIELD ANALYSIS

Analysis performed in the field is faster and more economical than analysis donein a laboratory. As analytical techniques are constantly improving and lighter andmore portable equipment is being developed, more analytical work can be carriedout directly in the field. Test methods are now available for measuring physicalproperties of oil such as viscosity, density, and even flash point in the field. Test kitshave also been developed that can measure total petroleum hydrocarbons directlyin the field. While these test kits are less accurate than laboratory methods, they area rapid screening tool that minimizes laboratory analysis and may provide adequatedata for making response decisions.

DETECTION AND SURVEILLANCE

Detection and Tracking Buoys and Systems

As oil spills frequently occur at moorings and docks, buoys and fixed-pointmonitoring systems have been developed to ensure rapid response at these sites.These systems detect the oil on water and transmit a radio signal to an oil spillresponse agency. Fluorescence is one method used to detect oil in these systems.An ultraviolet light is focused on the water surface and any oil that is presentfluoresces, or absorbs the ultraviolet light and re-emits it as visible light. Thisfluorescing phenomenon is relatively unique to oil and provides a positive detectionmechanism.

Figure 11 Chromatogram of a light crude oil.


In another detection method, an oil sorbent is used that changes in physicalproperties when it absorbs oil and thus triggers a device. An example of this wouldbe a sorbent that loses it strength when oil is absorbed. The sorbent is placed incontact with a spring and a switch, which is activated when oil enters the sorbent.This type of device is not effective for fast response. Other detection units aretriggered by the differential light reflection or absorption properties of oil.

Photo 41 Field analysis is becoming more common with the development of test kits. Thischemist is measuring the amount of oil in soil. (Environment Canada)

Photo 42 Spill-tracking buoys are used to track actual spills or to assess trajectories beforea spill occurs. (Environment Canada)


As these systems monitor a specific small area of water, they must be locatedwhere a spill would be likely to enter that area. It is difficult to determine this entrypoint in most situations. Furthermore, technologies available today are not sensitiveto quantities of oil released and thus may be triggered by very small amounts of oil.For these reasons, these systems are not used extensively.

As an oil spill moves with the winds and surface currents, the slick or portionsof it may move and responders may not always know its position, especially indarkness or fog. Buoys have been developed that move on the water in a mannersimilar to oil. These buoys transmit a position signal directly to receivers locatedon aircraft or ships or to a satellite that corresponds to the position of the oil slick.Some of these buoys receive Global Positioning System (GPS) data from satellitesand transmit this with the signal. The position of the spill can then be determinedusing a remote receiver. For this type of device to be effective, however, the buoymust respond to both the wind and surface currents in the same way as the oil would.Although this precision in response is difficult to achieve, devices are available thatcan successfully track a range of crude oils and Bunker C.

Visual Surveillance

Oil spills are often located and surveyed from helicopters or aircraft using onlyhuman vision. There are some conditions, however, such as fog and darkness, in

Photo 43 This device, a fluorometer in an underwater vessel, is used to measure the amountof oil in the water column. (Environment Canada)


which oil on the surface cannot be seen. Very thin oil sheens are also difficult todetect as is oil viewed from an oblique angle (less than 45°) especially in misty orother conditions that limit vision. Oil can also be difficult to see in high seas andamong debris or weeds and it can blend into dark backgrounds, such as water, soil,or shorelines.

In addition, many naturally occurring substances or phenomena can be mistakenfor oil. These include weeds and sunken kelp beds, whale and fish sperm, biogenic

Figure 12 Appearance of oil on a calm water surface.


or natural oils such as from plants, glacial flour (finely ground mineral material,usually from glaciers), sea spume (organic material), wave shadows, sun glint andwind sheens on water, and oceanic and riverine fronts where two different bodiesof water meet, such as a river entering another body of water.

A very thin oil sheen as it appears on water is shown in Figure 12. This figurealso shows the thickness and amount of oil that could be present under suchcircumstances.

REMOTE SENSING

Remote sensing of oil involves the use of sensors other than human vision todetect or map oil spills. As already noted, oil often cannot be detected in certainconditions. Remote sensing provides a timely means to map out the locations andapproximate concentrations of very large spills in many conditions. Remote sensingis usually carried out with instruments onboard aircraft or by satellite. While manysensors have been developed for a variety of environmental applications, only a feware useful for oil spill work. Remote sensing of oil on land is particularly limitedand only one or two sensors are useful.

Visual and Ultraviolet Sensors

Many devices employing the visible spectrum, including the conventional videocamera, are available at a reasonable cost. As these devices are subject to the sameinterferences as visual surveillance, they are used primarily to document the spillor to provide a frame of reference for other sensors. A sub-set of sensors operatingin the ultraviolet spectrum may be useful for mapping out a very thin sheen.

Photo 44 This photograph shows the limitations of visual imagery. The wind patterns andshadows can be mistaken for oil, although no oil is present. (Environment Canada)


Infrared Sensors

Thick oil on water absorbs infrared radiation from the sun and thus appears ininfrared data as hot on a cold ocean surface. Unfortunately, many other false targetssuch as weeds, biogenic oils, debris, and oceanic and riverine fronts can interferewith oil detection. The advantage of infrared sensors over visual sensors is that theygive information about relative thickness since only thicker slicks, probably greaterthan 100 µm, show up in the infrared.

Infrared images are sometimes combined with ultraviolet images, which showthe thin oil sheens, to yield a relative thickness map of an oil spill. This is referredto as an IR/UV overlay map. Infrared imagery also has some use at night since theoil appears “colder” than the surrounding sea. The oil is not detected at night in theinfrared as it is during the day.

Infrared sensors are relatively inexpensive and widely used for supportingcleanup operations and directing cleanup crews to thicker portions of an oil spill.They are also often used on cleanup vessels. The oblique view from a ship’s mastis often sufficient to provide useful information on where to steer the vessel for bestoil recovery over a short range.

Photo 45 This is a camera image of oil escaping from a barge that is being recovered fromthe ocean floor. (Environment Canada)


Laser Fluorosensors

Oils that contain aromatic compounds will absorb ultraviolet light and give offvisible light in response. Since very few other compounds respond in this way, thiscan be used as a positive method of detecting oil at sea or on land. Laser fluorosensorsuse a laser in the ultraviolet spectrum to trigger this fluorescing phenomenon and asensitive light-detection system to provide an oil-specific detection tool. There isalso some information in the visible light return that can be used to determinewhether the oil is a light or heavy oil or a lubricating oil.

Laser fluorosensors are the most powerful remote sensing tools available becausethey are subject to very few interferences. Laser fluorosensors work equally well on

Photo 46 This composite image of an oil slick in both infrared and ultraviolet shows therelative thickness of various areas. The orange areas are the thicker portionsmapped in infrared and the thin portions in blue are mapped in ultraviolet. (Envi-ronment Canada)

Photo 47 A scientist services a laser fluorosensor located inside the aircraft. (EnvironmentCanada)


water and on land and are the only reliable means of detecting oil in certain ice andsnow situations. Disadvantages include the high cost of these sensors and their largesize and weight.

Passive Microwave Sensors

The passive microwave sensor detects natural background microwave radia-tion. Oil slicks on water absorb some of this signal in proportion to their thickness.While this cannot be used to measure thickness absolutely, it can yield a measureof relative thickness. The advantage of this sensor is that it can detect oil throughfog and in darkness. The disadvantages are the poor spatial resolution and rela-tively high cost.

Thickness Sensors

Some types of sensors can be used to measure the thickness of an oil slick. Forexample, the passive microwave sensor can be calibrated to measure the relativethickness of an oil slick. Absolute thickness cannot be measured for the followingreasons: many other factors such as atmospheric conditions also change the radiationlevels; the signal changes in cyclical fashion with spill thickness; and the signalmust be averaged over a relatively wide area and the slick can change throughoutthis area.

The infrared sensor also measures only relative thickness. Thick oil appearshotter than the surrounding water during daytime. While the degree of brightnessof the infrared signal changes little with thickness, some systems have been adjustedto yield two levels of thickness. A third thickness level on the thinner outer edgesof fresh slicks shows up “colder” in the infrared as a result of light interference.

Sensors using lasers to send sound waves through oil can measure absolute oilthickness. The time it takes the sound waves to travel through the oil changes littlewith the type of oil and thus the measurement of this travel time yields a reliablemeasurement of the oil’s thickness. This type of sensor is large and heavy and isstill considered experimental.

Radar

As oil on the sea calms smaller waves (on the order of a few centimetres inlength), radar can detect oil on the sea as a calm area. The technique is highly proneto false targets, however, and is limited to a narrow range of wind speeds (approx-imately 2 to 6 m/s). At winds below this, there are not enough small waves to yielda difference between the oiled area and the sea. At higher winds, the waves canpropagate through the oil and the radar may not be able to “see” into the troughsbetween the waves. Radar is not useful near coastlines or between head lands becausethe wind “shadows” look like oil. There are also many natural calms on the oceansthat can resemble oil. Despite its large size and expense, radar equipment is partic-ularly well suited for searches of large areas and for work at night or in foggy orother bad weather conditions.


Satellites

While many satellites provide images in the visible spectrum, oil cannot be seenin these images unless the spill is very large or rare sea conditions are prevalent thatprovide a contrast to the oil. Oil has no spectral characteristics that allow it to beenhanced from the background.

Several radar satellites are now available that operate in the same manner asairborne radar and share their many limitations. Despite these limitations, radarimagery from satellite is particularly useful for mapping large oil spills. Arrange-ments to provide the data within a few hours are possible, making this a useful option.

Photo 48 A radar satellite image of some oil slicks. The slicks are approximately 2 kilometreslong and the white spots are ships. (Environment Canada)


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