As oil exploration moves increasingly offshore and into deeper waters of the world's oceans, the risk of large scale leaks and spills are significantly increasing. Oil spills at sea come from a variety of sources and are often difficult to detect quickly. They can occur naturally from oil seeping through fractures on the ocean floof. Manmade spills occur through tanker accidents, or submarine pipeline ruptures when transporting oil. They are also increasingly occurring at offshore drilling platforms when oil and gas exploration and production activities are being conducted, often at depths that make it extremely difficult to cap the well. Oil spills on land, where the majority of wells are still located, can usually be detected and stopped relatively quickly and tend not to spread very far beyond the immediate area of the wellhead. Clean up operations are therefore accomplished speedily and environmental damage is limited.
It is much more difficult to detect a spill and verify its extent when the spill or leaks occur at sea. In addition the spill can spread far and wide from the additional site due to the influence of ocean currents and weather conditions. In order to contain the spill and carry out effective and timely clean ups a comprehensive survey of the affected areas is essential. Remote sensing applications are playing an increasingly important role in monitoring the ecological effects of these oil spills because they can cause significant damage to coastal wetlands and fishing grounds.
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Most remote sensing applications involve having the sensors located vertically or obliquely over the target at various distances and this is their primary advantage as, providing the sensor has a high degree of resolution, the total area that can be covered with sufficient detail increases with increasing altitude. With current technology it is possible to routinely monitor marine spills from the air, and from orbiting spacecraft. Remote sensing data can be used to plot the rate and direction of an oil spill slick by multiple measurements over a period of time, by linking the data collected to drift prediction modeling software. Sensors can also be used to determine the physical characteristics and estimate the thickness of the oil on the sea surface This data, in turn, is used to facilitate the clean-up and control activities.
Primary Remote Sensing Technology used in Marine Oil Spills
Remote sensing devices used include infrared video and photography, thermal infrared imaging, laser fluorosensors, Ultraviolet/Visible (UV/VIS) spectrophotometers, and radar. The remote sensing equipment is normally carried by aircraft or installed in orbiting satellites at varying altitudes, although ship borne remote sensors also play a role in close range monitoring.
Airborne oil spill remote sensing is normally divided into two different modes of operation:
1. Large Area detection
2. Close-range monitoring.
Large Area detection
Large Area detection of oil spills is usually performed by two basic types of radar:
1. Synthetic Aperture Radar (SAR) and
2. Side-Looking Airborne Radar (SLAR)
These use cloud-penetrating X-band radar techniques. SAR uses the forward motion of the aircraft to synthesize a very long antenna, thereby providing a greater range and resolution than the SLAR.
Oil spill detection using airborne radar is generally based on the principle that oil spills, can reduce the radar back-scatter due to dampening of gravity-capillary waves of the sea surface, making it visible in certain spectral bands.
Capillary waves on the ocean reflect radar energy, producing a whitish image known as sea clutter. Since oil on the sea surface dampens some of these capillary waves, an oil slick can be detected as a darker area on the sea surface. Therefore, black and white images from a radar detector are very effective at locating and monitoring oil spills especially those that cover a large area. It is by no means a perfect tool as there are a number of interferences that can appear to be oil slicks, but, in fact, are not. These can include fresh water currents moving through the salt water of the sea, beds of seaweed, and calm areas of the ocean caused by wind action, or behind outcrops of land.
Radar is also limited by wave height. Waves that are too low will not produce enough sea clutter to contrast to the oil, and very high waves will scatter radar so much that oil detection in deep wave troughs is masked.
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However radar is an important tool for oil spill remote sensing because it is the only effective sensor that can be used to search large areas and it can be used at night and under cloudy or foggy conditions.
Radar has also been used to measure currents and predict oil spill movements by using multiple scans over a period of time in order to estimate the forward movements at the frontal edges of the slick(Forget and Brochu 1996).
Close Range Monitoring
Once a survey has been conducted by airborne radar on the possible extent of a suspected oil spill , then a switch can be made to close range sensors to provide on-site monitoring of the oil, and the impact of clean up efforts.
Close-range monitoring of oil spills includes estimating the thickness of the oil layer on the sea surface and the composition and physical characteristics of the oil involved.This information can be used to identify the optimum mechanism for dispersion and removal of the oil, and also may help to identify the original source of the oil. The sensing technology used at close range will typically be operated in aircraft and helicopters at altitudes of 300-1,000 metres, and will cover relatively small areas within the overall slick. There are a number of developed near-range sensors, such as infrared (IR)/ ultraviolet (UV) line scanners, visible line scanners, camera systems, microwave radiometers (MWRs) and laser fluorosensors (LFSs).
Infrared detection of oil slicks depends on the fact that oil appears cooler than the surrounding water.
This mechanism is not fully understood. The current theory is that a thin layer of oil on the water surface causes a reduction of the thermal radiation emitted by the water. Infrared devices can not detect water-in-oil emulsions under most circumstances (Bolus 1996). This is probably a result of the high thermal conductivity of such emulsions as they contain about 70% water and so do not exhibit a detectable temperature difference. Most infrared sensing of oil spills takes place in the thermal infrared at wavelengths of 8 to 14 microns.
Ultraviolet sensors can be used to map even thin oil sheens (<0.01 microns) as oil slicks display high reflectivity of ultraviolet (UV) radiation..
Combining IR and UV can provide a more positive indication of oil than using either technique alone. Overlaid ultraviolet and infrared images can be used to map the relative thickness of oil spills.
Laser fluorosensors are active sensors that cause certain compounds in petroleum oils that absorb ultraviolet light to become electronically excited. This, in turn, results in a process known as fluorescence emission, in the visible region of the spectrum. There are few compounds that react in this way and, therefore, fluorescence indicates a high lilelihood that oil is present. Other natural compounds that fluoresce, such as chlorophyll, do so at different wavelengths so do not interfere with the oil emission signatures. As different types of oil yield slightly signatures, it is possible to distinguish between different types of oil.(Diebel et al. 1989; Geraci et al. 1993). The fluorosensor is also used for detecting oil in certain ice and snow situations.
Microwave Radiometers Sensors. The ocean emits microwave radiation. Oil on the ocean emits stronger microwave radiation than the water and thus appears as a bright object on a darker sea. The emissivity factor of water is 0.4 compared to 0.8 for oil (O'Neil et al. 1983; Ulaby et al. 1989). A microwave radiometer is a passive sensor that can detect this difference in emissivity and indicates the possible presence of oil. As the emissivity will change depending on the thickness of the oil layer the MWR could be used to estimate the thickness of the slick. Theo Hengstermann, Nils Robbe (Optimare Company of Denmark)
Satellite Remote Sensing
With the continuing development of high resolution satellite sensing devices, much more information can now be quickly obtained about major oil spills.
But before remote sensing from aircraft or satellites could become an effective tool for monitoring oil spills a significant problem area had to be overcome.
A stationary reflectance spectrometer, breaks the light into discrete wavelengths. However, when such a spectrometer is flown on an aircraft or spacecraft, a problem with recording the light arises because the scene moves past the lens at high speed. In older detectors the processor couldn't sample the dispersed light fast enough to resolve it into the closely-spaced wavelengths needed for a spectral curve capable of analysis. The light was recorded as broad bands.
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The technology for a spectrometer that could scan effectively at narrow wavelength intervals while moving over terrain at high speed, came with the development of Charge-Coupled Detectors (CCD's). A CCD is a semi-conducting chip that is sensitive to photons of light. Radiation produces an electron charge on the chip proportional to photons received. This is dependent on the intensity and exposure time. The charge is rapidly removed and the CCD is reset for the next set of photons received.
As the flight proceeds, a huge amount of spatial and hyperspectral data is produced that can then be analysed using powerful computer systems and dedicated software. The data can be manipulated to produce images using individual narrow spectral bands associated with small plots on the ground.
To illustrate the importance of remote sensing in helping to monitor and mitigate the effects of large oil spills, the activities related to the Deepwater Horizon oil spill in April 2010 provide a practical example:
What became the worst oil spill in American history occurred in the Gulf of Mexico 50 miles from Louisiana's Mississippi River delta. The drilling rig, Deepwater Horizon, operated by British Petroleum (BP) exploded from a gas leak on April 20, 2010 and sank, killing 11 workers. The rig was drilling at the ocean floor about 1500 meters below the surface and, had reached a reservoir of oil at a depth of more than 4000 meters. The oil was mixed with natural gas and under high pressure. When a leak occurred at the well head and the Blow Out Preventer (BOP) failed, Hundreds of thousands of barrels of oil leaked from the well head 1500 meters below the surface and caused a slick toform on the surface headed towards the Gulf coast. The land in the delta is just above sea level - low and flat and easily breached by high waves.
This was the first failure of an oil well in deep water. That depth, nearly a mile, made it extremely difficult to cap the escaping oil.
Using remote cameras installed in unmanned submersibles it was possible to observe the oil leaking from the wellhead in real time, and also to facilitate the various attempts to cap the well.
Aerial flights were carried out over the oil spill, using remote sensing technology designed to obtain detailed spectral measurements of the properties of the oil as it spread across the Gulf waters.
In this instance, a sophisticated and sensitive remote sensing system known as AVIRIS (Airborne Visible/Infrared Imaging Spectrometer ) was one of the primary means of acquiring data related to the oil spill. AVIRIS used a hyperspectral imaging system capable of producing a continuous spectral curve. It had two sets of CCD arrays, one with silicon chips for the visible range and the other with Indium-Antimony chips for wavelengths in the Near-Infra Red to Short-Wave-Infra Red. A refrigeration unit cooled the detectors with liquid nitrogen to optimize its sensitivity.
A typical AVIRIS operation, produced a spatial resolution of about 20 meters, but that was improved to five meters by flying at lower altitudes, albeit at the expense of smaller areas covered.
Using this system natural and false colour images of oil streaks within the spill were produced, together with other data that was used to facilitate containment, dispersal and removal operations.
Remote sensing of oil spills to facilitate monitoring, containment and removal activities has evolved into a key element for the protection of the marine environment. A number of specialised oil spill remote sensors are now available. These sensors have become well established during the last three decades.