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Remote Sensing involves techniques that use sensor devices to detect and record signals emanating from target(s) of interest not in direct contact and at a distance from the sensor.
Techniques range from simple visual interpretation of a sensed scene to methods of analysis that utilize complex algorithms applied to measurements. Sensors are primarily optico-mechanical and electronic components. Detection requires that the sensor is able to properly respond to the signal, and to measure its quantitative properties. Recording requirements are necessary to enable the signal to be acquired and saved in a usable format for subsequent analysis. Most commonly, signals are acquired in the form of electromagnetic (EM) radiation (as photons) that represents discrete wavelength intervals or bands (e.g., X-rays; visible light; radio waves) within the EM spectrum. However, remote sensing is also an appropriate term when applied to acoustical devices, or to detectors that respond to magnetic force fields, The target of interest is at a distance from, and is not physically touching, the sensor. Therefore, increasing distance enables a wider field of view to be sensed (typically sensor optics are used to bring into focus all resolvable objects in an area of observation) but at the expense of better resolution (showing greater detail). Most remote sensing applications involve having the sensors located vertically or obliquely over the target at various distances and this is the primary advantage of remote sensing applications
The total area covered is considerably larger if observations are made from a tall building or a mountain top. This increases even more - to perhaps hundreds of square miles - from an aircraft at 30000 feet. From a vertical or high oblique perspective (e.g. from a mountain or a skyscraper), a scan of the surface below is notably different than from a point directly on that surface. Many surface features become clearer from these viewpoints. This is why remote sensing is carried out from airplanes and spacecraft with onboard sensors that survey and analyse surface features over extended areas from above. It is a cost-effective way of maintaining and updating information about the physical surface of the Earth.
Until the 1960s, remote sensing was confined to 'aerial photography'. Now, the term is most often applied to imagery from satellite or aircraft remote sensing devices. Satellite remote sensors (and similar technology in aircraft) have these major advantages over aerial photography: 1) they provide worldwide coverage; 2) they allow quantitative analysis of the data (which are acquired in digital formats), so that the objects in the scenes can be readily identified and analysed; and 3) the coverage can be repeated easily and frequently.
Below are some major applications of remote sensing technology:
1. Imaging, identification, and analysis of the features exposed on land surfaces, gathering data about landforms, geology, topography, urban regions, agriculture, forests, rivers and lakes.
2. Imaging of the ocean surfaces, their physical properties, and sea states.
3. Imaging of the seafloor and of creatures within the water (mainly by sonar).
4. Imaging of meteorological conditions and determination of various atmospheric properties.
Most remote sensing systems are built around cameras, scanners, radiometers, Charge Coupled Device (CCD)-based detectors, radar, etc. of various kinds. These normally look at their targets from a distance. Geophysical instruments operating on the Earth's surface or in boreholes are also remote sensing devices, as are ground-based telescopes, and cameras.
It can therefore be seen that remote sensing is a widespread technique for observing and acquiring data, in order to characterize areas of scientific and technical interest and importance.
As examples, remote sensing applications are used in monitoring of agricultural productivity, deforestation, changing land use, growth of cities, effects of tsunamis, earthquakes, fires, short term weather behaviour and long term climate changes, global warming, communications , and military surveillance.
Remote Sensing of Marine Oil Spills
Accidents involving petroleum and natural gas extraction, transportation, or processing are frequent enough to be of major concern to environmentalists. Oil spills on land, where most oil wells are located, usually can be stopped quickly. Most spillage is confined to the site immediately around the wellhead and damage to the environment can be cleaned up relatively easily.
A much more difficult oil spill is one that contaminates the ocean from a failure at an offshore drilling site, from tanker accidents, or from submarine pipelines. Oil can also reach marine surfaces from so-called "oil seeps", which are natural leakage of oil from fractures under the ocean floor. Oil is also added to the sea surface in the form of leaks from oil fired ships.
These spills can have a significant and adverse effect on coastal wetlands and on offshore fisheries. Remote sensing applications are now playing a major role in monitoring the ecological effects of these oil spills. Such spills are now routinely monitored from ships, the air, and from orbiting spacecraft. For ocean spills, in particular, remote sensing data can provide information on the rate and direction of oil movement through multi-temporal imaging, and provide input to drift prediction modeling software and may facilitate the targeting, clean-up and control efforts. Remote sensing devices used include the use of infrared video and photography from aircraft, thermal infrared imaging, airborne laser fluorosensors, airborne and space-borne optical sensors, as well as airborne and spaceborne SAR (Synthetic Aperture Radar). Ultraviolet (UV) radiation is also an effective tool for monitoring oil on water because petroleum responds by fluorescing.
Airborne oil spill remote sensing is normally divided into two different modes of operation:
1. Far-range detection
2. Near-range monitoring.
Far-range 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) real aperture type
These use cloud-penetrating X-band radar techniques.SAR uses the forward motion of the aircraft to synthesize a very long antenna, thereby achieving very good spatial resolution, which is independent of range. The SAR has greater range and resolution than the SLAR.
Oil spill detection using airborne radar is generally based on the principle that oil spills, as well as biogenic surface films (even monomolecular films) and hydrodynamic effects, may reduce the radar back-scatter due to dampening of gravity-capillary waves of the sea surface, making it visible in certain spectral bands. Therefore, radar imagery, especially in the black and white mode, is very effective at locating and monitoring oil spills over their full extent.
Capillary waves on the ocean reflect radar energy, producing a "bright" image known as sea clutter. Since oil on the sea surface dampens some of these capillary waves, the presence of an oil slick can be detected as a "dark" sea. Unfortunately, oil slicks are not the only phenomena that are detected in this way. There are many interferences or false targets, including fresh water slicks, wind slicks (calms), wave shadows behind land or structures, and weed beds.
Radar is also limited by sea state. Sea states that are too low will not produce enough sea clutter in the surrounding sea to contrast to the oil and very high seas will scatter radar sufficiently to block detection inside the troughs.
However radar is an important tool for oil spill remote sensing because it is the only sensor that can be used for searches of large areas and it is one of the few sensors that can be used at night and through clouds or fog.
Radar has also been used to measure currents and predict oil spill movements by observing frontal movements (Forget and Brochu 1996). Studies have also shown that frontal currents and other features can be detected by SAR (Marmorino et al. 1997).
Near Range Monitoring
Possible areas of oil spill detected by airborne radar are subsequently investigated on-site using near-range sensors.
Near-range monitoring of oil spills includes mapping of relative and absolute oil layer thickness, as well as classification of the type of oil. This mode of operation is typically limited to swaths of several hundreds of metres at flight altitudes in the range of 300-1,000 metres. There are a number of well-established 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 surrounding water most of the time.
The reason for the appearance of the "cool" slick is not fully understood. The
most plausible theory is that a moderately thin layer of oil on the water surface causes destructive interference of the thermal radiation waves emitted by the water, thereby reducing the amount of thermal radiation emitted by the water.
Infrared devices can not detect emulsions (water-in-oil emulsions) under most circumstances (Bolus 1996). This is probably a result of the high thermal conductivity of
emulsions as they typically contain 70% water and thus do not show a temperature difference. Infrared cameras are now very common and commercial units are available from several manufacturers. 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 sheens of oil as oil slicks display high reflectivity
of ultraviolet (UV) radiation even at thin layers (<0.01 microns). Overlaid ultraviolet and infrared images are often used to produce a relative thickness map of oil spills.
Combining IR and UV can provide a more positive indication of oil than using either technique alone.
Laser fluorosensors are active sensors that take advantage of the fact that certain compounds in petroleum oils absorb ultraviolet light and become electronically excited. This excitation is rapidly removed through the process of fluorescence emission, primarily in the visible region of the spectrum. Since very few other compounds show this tendency, fluorescence is a strong indication of the presence of oil. Natural fluorescing substances, such as chlorophyll, fluoresce at sufficiently different wavelengths than oil to avoid confusion. As different types of oil yield slightly different fluorescent intensities and spectral signatures, it is possible to differentiate between classes of oil.
Most laser fluorosensors used for oil spill detection employ a laser operating in the
ultraviolet region of 300 to 355 nm (Diebel et al. 1989; Geraci et al. 1993). With this wavelength of activation, there exists a broad range of fluorescent response for organic matter, centered at 420 nm. Chlorophyll yields a sharp peak at 685 nm. The fluorescent response of crude oil ranges from 400 to 650 nm with peak centers in the 480 nm region.
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 passive device can detect this difference in emissivity and could therefore be used to detect oil. In addition, as the signal changes with thickness, in theory, the device could be used to measure thickness. Passive microwave radiometers may have potential as all-weather oil sensors.
Theo Hengstermann, Nils Robbe (Optimare Company of Denmark)
Visible. In the visible region of the electromagnetic spectrum (approximately 400 to 700 nm), oil has a higher surface reflectance than water, but also shows limited nonspecific absorption tendencies. Oil generally manifests throughout this visible spectrum. Sheen shows up silvery and reflects light over a wide spectral region down to the blue. As there is no strong information in the 500 to 600 nm region, this region is often filtered out to improve contrast (O'Neil et al.1983). Overall, however, oil has no specific characteristics that distinguish it from the background (Brown et al. 1996). The use of visible techniques in oil spill remote sensing is largely restricted to documentation of the spill because there is no mechanism for positive oil detection.
Satellite Remote Sensing
With the continuing development of high resolution satellite sensing devices, much more information is now available over short turnaround times above major oil spills.
To illustrate the importance of remote sensing in helping to monitor and combat large scale oil spills the activities related to the Deepwater Horizon oil spill in April 2010 provide a practical example:
Until the BP oil spill of 2010, the worst U.S. oil spill was in 1989 off Prince William Sound on the Alaskan coast. The tanker Exxon Valdez ran aground against a reef and split open. Almost 11 million gallons of oil leaked into the sea. Most monitoring and tracking of the resulting oil spill was done from aircraft and ships.
As observed above, oil and other petroleum products can be inadvertently introduced into the sea in several ways. One source is at or below drilling platforms. For the U.S., the most numerous of these is in the western half of the Gulf of Mexico. Oil also reaches the ocean surface in the Gulf simply from natural occurring seeps.
What has become the worst oil spill in American history occurred 50 miles offshore 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. At the time of this event, the drilling, which started at the ocean floor some 1500 meters below the surface of the sea, had reached a reservoir of oil at a depth exceeding 4000 meters. The oil was mixed with natural gas and under high pressure. The Deepwater Horizon well was inadequately sealed, and natural gas built up inside it. This drove both drilling mud and oil itself out onto the ocean floor, initiating the leak. When workers on the rig tried to activate the well's blowout preventer (BOP), it failed. An attempt to activate the blowout preventer, using undersea robots, also failed. The oil, driven by the entrained gas, leaked from the well head 1500 meters below the surface. Hundreds of thousands of barrels of oil came to the surface to form a slick headed towards the Gulf coast. The land in the distributaries is just above sea level - low and flat - and easily breached by high waves during a storm.
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 initiated 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.
The instrument that gathered the data was AVIRIS (Airborne Visible/Infrared Imaging Spectrometer which is a hyperspectral imaging system capable of producing a continuous spectral curve. AVIRIS can produce both natural and false color images of the oil streaks within the spill.
A stationary reflectance spectrometer, breaks the light into wavelengths emanating from the fixed view. 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 to construct a spectral curve and the light was recorded as broad bands.
The technology for a scanning spectrometer that could sweep across moving terrain, while sampling at narrow wavelength intervals, came with the development of Charge-Coupled Detectors (CCD's). A CCD is a microelectronic semi-conducting metal chip that detects light. Radiation produces an electron charge on the chip in proportion to the number of photons received, which is governed by the intensity and exposure time. The charge must be rapidly removable, resetting the CCD for the next influx of photons.
As the instrument proceeds along its flight path or orbit, the final result is a vast collection of data that has both spatial and hyperspectral inputs. The data can be manipulated to produce images using individual narrow spectral bands associated with small plots on the ground. Spectral data for any pixel position across the width to the wavelengths sampled lengthwise, can be used to plot a spectral curve for that piece of the surface. This, in a general way, describes how hyperspectral imaging spectrometers operate. JPL's AVIRIS uses diffraction gratings with two sets of CCD arrays, one with silicon chips to sense in the visible range and the other with Indium-Antimony (InSb) chips for wavelengths in the Near-IR to Short-Wave-IR range. A refrigeration unit cools the detectors with liquid nitrogen for optimum performance. AVIRIS gathers its light through a 30° field of view, sending diffracted light to the 614 individual CCD's in the width (across flight) direction. An oscillating mirror scans the scene and sends the incoming radiation to sweep across the array. The spatial resolution derived from this depends on the platform height. A typical operation, produces a spatial resolution of about 20 meters, but that can be improved to five meters by flying at lower altitudes, which, of course, narrows the width of the ground coverage.
Satellites and aircraft were the primary platforms for observing the effects of the spill over the open ocean and along the beaches and coastal wetlands. But the scope of the spill justified other ways of monitoring these effects. Helicopters, allowed observers to remain over small areas at a time, noting details that were relayed in real time to ground workers engaged in setting up protection barriers or in clean up operations.
However satellites can only be used to monitor surface oil. It is thought that much of the spill may have remained subsurface, dispersed and in plumes at various depths.
Remote sensing of oil spills in conjunction with their prevention and combat 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. Since the mid-1990s, there are new approaches to handling multi-sensor oil spill data, especially in terms of fusion and geospatial information systems (GIS) integration.