LIDAR operates on the same principles as RADAR except that it uses light rather than radio waves to collect information. There are three generic types of LIDAR:
Range finders are the simplest of the LIDARs and are used to determine the distance to a solid or hard target. When mounted in an aircraft and coupled with imaging instrumentation and a GPS, these instruments can provide 3-D topographic maps.
DIAL is used to measure chemical concentrations in the atmosphere (open air). A DIAL LIDAR uses a laser wavelength that is strongly absorbed by the target compound and a second nearby wavelength that is not absorbed by the target compound. The difference in intensity of the two return-signals can be used to calculate the concentration of the compound being investigated.
Doppler LIDAR is used to measure the velocity of a moving target. When light transmitted from the LIDAR hits a moving target, the wavelength of the light reflected/scattered off the target changes slightly. This is known as a Doppler shift. This type of LIDAR finds uses in speed limit enforcement and weather predictions (cloud and wind speed as well as density). A complete treatment of LIDAR can be found in Argall and Sica 2002.
LIDARs consist of a source transmitter, receiver, and detector system. All uses of LIDAR involve laser light operated in the UV, visible, or infrared wave range that is transmitted toward a target. The light interacts with the target where it is either absorbed or reflected/scattered back to a measuring device. They can be deployed in monostatic or bistatic configurations. The monostatic configuration is employed the most in contaminant profiling. The system can be set up in either a coaxial configuration where the laser beam is transmitted within the receiver's field of vision or in a biaxial fashion where the transmitting and receiving units are adjacent to each other (Exhibit 1).
In the bistatic configuration, which is seldom used, the light source and the detector are at some distance apart (Exhibit 2). The receiving optics gather reflected light from the source beam and direct it through a filter to the detector. The bistatic system can only detect light from a small layer of the atmosphere at a time and the detector must be frequently moved to obtain a contaminant profile (Argall and Sica 2002).
LIDARs use lasers for their source and hence do not require a sending telescope as do Fourier Transform Infrared (FTIR) spectroscopy and Ultraviolet Differential Optical Absorption Spectroscopy (UV-DOAS); however, they generally require a mechanism for widening the laser beam to make it "eye safe." For the DIAL system, an appropriate wavelength is chosen for the species to be measured along with a nearby wavelength that will not be absorbed by the target compound. The laser can be transmitted as a continuous-wave or pulsed source. Continuous waves are used when the signal is integrated over a long time period or when the target is close. Pulsed waves are at a much higher energy level than can be maintained during a continuous emission, and they are used for long-range remote sensing or when the signal integration time needs to be short. Tunable laser systems that allow for quickly changing wavelengths are available to measure a wider range of contaminants.
The receiver may have a Cassegrain or Newtonian telescope or a single fixed condensing lens that focuses the returning or transmitted light (depending upon system deployment). Before being directed to the detector, the backscatter light is usually processed to filter out background.
LIDAR detectors convert the returning light into electrical currents, which are then processed into photocounts. Photomultiplier tubes have been employed the most in DIAL systems, but semiconductor devices, such as mercury-cadmium-telluride (MCT) detectors and avalanche photodiodes are finding increasing use. To function properly MCT detectors have to be cooled to liquid nitrogen temperatures.
Unlike FTIR or UV-DOAS, DIAL produces a measure of discrete concentrations versus distance along the line of the beam path. This allows for the values to be superimposed on a map or other visual aid to show where low and high point concentrations are occurring. A series of closely placed measurements over a site can be plotted to show an estimate of the spatial distribution and quantity of a chemical (e.g., an isopleths map).
LIDAR has historically been used in the environmental field for measuring criteria pollutants in the upper atmosphere. As the equipment becomes more portable and less expensive, it is gaining acceptance for industrial and commercial applications. The proper detection and identification of a chemical species using DIAL is directly dependent upon it having a unique absorption frequency over the path being measured (i.e., there are no other chemicals present that have the same absorption frequency) and the availability of a laser source that emits that frequency. In the past, the lack of laser sources that emit specific bands has limited the number of chemicals that this technology can identify; however, as more reliable tunable lasers become available the list of detectable chemicals should increase. In addition to specific chemical detection, a DIAL system can be used in a more "open" mode much like a point source organic vapor analyzer. In this mode, a chemical family, such as alkanes, is measured by picking a band that is common to many and interpreting the results as an "average." The Alberta Research Council study given below is an example of this use.
Detection limits are largely dependent on the path distance, atmospheric conditions, and the instrument being used. However, an examination of the literature indicates that sub-ppm levels of a variety of chemicals at ranges of one kilometer or more are routine. Exhibit 3 contains detection limits and effective ranges for commonly measured chemicals.
Potential Detection Limits for Air Pollutants Using DIAL¹
|UV/Visible DIAL System
||Detection Limits² µg/m3
||Detection Range in Meters
|Nitric Oxide (NO)
|Nitrogen Dioxide (NO2)
|Sulfur Dioxide (SO2)
|Infrared Dial System
|Higher Alkanes (CxHx)
|Hydrogen Chloride (HCI)
|Nitrous Oxide (N2O)
1 Values are from National Physical Laboratory, United Kingdom
2 The detection limits apply at a range of 200 m for a 50-meter plume
Oil Spill Tracking on Open Water
Yamagishi et al. (2000) report on the use of LIDAR in detecting and tracking oil spills and sheens over open water. The instrument is mounted in a small plane that overflies the area of concern. The laser emits in the ultraviolet range, which causes a fluorescence response that is captured by an imaging camera. The wavelength response strength is measured and can be analyzed to determine the type of petroleum product released. In addition, the method can also be used to estimate the thickness of the oil on water.
Remote Sensing of Chemical and Biological Warfare Plant Effluents from Aircraft
Sandia National Laboratories is developing a multispectral LIDAR system for deployment on unmanned aircraft. The system is designed to detect and image effluents that are associated with the production of chemical and biological warfare agents. A tunable UV laser is envisioned for the source with the aircraft operating at between 1 and 10 kilometers above ground surface (Hargis 1998).
Remote Sensing for Natural Gas Pipeline Leaks from Aircraft
Lenz et al. (2005) report on a Department of Energy/Department of Transportation co-sponsored demonstration of the ability of airborne DIAL LIDAR to detect gas leaking from underground pipelines. The Airborne Natural Gas Emission LIDAR (ANGEL) system operates from a fixed-wing aircraft flying between 100 and 150 mph at a height of approximately 1,000 ft. It has two DIAL lasers tuned to detect methane and ethane. The system has a scanning component that is pre-programmed with the pipeline's location data to allow it to focus the lasers on the pipeline so that the aircraft does not have to be directly over the pipeline to compensate for aircraft yaw, pitch, and roll. A digital camera records the ground surface beneath the aircraft and provides the background for superimposing the laser results. The final product is a map showing the exact location of any gas leaks and an estimate of their concentration.
A portable and less complex system was also tested at the Department of Energy/Department of Transportation co-sponsored demonstration (U.S. DOE 2004). This system, which can be mounted on the undercarriage of a helicopter, consists of a methane-detecting DIAL LIDAR, digital imaging camera, and GPS unit. The pipeline is flown over by the helicopter at 30 mph and visual observation of pipeline markers is used to guide the aircraft. The location uncertainty of the equipment used at the demonstration was ï¿½100 ft.
Both systems were generally successful in identifying leaks of 500 standard cubic feet (scf) or more and much less successful with leaks of 100 scf or less. The tested prototypes have since been modified to improve performance.
Particulate Monitoring for Ore Handling Facilities
The Institut National d'Optique (Canada) has developed and demonstrated a LIDAR system for monitoring industrial particulates. To protect the more sensitive elements of the equipment, the laser, detector, computer, and acquisitions electronics are housed in a controlled environment with the LIDAR head (optical transmitter and receiver) connected by fiber optic cables to the laser and detector. The head is mounted on a control unit that can be remotely operated to scan up to 355 degrees laterally and ±90 degrees vertically. The demonstrations were at port facilities where various ores were being unloaded and fugitive dust control was required. The system successfully imaged particulate release and was used to identify and correct loading practices to reduce emissions. The value of this system over high-volume stationary air samplers is that it gives real-time data on how far the particulates are from the source, total average release, and the location of "hotspots" (Bï¿½langer et al. 2001).
Monitoring Contaminant Fluxes from Industrial Stacks
DIAL systems can be used to measure contaminant releases from industrial stacks. These measurements, combined with an estimate of the wind speed and direction, are used to estimate contaminant flux. Since the DIAL system measures instantaneous concentrations of a single contaminant across a cross section of the plume, the accuracy of the wind speed associated with the measurement at a specific time directly affects the accuracy of the flux estimate. In an experiment at a pulp and paper mill, Weibring et al. 1998(a) compared the method of using standard wind anemometers with one using a video camera to calculate SO2 flux. The wind speed was calculated by evaluating the plume change between video frames, that could be directly related in time to measurements made by the LIDAR. Since the stack was 100 meters high, the anemometer tower gave readings that were much closer to the ground than actual stack conditions. The researchers found that these readings were often affected by buildings and the adjacent forest and therefore underestimated the flux by a considerable degree. The camera technique, which only works with a visible plume, was found to be much more accurate.
Verification and Monitoring of Cap and Cover System Integrity
Heiser and Sedlacek (2006) report on the development of a DIAL LIDAR to determine the integrity of caps and covers. For wastes capped in place or for older landfills where a leachate collection system is absent and there is no generation of methane gas, the authors propose injecting a perfluorocarbon tracer into the capped area and using the open path LIDAR to determine if any of the gas escapes. Areas of high concentration of tracer gas could be pinpointed and repairs to the cover made.
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