Environmental Monitoring Using Airborne Midwave Infrared Mapping

People residing near industrial areas are affected by the gas plume released from nearby industrial plants in many ways. Since the gas cloud is able cover a large area, it is a challenging process to better understand the whole situation. Since the gas plume may consist of many different gases, a versatile instrument is required to address this situation.

Airborne infrared remote sensing is useful for characterization of the gas plume, and determining its direction and its spreading pattern over an area of interest. Infrared hyperspectral imaging provides reliable results using the characteristic infrared spectral features of each target. This article illustrates the advantage of using airborne midwave infrared hyperspectral imaging to measure gas emissions from industrial facilities as well as the ability of the Telops Hyper-Cam airborne platform to map urban areas consisting of various industrial plants.

Instrumentation and Flight Conditions

The Telops Hyper-Cam is a compact hyperspectral imaging device using Fourier transform infrared (FTIR) technology. The lightweight instrument has a unique combination of spectral, spatial, and temporal resolution, providing comprehensive characterization of substances of interest. The Hyper-Cam is equipped with a Focal Plane Array (FPA) detector consisting of 320x256 pixels over a basic 6.4°x5.1° field of view (FOV). The spectral resolution is user-selectable of up to 0.25 cm^-1 over the spectral range of 3.0-5.0µm.

Figure 1. The Telops Hyper-Cam Airborne Platform

The Telops Hyper-Cam Airborne Platform (Figure 1) facilitates extending the capabilities of the ground-based Telops Hyper-Cam for airborne applications. It is equipped with an inertial motion unit (IMU) and a global positioning system (GPS) for tracing and georeferencing of the aircraft actions in flight. It also features an image motion compensation (IMC) mirror that negates the aircraft movements during data acquisition using the GPS/IMU data. All data consist of the relevant data for orthorectification and stitching. A radiative transfer model developed by Telops is used to perform chemical imaging.

All flights were performed at speed of 110 knots and an altitude of 3000m, enabling a ground pixel size of 1 m2/pixel. The spectral resolution used was 9 cm^-1, which provided 110 spectral bands in total over the entire range covered by the FPA detector. Relative humidity and outside temperature at ground level were 68% and 2°C, respectively.

Experimental Results

Atmospheric Transmittance in the Mid-Wave Infrared Spectral Range

The atmosphere is composed of several infrared-active constituents in the spectral range of 3-5µm, including methane (CH₄), nitrous oxide (N₂O), CO₂, and H₂O. The ground surface acts like a blackbody source in a model environment and its radiance can be estimated by a Planck curve L_bkg at ground temperature. The product of this quantity and the atmospheric transmittance (T_atm) provides the fraction of the energy remained unabsorbed by the infrared- active atmospheric components. The self-emission pertaining to the absorbed energy is described by the term L_atm (l - T_atm) in Equation 1 as follows:

    L_tot = L_bkgτ_atm+ L_atm (1— τ_atm)      Equation (1)

Since the ground temperature is considerably higher than the temperature of the atmospheric components, their self emission is dominated by the absorption of the background radiance. As a result, the total spectral radiance L_tot quantified by a mid-wave infrared airborne sensor will be lower when compared to Lbkg at wavenumbers related to molecular absorption/emission bands of CH₄, N₂O, CO₂, and H₂O (Figure 2). Figure 2 depicts a typical pixel from a midwave infrared hyperspectral datacube captured at a 3000m altitude and the non-linear regression of Equation 1. The simulation is in line with the measurement and the dip between 2300 -2400 cm^-1 is a radiometric calibration artifact due to CO₂ and was not included in the fits.

Figure 2. Absorption spectra of the main infrared-active atmospheric components (top) and a typical midwave infrared spectrum (black curve) recorded at a 3000m altitude (bottom).

Airborne Measurements of Aluminum Smelter

Airborne measurements were performed over an operating aluminum smelter releasing CO gas. The phenomenology pertaining to the observation of a gas cloud from an airborne sensor is depicted in Figure 3. Only part of the background radiance (Lbkg) will only be transmitted by the gas plume from the smokestack (τ_plume) due to the presence of the infrared-active components in the emission gases. The infrared self-emission from these gases is defined by the term L_plume (1- τ_plume).

Figure 3. Phenomenology associated with airborne infrared hyperspectral imaging of a gas plume from a ground smokestack.

Since the gas emissions from the aluminum smelter are considerably hotter than their environment, the overall spectral radiance (L_tot) will be greater than L_bkg at wavenumbers related to the molecular absorption/emission bands pertaining to infrared-active component in the gas cloud. The ensuing radiative transfer equation is expressed as follows:

   L_tot = [L_bkg τ_plume+ L_plume (1- τ_plume)]τ_atm+L_atm (1- τ_atm)      Equation (2)

Figure 4. Airborne midwave infrared spectra of pixels corresponding to the gas plume of the aluminum smelter (black curve) and a typical background area (green curve).

Figure 4. llustrates the characteristic infrared spectrum pertaining to a pixel located close to one of the smokestacks. The red curve in the top picture represents the best fit for Equation 2. The absorption spectra of CO and OCS are shown in the bottom picture for comparison purposes. The presence of OCS can be justified by anode consumption in the aluminum electrolysis. The transmittance value (τ) varies in proportion to the concentration (c), the path length (l), and the infrared spectral response (ε) as expressed in Equation 3.

   τ = e-^(elc)

Figure 5 shows the results of quantitative airborne chemical imaging of CO and OCS.

Figure 5. Quantitative chemical imaging of carbon monoxide (CO, red cloud) and carbonyl sulfide (OCS, yellow cloud) from an operating aluminum smelter (Left). A schematic view of the main structures of the aluminum smelter is shown on the right.

Airborne Measurements of Waste Incinerator and Paper Mill

Here, airborne measurements were taken over an operating paper mill and a waste incinerator, where the gas emissions contain a large amount of water vapor. Figures 6 and 7 illustrate the chemical imaging of water vapor and aerosol plumes from the waste incinerator and the paper mill, respectively. The low atmospheric transmittance in the spectral range related to carbon dioxide makes measurement of ground sources of carbon dioxide for the waste incinerator difficult from a remote location by an airborne mid-wave infrared sensor.

Figure 6. Chemical imaging of water vapor (H₂O, yellow cloud) and water aerosol cloud (red cloud) from a waste incinerator (top). Visible image (bottom) is shown for comparison purposes.

Figure 7. Chemical imaging of water vapor (H₂O, yellow cloud) and water aerosol cloud (red cloud) from a paper mill top). Visible image (bottom) is shown for comparison purposes.

Conclusion

The airborne infrared hyperspectral data facilitated successful determination of various chemicals released by different industrial plants. Quantitative chemical imaging of the constituents was performed with a good accuracy. The results demonstrate the advantage of using airborne mid-wave infrared hyperspectral imaging in the characterization of gas clouds over large areas.

About Telops

Telops Inc., located in Quebec City, is a leading supplier of hyperspectral imaging systems and high performance infrared cameras for the defence, industrial, environmental and research industries. Telops also offers R&D services for optical systems technology development in order to respond to the specific needs of its customers.

Since its beginnings in 2000, Telops has distinguished itself with the quality of its technical personnel and its innovative approach to many technological challenges in the optics field. Today, the expertise of Telops scientists, engineers and technologists in optical technology, and the performance of its infrared cameras and hyperspectral imagers are recognized internationally.

While being headquartered in Canada, Telops caters to an international market using an efficient network of distribution and representation.

Whether you are looking for equipment, expertise or subcontracting for your optical system projects, Telops will turn your high expectations into success.

This information has been sourced, reviewed and adapted from materials provided by Telops.

For more information on this source, please visit Telops.