Thermal Infrared Hyperspectral Studies of Materials in the Field

Purpose. Remotely-sensed hyperspectral data recorded by vehicle, rover, airborne and satellite-carried spectrometers are used for stand-off material identification. The minimum amount of a material that can be identified depends in part on the spectral band strength (spectral contrast) that the material exhibits at the spectral resolution of the instrument, the amount of material present, and the signal-to-noise ratio (SNR) of the spectrum. Weathering and surface roughness affect the band contrast of materials. To improve understanding of these effects, we study signatures of targets measured both in the laboratory and field. Currently we focus on carbonates because they exhibit a wide range of surface texture at several scales, and are of interest to both Department of Defense (DoD) and planetary researchers. Targets studied include weathered, indurated carbonate (calcrete), limestone, and a carbonate rich soil. We use:

  • Laboratory spectra (2.5 - 200 Ám)
  • Van-mounted field spectrometer data (3-5 and 7-13 Ám).
  • Uniquely high quality airborne spectrometer data (Spatially Enhanced Broadband Array Spectrograph System, SEBASS, 3-5 and 7-13 Ám)

    This study may explain why carbonates, which are expected to be present on Mars, have not been detected using the Mars Global Surveyor Thermal Emission Spectrometer (TES). In addition, we study data fusion, path-length, viewing geometry, proximity, and reflected downwelling radiance effects for solid phase targets measured from vehicle/rover, airborne, and satellite perspectives.

    This work joins expertise and technology developed under both Department of Defense and NASA programs, and is sponsored by the Lunar and Planetary Institute (a non-profit research institute), and The Aerospace Corporation (a non-profit FFRDC). We welcome inquiries about this work, and questions should be addressed to Laurel Kirkland (kirkland@lpi.usra.edu, 281-486-2112).

    Field site. The site described here is the Mormon Mesa, near Mesquite, Nevada. Mormon Mesa has a cap rock of strongly indurated calcite (calcrete), overlain by loamy soil rich in carbonate and quartz, and significant coverage by fragments of calcrete [Gardner, 1972].

    Laboratory spectrometer. We measure reflectance spectra over the 2.5-200 Ám wavelength regions with a Nicolet Magna 550 Fourier transform infrared (FTIR) spectrometer with a DTGS detector. Biconical reflectance spectra (2.5-200 Ám) use a Harrick Scientific "praying mantis" diffuse reflectance attachment, and a Labsphere Infragold standard as the background spectrum. A solid substrate beamsplitter and DTGS detector with a polyethylene window are used to record spectra from 9 to 200 Ám. A region of overlap (9-25 Ám) between the coverages of the KBr (2.5-25 Ám) and solid substrate beamsplitters allow spectra from both regions to be scaled if bands existed in the region of overlap. Hemispherical reflectance measurements (2.5-25 Ám) use a 3" diameter Labsphere integrating sphere lined with Infragold to determine absolute emissivity via Kirchhoff's Law (emissivity = 1- reflectance). The wall of the sphere (with the sample in place) is the background spectrum. The upward facing weathered sample surfaces (as marked in the field) are examined to better correlate laboratory with field spectra.

    Van-mounted field spectrometers. Van-mounted interferometer spectrometers used include a Brunswick Model 21 (M21) and a Block Engineering Model 100 (M100). The Aerospace Corporation has a mature program for stand-off detection and identification of materials using these spectrometers. They have well-developed, precisely controlled viewing geometry and georeferencing capabilities, whose accuracy and high quality have been demonstrated in extensive field tests. The spectrometers view the scene through an optical column that includes a mirror that raster scans in 2 dimensions in order to build an image. We include measurements to study field data measured in short-distance environments and next to buildings and objects to improve understanding of path-length, viewing geometry, proximity, and reflected downwelling radiance effects for solid phase targets measured from this perspective. We use the results to develop the necessary measurement protocols and instrumentation to identify materials in the field from a vehicle/rover perspective, and for data fusion with airborne and satellite measured data. Field spectrometers also provide a cross-check for the airborne spectra.

    Airborne spectrometer. SEBASS measures in two channels, from 2.42 to 5.33 Ám, and 7.57 to 13.51 Ám. Each channel measures a spectrum with 128 points. For the work described here, SEBASS operates as a line scanner in the "pushbroom" mode from an aircraft. Each image is 128 pixels wide (cross-flight direction), and the image length (along the flight direction) varies, but is typically 2000 pixels. The instantaneous field of view (FOV) is 1 milliradian per pixel, and thus is 128 milliradians crosstrack.
    SEBASS uses two 128x128 detector arrays, and a prism for the dispersive element. To increase the signal to noise ratio (SNR), the entire optical bench is cooled to 10K using liquid helium. The long wavelength channel has a spectral resolution of ~0.050 Ám per pixel, and the short wavelength channel has a spectral resolution of ~0.025 Ám per pixel.
    Additional information is in the reference: Hackwell, J. A. et al., SPIE Vol. 2819, pages 102-107, 1996, and in the references below.

    Addition information is in:

    This link gives additional information on conducting field spectrometer measurements, and on field spectrometers.


     

    SEBASS in the Otter, with Cameron Purcell (left) and Eric Keim.


     

          SEBASS in the Otter


     

    Eastern edge of the Mormon Mesa, showing typical calcrete mesa cap rock, and the loose material that has fallen from the cliff. These regions were imaged by SEBASS and the field spectrometers. Eric Keim and Cameron Purcell are visible in the upper right.


     

    Typical mesa material consists of calcrete fragments a reddish, loamy, quartz and carbonate rich soil.


    Typical mesa material, from the same region shown above. This region was measured by SEBASS and the field spectrometers. We also collected field samples, and have characterized these in the laboratory (Paul Adams and Allan Treiman). This combination of airborne, field, and laboratory spectral measurements and sample characterization will be utilized to improve our understanding of thermal infrared spectra of weathered materials, especially carbonates. Shown are Eric Keim and Cameron Purcell (of The Aerospace Corporation) taking a differential GPS measurement.


    Field spectrometer van. Visible on top of The Aerospace Corporation's van is the column containing the scanning mirror for the 7 - 14 Ám spectrometer (right); the short wavelength spectrometer (3 - 5 Ám, center); and the short wavelength spectrometer calibration blackbody target (left). Don Stone (The Aerospace Corporation) is entering the van, and Cameron Purcell is in the background taking a GPS measurement. Typical calcrete material and reddish soil are visible on the flat mesa top.


     

    Partial SEBASS image of Mormon Mesa, shown offset from the main Mesa paved road for clarity. The SEBASS image is processed so that red=carbonates, and blue=quartz. The asphalt road has limestone in the aggregate, which gives it a strong carbonate signature, and the arroyos also have limestone which gives a strong signature. However, the calcrete gives an unexpectedly weak signature. This shows that carbonates are not always as straightforward to locate using remotely sensed spectra as is generally assumed based only on laboratory spectral measurements. This illustrates the importance of extending spectral studies from the lab into the field.


    Inside the field spectrometer van. The silver column in the mid-right contains the optical train that allows the spectrometer to view outside the van. The optical train incorporates a mirror that can scan in two dimensions. A spectrum is visible on the green screen in mid-left. The joy stick in front of the screen allows the user (here, Ken Herr) to "drive" the spectrometer field of view, which is viewable on the small, gray screen above and to the right of the green screen. This spectrometer records spectra in the 7 - 14 Ám region.


    Measuring the Mormon Mesa cliff. Line scans of the cliff were recorded using the field spectrometers. The short wavelength spectrometer is visible mounted on top of the van, and the housing for the long wavelength spectrometer scanning mirror. Allan Treiman is to the left with Don Stone behind the open van door, and Jess Valero pointing up at the cliff.


    SEBASS team patch. Larger image here.


    LINKS

    Lunar and Planetary Institute

    The Aerospace Corporation

    Laurel Kirkland's Home Page