An Overview of Thermal Infrared Remote Sensing with Applications to Geothermal and Mineral Exploration in the Great Basin, Western United States
Published:January 01, 2009
James V. Taranik, Mark F. Coolbaugh, R. Greg Vaughan, 2009. "An Overview of Thermal Infrared Remote Sensing with Applications to Geothermal and Mineral Exploration in the Great Basin, Western United States", Remote Sensing and Spectral Geology, Richard Bedell, Alvaro P. Crósta, Eric Grunsky
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The physics of thermal infrared aerospace measurements is based on Planck’s Radiation law, Wien’s Displacement law, and Kirchoff’s law. The electromagnetic spectrum for thermal infrared aerospace measurements includes measurements beyond the reflected short- (2.5 μm) to the long-wave infrared (14 μm).
Thermal infrared sensors measure thermal emission from the Earth’s surface in single wavelength bands (broadband), tens of bands (multiband), and in hundreds of bands (hyperspectral). Broadband thermal infrared measurement techniques include surface temperature mapping and thermal inertia mapping. Multiband and hyperspectral techniques involve mapping of changes in thermal emission at different wavelengths (emissivity mapping). Today, broadband surface temperature mapping is mostly done with satellite sensors. Thermal inertia mapping is done using broadband measurements taken during the day and night. Emissivity mapping is done using tens to hundreds of bands, and it requires sensors capable of measuring small changes in radiant emittance. Sensor systems discussed in this study include: Thermal Infrared Multispectral Scanner (TIMS), the Moderate Resolution Imaging Spectroradiometer (MODIS), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Simulator (MASTER), the Spatially Enhanced Broadband Array Spectrograph System (SEBASS) and the ASTER satellite sensor.
Several areas of Nevada, such as Brady’s Hot Springs, Steamboat Springs, Geiger Grade, and Virginia City, were used as sites for demonstrating the geologic applications of thermal infrared remote sensing. Corrected day and night images over Steamboat Springs were acquired by TIMS. These day-night images were combined together to produce a final processed temperature image, in which the temperature effects of albedo, topographic slope, and thermal inertia were minimized to facilitate the detection of geothermal anomalies. Spectral variations in emitted thermal energy were detected over the Geiger Grade and Virginia City areas using the MODIS-ASTER Simulator (MASTER) and (SEBASS). MASTER thermal infrared image data allowed two primary mineralogic units in the Steamboat Springs area to be identified: sinter and/or chalcedony deposits and quartz-alunite alteration, which have spectral emissivity features around 9.0 μm; and clay-rich soil and clay alteration, which have spectral emissivity features around 9.7 μm. The higher spatial and spectral resolution SEBASS data allowed six different alteration assemblages to be identified: quartz, alunite, pyrophyllite, feldspar, kaolinite, and montmorillonite and/or illite.
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Remote Sensing and Spectral Geology
Two recent papers, “Utility of high-altitude infrared spectral data in mineral exploration: Application to northern Patagonia Mountains, Arizona,” by Berger et al. (2003), and “Mapping hydrothermally altered rocks at Cuprite, Nevada, using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a new satellite-imaging system,” by Rowan et al. (2003), make a distinctive mark on the use of airborne and satellite hyperspectral imaging as an exploration tool.
These two papers deal with imaging of the Earth’s surface using the visible (0.4 μm) to near infrared (2.5 μm) part of the electromagnetic spectrum to map various mineral species. Depending on their structure and molecular bonding, minerals reflect and absorb the electromagnetic spectrum in unique ways. A large group of minerals have distinct electromagnetic signatures that make it possible to identify them from imaging systems that map the range of the electromagnetic spectrum between 0.5 and 2.5 μm.
These papers represent two distinct approaches. The first paper, by Berger et al., discusses the use of the AVIRIS (Airborne Visible Infrared Imaging Spectrometer) scanner, which provides high-resolution reflectance measurements in the spectral domain (224 channels between 0.4 and 2.45 μm) and variable spatial resolution (20 m), dependent on aircraft altitude. The second paper, by Rowan et al., discusses the use of the ASTER satellite scanner, which offers a limited range of spectra at three spatial resolutions (15, 30, and 90 m). ASTER measures reflectance radiation in 3 bands within the 0.52- to 0.86-μm range (visible-near-infrared) at 15-m spatial resolution, and 6 bands between 1.00 and 2.43 μm (short wave infrared) at 30-m spatial resolution. Emitted radiation is measured in 5 bands between 8.125 and 11.650 μm (thermal infrared) with a 90-m spatial resolution.
The main advantage of the AVIRIS sensor is the level of spectral detail, which provides accurate measurements of reflectance and absorption features of minerals that enables detailed mineral mapping. Its main disadvantages, however, are the extensive processing required to make the reflectance spectra useful, and its limited spatial coverage and acquisition cost based on programmed flights. In contrast, the main advantage of the ASTER sensor is that it measures key portions of the visible, near-infrared, and thermal infrared spectra of minerals for large-scale mapping projects, whereas its main disadvantage is that the data represent only portions of the electromagnetic spectrum and some minerals cannot be distinctively mapped. In addition, the lower spatial resolution in the near-and thermal infrared portions of the spectrum makes it more difficult to map at detailed scales.