Characterization and initial field test of an 8–14 μm thermal infrared hyperspectral imager for measuring SO2 in volcanic plumes

• Published in: Bulletin of volcanology Vol. 78; no. 10
• Main Authors: Gabrieli, Andrea, Wright, Robert, Lucey, Paul G., Porter, John N., Garbeil, Harold, Pilger, Eric, Wood, Mark
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2016
• Subjects: Earth and Environmental Science; Earth Sciences; Geology; Geophysics/Geodesy; Mineralogy; Research Article; Sedimentology; Imaging interferometer; Thermal InfraRed (TIR); SO; Volcanic gases; Hyperspectral
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-016-1068-6

The ability to image and quantify SO 2 path-concentrations in volcanic plumes, either by day or by night, is beneficial to volcanologists. Gas emission rates are affected by the chemical equilibria in rising magmas and a better understanding of this relationship would be useful for short-term eruption prediction. A newly developed remote sensing long-wave thermal InfraRed (IR) imaging hyperspectral sensor – the Thermal Hyperspectral Imager (THI) – was built and tested. The system employs a Sagnac interferometer and an uncooled microbolometer in rapid scanning configuration to collect hyperspectral images of volcanic plumes. Each pixel in the resulting image yields a spectrum with 50 samples between 8 and 14 μm. Images are spectrally and radiometrically calibrated using an IR source with a narrow band filter and two blackbodies. In this paper, the sensitivity of the instrument for the purpose of quantifying SO 2 using well constrained laboratory experiments is evaluated, and initial field results from Kīlauea volcano, Hawai’i, are presented. The sensitivity of THI was determined using gas cells filled with known concentrations of SO 2 and using NIST-traceable blackbodies to simulate a range of realistic background conditions. Measurements made by THI were then benchmarked against a high spectral resolution off-the-shelf Michelson FTIR instrument. Theoretical thermal IR spectral radiances were computed with MODTRAN5 for the same optical conditions, to evaluate how well the (known) concentration of SO 2 in the gas cells could be retrieved from the resulting THI spectra. Finally, THI was recently field-tested at Kīlauea to evaluate its ability to image the concentration of SO 2 in a real volcanic plume. A path-concentration of 7150 ppm m was retrieved from measurements made near the Halema’uma’u vent.