Remote Measurements of Volcanic Gases Using Thermal Infrared Hyperspectral Imaging

Abstract

Remote measurements of sulfur dioxide (SO2) emission rate (mass flux), emitted by active volcanoes are important to predict volcanic behavior and for public health. However, an accurate method to carry out these measurements during both day and nighttime has not been established yet. Many volcanic gases of interest have distinctive spectral features in the long wave Thermal InfraRed (8-14 μm), which have potential to be detected by imaging interferometry. The aims of this dissertation project are threefold. Firstly, a newly developed Thermal Hyperspectral Imager (THI) was tested for SO2 gas measurement capabilities. The system employs a Sagnac interferometer and an uncooled microbolometer in rapid scanning configuration to collect hyperspectral images. Each pixel in the resulting image yields a spectrum with 50 samples between 8 μm and 14 μm. It was found that imaging interferometry can detect SO2 spectral features at high spatial, spectral and temporal resolution with high signal to noise. Secondly, the dissertation focused on how to convert measured SO2 gas radiance into SO2 path-concentrations, needed to derive flux information. It was found that existing techniques for SO2 gas path-concentrations retrievals, which are based on comparing spectral radiance for portions of the image that correspond to the plume and for adjacent regions of the image that do not (i.e., “clear sky”) to isolate the contribution that the plume makes to the measured radiance from the background, do not work if the plume is imaged against a cloudy background with strong heterogeneities. Therefore, a new method was developed. The new algorithm is based on Partial Least Squares Regression techniques to allow for single point SO2 path-concentration retrievals. It provided good results under both clear and cloudy sky background conditions, but only under low atmospheric water vapor column amounts. Finally, temporal trends of SO2 fluxes were evaluated during day and nighttime at the summit of Kīlauea volcano under both clear and cloudy sky conditions. The new camera and techniques described in this dissertation provide an effective tool for monitoring SO2 fluxes remotely under a variety of background conditions (clear and cloudy sky, cold and hot backgrounds).