Development of hyperspectral thermal remote sensing for geologic applications

The utility of multichannel thermal infrared imaging techniques was first demonstrated for mapping geologic units in the late 1970’s by Anne Kahle and Larry Rowan. They analyzed six thermal channels of Daedalus 24-channel scanner data over the East Tintic Mining District in Utah. The thermal channels were found to measure broad spectral variations in the emissivity of igneous rock types. Analysis of spectral emissivity allowed the gross composition Tintic rocks, and their associated silicic alteration, to be detected in areas not well discriminated by reflectance data. NASA funded the development of the Thermal Infrared Multispectral Scanner (TIMS) in 1981. NASA funded experiments with TIMS over the next two decades showed that gross variations in silicate mineralogy could be mapped in igneous, sedimentary and metamorphic rock terrains, including products of hydrothermal alteration, especially silicic alteration.
In 1993, John Hackwell of The Aerospace Corporation began development of the Spatially Enhanced Broadband Array Spectrograph System (SEBASS). SEBASS made its first flight in 1995 for an Aerospace-sponsored research program. Beginning in 1998, The Aerospace Corporation was encouraged to make SEBASS data available for civilian use by geoscientists. In 1999, SEBASS data were acquired over a number of sites in the Western United States. From 2001 to 2004 SEBASS data were not made available. However, beginning in 2005 SEBASS data were again acquired for the Arthur Brant Laboratory for Exploration Geophysics over sites in Nevada and California. The SEBASS sensor is a major technological breakthrough because instead of optical-mechanical scanning, the sensor uses two solid-state detector arrays cooled to 10 Kelvin by liquid helium.
SEBASS collects data in 256 spectral channels using two separate focal planes for the MWIR (3.0 to 5.5μm) and LWIR (7.8 to 13.5μm). The total field of view is 7.3 degrees and the instantaneous field of view is 1.1 milliradian. SEBASS has been flown in a Twin-Otter aircraft normally at an altitude above terrain that will give 2-meter ground instantaneous field of view. Data are provided in spectrally and radiometrically calibrated hyperspectral data cubes for analysis in Environment for Visualizing Images (ENVI) format and it is released to the mineral exploration community on a controlled basis. Analysis of SEBASS data coupled with mapping and sampling revealed hydrothermal alteration minerals commonly associated with base and precious metal deposits, as well as some newly identified minerals such as hydrous sulfate crusts around fumaroles at Steamboat Springs and acid-sulfate weathering minerals (e.g., jarosite) forming on mine dumps in Virginia City.
Identification of these alteration minerals is important for characterizing mineral occurrences, particularly with the presence of indicator minerals like alunite, pyrophyllite, kaolinite, illite/sericite, and chlorite.