Synergy of active and passive airborne thermal infrared systems for surface compositional mapping

Abstract

NASA thermal infrared multispectral scanner (TIMS) and Commonwealth Scientific Industrial Research Organisation mid‐infrared airborne CO2 laser spectrometer (MIRACO2LAS) data were acquired over the Mount Fitton area, South Australia, in order to evaluate their combined use for geological mapping and mineral exploration. TIMS is a passive, imaging system with six spectral bands in the thermal infrared wavelength region (8–12 μm), whereas MIRACO2LAS is an active, profiling system with ∼100 spectral bands in the 9–11 μm spectral range. The TIMS and CO2 laser data (emissivity variations for TIMS and apparent reflectance for MIRACO2LAS) were processed to enhance spectral information related to the surface composition. This spectral information was compared with existing geological maps and field emissivity spectra. Known geological units were well discriminated in the TIMS imagery, including a range of quartz‐rich and carbonate‐rich sedimentary units, as well as several previously unmapped areas of alteration in the carbonate rocks. However, the broadband spectral resolution of TIMS did not allow identification of discrete mineral constituents. In contrast, the high spectral resolution MIRACO2LAS data provided diagnostic spectral information about a range of minerals present including quartz, dolomite, talc, and tremolite, albeit, along discrete profiles. The widths of some of these diagnostic spectral features were less than 0.2 μm wide, which is half the resolution of the TIMS band passes (0.4 μm). The MIRACO2LAS spectra closely matched the shape, location, and depth of spectra of field samples measured by both a field emission spectrometer and a laboratory laser spectrometer. Pure mineral spectra measured by the (bidirectional) laboratory laser spectrometer also closely matched those measured by a conventional laboratory spectrometer measuring directional hemispherical reflectance. These results indicate that future remote thermal infrared systems designed for improved geological mapping and mineral exploration should incorporate both an imager for mapping lithological units and a high spectral resolution profiler for identifying dominant mineral constituents. Recently, a hyperspectral imaging thermal infrared instrument has been developed termed Spatially Enhanced Broadband Array Spectrograph System (SEBASS) [Hackwell and Warren, 1997]. This instrument should allow spectral identification of an entire scene [Gillespie et al., 1997], although the swath width is currently limited to 128 pixels as opposed to the 752 pixels of TIMS after panoramic correction.