SYNERGY OF ACTIVE AND PASSIVE AIRBORNE THERMAL INFRARED SYSTEMS FOR SURFACE COMPOSITIONAL MAPPING

NASA thermal infrared multispectral scanner (TIMS) and Commonwealth Sc ientific Industrial Research Organisation mid-infrared airborne CO2 la ser spectrometer (MIRACO(2)LAS) data were acquired over the Mount Fitt on 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 reg ion (8-12 mu m), whereas MIRACO(2)LAS is an active, profiling system w ith similar to 100 spectral bands in the 9-11 mu m spectral range. The TIMS and CO2 laser data (emissivity variations for TIMS and apparent reflectance for MIRACO(2)LAS) were processed to enhance spectral infor mation related to the surface composition. This spectral information w as 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 car bonate rocks. However, the broadband spectral resolution of TIMS did n ot allow identification of discrete mineral constituents. In contrast, the high spectral resolution MIRACO(2)LAS data provided diagnostic sp ectral information about a range of minerals present including quartz, dolomite, talc, and tremolite, albeit, along discrete profiles. The w idths of some of these diagnostic spectral features were less than 0.2 mu m wide, which is half the resolution of the TIMS band passes (0.4 mu m). The MIRACO(2)LAS spectra closely matched the shape, location, a nd depth of spectra of field samples measured by both a field emission spectrometer and a laboratory laser spectrometer. Pure mineral spectr a measured by the (bidirectional) laboratory laser spectrometer also c losely matched those measured by a conventional laboratory spectromete r measuring directional hemispherical reflectance. These results indic ate 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 p rofiler for identifying dominant mineral constituents. Recently, a hyp erspectral imaging thermal infrared instrument has been developed term ed Spatially Enhanced Broadband Array Spectrograph System (SEBASS) [Ha ckwell and Warren, 1997]. This instrument should allow spectral identi fication of an entire scene [Gillespie et al., 1997], although the swa th width is currently limited to 128 pixels as opposed to the 752 pixe ls of TIMS after panoramic correction.