Mapping surface mineralogy and scattering behavior using backscattered reflectance from a hyperspectral midinfrared airborne CO 2 laser system (MIRACO2LAS)

Airborne, high-spectral resolution, thermal-infrared (TIR) MIRACO ₂LAS reflectance data were evaluated for mapping surface mineralogy and scattering behavior for a variety of semi-arid, geological test sites in Australia. MIRACO₂LAS is a rapidly tuned, airborne CO₂ laser system that measures backscattered (bidirectional) reflectance at 100 wavelengths between 9.1 and 11.2 μm for 2-m footprints in line profile mode. An operational methodology is described that permits reduction of the raw airborne signal-to-ground reflectance. This ground reflectance has two major properties, namely, wavelength-dependent mineralogical variations and reflection albedo variations related to surface roughness. Comparisons between the airborne MIRACO₂LAS spectra and laboratory directional hemispherical reflectance (DHR) spectra show the same spectral shapes, though differences in average reflectance (albedo) occur for some types of rocks. The minerals identified using MIRACO₂LAS include silicates (for example, quartz, microcline, plagiodase, almandine, spessartine, talc, tremolite, and kaolinite) and carbonates (dolomite and magnesite) as well as vegetation (dry and green). Many of the diagnostic spectral features that allow identification of these materials are narrow (<0.2 μm), making them difficult to detect with broadband TIR systems, like the airborne TIMS and satelliteborne ASTER. Based on an empirical relationship between the minimum and maximum reflectance established using laboratory DHR spectra, a method is proposed that allows the use of MIRACO₂ LAS data to identify surfaces that are characterized by Lambertian or specular scattering. The MIRACO₂LAS results show that Lambertian-type scatterers include soils and many types of isotropic rocks