ADVANCES IN LITHOLOGIC MAPPING BY USING OPTICAL REMOTE SENSING MEASUREMENTS

Advances in optical remote sensing provide geoscientists with improved methods for identifying and mapping lithologies. The level of lithologic definition depends mainly on the wavelength range and spectral resolution of the instrument. Hyperspectral imaging systems typically record reflected solar energy in more than 100 narrow bands in the 0.4 to 2.5 micrometer wavelength region, permitting identification of specific minerals which exhibit absorption features within this wavelength region. Airborne hyperspectral systems have been collecting high signal-to-noise data for several years, and NASA EO-1 Hyperion results have demonstrated that this level of mineralogic determination is feasible from orbital platforms. The Advanced Spaceborne Thermal Emission and Reflection (ASTER) radiometer is an especially noteworthy advance in geological remote sensing, because it records emitted, as well as reflected, energy with spectral resolution that is adequate for mapping several important lithologic categories throughout the 60 km x 60 km scenes. ASTER records energy in 3 bands in the 0.55 to 0.8 micrometer wavelength region (VNIR) with 15-m resolution, including stereoscopic images; 6 bands in the 1.65 to 2.4 micrometer region (SWIR) with 30-m resolution; and 5 bands of emitted energy in the 8.1 to 11.7 micrometer region (TIR) with 90-m resolution. Spectral-mineral groups which can be mapped using ASTER data include carbonates, Al-OH, Fe,Mg-OH, H-O-H and Fe-oxide groups. Some important distinctions being made within these groups are challenging, even at the outcrop, such as limestone (calcite) versus dolomite, and alunite-kaolinite versus illite-smectite. Analysis of the 5-band spectral emittance data permits additional lithologic distinctions, such as siliceous, intermediate, and mafic rocks, and estimates of silica content appear to be feasible. ASTER stereoscopic images are sources of geomorphological information and digital elevation models (DEM). Surface reflectance and emittance data, as well as DEMs, are presently available for geoscience research at no cost, and image analysis software that is well-suited for geologic analysis is available.