Mapping Evaporitic Minerals in Sud Lipez Salt Lakes, Bolivia, Using Remote Sensing
Published:January 01, 2009
Fernando Caceres, Hamid Ali-Ammar, Eric Pirard, 2009. "Mapping Evaporitic Minerals in Sud Lipez Salt Lakes, Bolivia, Using Remote Sensing", Remote Sensing and Spectral Geology, Richard Bedell, Alvaro P. Crósta, Eric Grunsky
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This work focuses on the analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images collected over salt lakes occurring in the Sud Lipez region (southwestern Bolivia). Thanks to its extreme aridity, lack of vegetation cover, high latitude (>4,000 m) and wide variety of brine geochemistry, this region proves to be an ideal place to demonstrate the capabilities of ASTER visible, near, and short-waved infrared bands in mapping mineral occurrences. ASTER images obtained over two major salt lakes, namely Capina and Pastos Grandes, have been corrected and properly calibrated using field spectrometry. Their further processing using pixel purity indices reveals end-member spectra that closely match those of mineral species—such as gypsum, calcite, and ulexite—known to occur in this context. The classification of the multispectral scenes on the basis of these selected end members reveals that a detailed mineral mapping in such evaporitic environments can be achieved with high confidence and good spatial accuracy. When compared to the very schematic geologic maps recently made available from field work, the remote sensing data indicate a very good correlation while revealing instructive patterns of facies variations that can serve as a basis for further field exploration work. The potential of the Bolivian section of the Andes cordillera for borate deposits is high but still poorly explored. Borate concentrations often form discontinuous pods and lenses within sediments but can locally evolve into continuous decimetric strata. Though most of the time such deposits are covered by a burden of sedimentary and evaporitic materials, it is believed that multispectral remote sensing can be extremely useful in spotting limited outcrops of boron minerals.
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Remote Sensing and Spectral Geology
Two recent papers, “Utility of high-altitude infrared spectral data in mineral exploration: Application to northern Patagonia Mountains, Arizona,” by Berger et al. (2003), and “Mapping hydrothermally altered rocks at Cuprite, Nevada, using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a new satellite-imaging system,” by Rowan et al. (2003), make a distinctive mark on the use of airborne and satellite hyperspectral imaging as an exploration tool.
These two papers deal with imaging of the Earth’s surface using the visible (0.4 μm) to near infrared (2.5 μm) part of the electromagnetic spectrum to map various mineral species. Depending on their structure and molecular bonding, minerals reflect and absorb the electromagnetic spectrum in unique ways. A large group of minerals have distinct electromagnetic signatures that make it possible to identify them from imaging systems that map the range of the electromagnetic spectrum between 0.5 and 2.5 μm.
These papers represent two distinct approaches. The first paper, by Berger et al., discusses the use of the AVIRIS (Airborne Visible Infrared Imaging Spectrometer) scanner, which provides high-resolution reflectance measurements in the spectral domain (224 channels between 0.4 and 2.45 μm) and variable spatial resolution (20 m), dependent on aircraft altitude. The second paper, by Rowan et al., discusses the use of the ASTER satellite scanner, which offers a limited range of spectra at three spatial resolutions (15, 30, and 90 m). ASTER measures reflectance radiation in 3 bands within the 0.52- to 0.86-μm range (visible-near-infrared) at 15-m spatial resolution, and 6 bands between 1.00 and 2.43 μm (short wave infrared) at 30-m spatial resolution. Emitted radiation is measured in 5 bands between 8.125 and 11.650 μm (thermal infrared) with a 90-m spatial resolution.
The main advantage of the AVIRIS sensor is the level of spectral detail, which provides accurate measurements of reflectance and absorption features of minerals that enables detailed mineral mapping. Its main disadvantages, however, are the extensive processing required to make the reflectance spectra useful, and its limited spatial coverage and acquisition cost based on programmed flights. In contrast, the main advantage of the ASTER sensor is that it measures key portions of the visible, near-infrared, and thermal infrared spectra of minerals for large-scale mapping projects, whereas its main disadvantage is that the data represent only portions of the electromagnetic spectrum and some minerals cannot be distinctively mapped. In addition, the lower spatial resolution in the near-and thermal infrared portions of the spectrum makes it more difficult to map at detailed scales.