ASTER Brightness and Ratio Codes for Minerals: Application to Lithologic Mapping in the West-Central Powder River Basin, Wyoming¤
Published:January 01, 2009
Jason L. Perry, Robert K. Vincent, 2009. "ASTER Brightness and Ratio Codes for Minerals: Application to Lithologic Mapping in the West-Central Powder River Basin, Wyoming", Remote Sensing and Spectral Geology, Richard Bedell, Alvaro P. Crósta, Eric Grunsky
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The ASTER sensor, which has been aboard the TERRA satellite since late 1999, has 15 spectral bands that cover 14 different wavelength regions in the 0.52- to 11.65-μm range. Because there are only three primary colors that humans can directly observe simultaneously, displays of triplets of spectral parameters as red, green, and blue are commonly used to enhance the appearance of specific minerals of interest, where they are exposed on the ground. To aid this process, we have created 14 brightness codes (one for each different ASTER spectral band) and 46 spectral ratio codes (36 nonreciprocal spectral ratios of nine spectral bands between 0.52–2.43 μm and 10 nonreciprocal spectral ratios of five spectral bands between 8.125–11.65 μm), which divide the mineral library spectra into deciles. Each decile of each spectral band or ratio is labeled from 9 for the highest decile, down to 0 for the lowest decile. A triplet combination with codes of 9, 0, 0 can be displayed as red, blue, green (RGB), respectively, which makes the mineral of interest red in the resulting image, with few (usually well less than 10% of the minerals in the library) false positives. Examples of how spectral ratio codes may be applied are demonstrated using ASTER data of the west-central Powder River basin in Wyoming for the enhancement of hematite (in red beds), gypsum, quartz (in sandstone), and calcite (in limestone). Supervised classification derived from training sets identified on spectral ratio images selected on the basis of ratio codes of ASTER data produced a lithologic map of the study area in the west-central Powder River basin that had an accuracy of 63.3 percent, compared with field data. We conclude that the supervised classification provides a more accurate lithologic map than we could have produced by using a traditional geologic map from which to pick training sets.
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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.