CALIBRATION AND REGIONAL MAPPING OF ARGILLIC AND PHYLLIC ALTERATION USING ADVANCED SPACEBORNE THERMAL EMISSION AND REFLECTION RADIOMETER (ASTER) DATA, HYPERION DATA, AND ENVIRONMENT FOR VISUALIZING IMAGES (ENVI) LOGICAL OPERATOR ALGORITHMS

A method for regional mapping of phyllic and argillic hydrothermal alteration using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data has been developed. ASTER measures reflected radiation in 3 bands between 0.52 and 0.86 μm and in 6 bands from 1.6 to 2.43 μm with 15-m, and 30-m resolution, respectively. Environment for Visualizing Images (ENVI) logical operators are used to perform multiple band ratios and thresholds that can be applied to a scene in one algorithm, thus eliminating separate production and application of vegetation and dark pixel masks. Argillic and phyllic logical operator algorithms mask green vegetation using a band 3/2 ratio that defines the 0.66 μm chlorophyll absorption feature, thresholds band 4 to eliminate pixels with low albedo that contain abnormally high band 5 and band 9 values, and maps argillic and phyllic alteration using band ratios 4/5, 4/6, 5/6, and 6/7 that delineate the Al-OH absorption features at 2.17 μm and 2.20 μm. A byte image is produced for each alteration type and converted to vector format for use in other datasets.

Accurate calibration of ASTER data is critical because ASTER ratio values for each logical operator were determined from library and image spectra that have been calibrated using in situ reflectance data that define Al-OH absorption features in the 2.17 to 2.20 μm region. ASTER band 5 reflectance values at 2.17 μm are typically 5 to 12 percent lower than reflectance data measured in the field. Hyperion satellite data are used to correct the band 5 anomaly. Hyperion data, which have 196 spectral bands in the 0.45 to 2.4 μm region, are converted to reflectance, resampled to ASTER bandpasses, and used to calibrate ASTER band 5 using difference values from band 5/6 ratios of ASTER and Hyperion-convolved ASTER data.

The calibration and mapping method has been tested at Cuprite, Nevada using an ASTER simulated Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) dataset and ASTER AST07 reflectance dataset. Of total alteration mapped, 95 percent of the argillic alteration and 72 percent of muscovite-bearing phyllitic schist mapped in the AST07 dataset was mapped in the AVIRIS-convolved ASTER dataset. This mapping method has been applied to a 62-scene mosaic in the High Zagros Mountains, Iran and a 30-scene mosaic in eastern Afghanistan.