Assessment of new spectral indices and multi-seasonal ASTER data for gypsum mapping

Abstract

Advanced spaceborne thermal emission and reflection radiometer (ASTER) data and different spectral indices were employed to map gypsum minerals. However, most proposed gypsum indices are designed based on the 2.21 μm gypsum absorption, overlapping with most hydroxy-bearing minerals. Moreover, the seasonal mutual transformation between gypsum, bassanite, and anhydrite may lead to seasonal reflectance variability of gypsum formation pixels, affecting the classification accuracy of gypsum indices. In this research, the feasibility of 2.26 μm (ASTER band7) reflectance absorption for gypsum mapping was assessed, using lab and ASTER reflectance. On the basis of this, two new ASTER gypsum spectral indices (GI1: B4*B8/B6*B7; GI2: B4*B8/B7*B7) were proposed and applied to exclude the interference of hydroxyl-bearing minerals effectively. Seasonal reflectance variability of gypsum formation pixels was confirmed, and it causes the accuracy difference of gypsum indices for multi-seasonal ASTER data. The GI1 achieves the most robust accuracy for multi-seasonal ASTER data with average areas under receiver operator characteristic (ROC) curve of 98.5% and 98.7% for summer and winter ASTER data. Therefore, the GI1 can be used for gypsum mineral mapping, especially in the areas where clay minerals and other hydroxyl-containing minerals are widely distributed.