Mapping evaporate minerals by ASTER

Evaporate minerals are important industrial raw materials that have been used in diverse industries for many years. As one of the most extensively used evaporate minerals, gypsum is an important raw material in the construction, agriculture, textile, dentistry and chemical industries, resulting in a massive increase in demand of these minerals in recent years. The aim of this study was to demonstrate the responses of common remote sensing mapping techniques and further develop some of them while evaluating their success in well-known gypsum outcrops using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery. The region selected for the test area was Ankara Bala, which has gypsum outcrops with operational mines mapped in detailed studies by the General Directorate of Mineral Research and Exploration (MTA). The methods of band ratioing (BR), decorrelation stretch (DS), feature-oriented principal component analysis (FOPCA) and thermal indices were tested to map the mineral gypsum. For the BR method, the ratio 4: 9, for DS the Red-Green-Blue (RGB) composite 1-4-8, for FOPCA Principal Component (PC) 3 and for thermal infrared (TIR) indices the previously known Quartz Index (QI) modified as the Sulfate Index (SI) were found to be successful in general terms for evaporate mapping. For an absolute accuracy assessment the results of these methods were checked in the field and, from the areas where the results showed common anomalies, samples were taken for field spectrometry analyses and X-ray analyses. For a relative accuracy assessment all of the results were compared with each other to evaluate the differences and their successes. We found that all of the methods were successful in mapping evaporates; however, despite its lower spatial resolution, the TIR data from ASTER when used as the SI yielded a more refined result than the other methods