Comparison of Band Ratioing and Spectral Indices Methods for Detecting Alunite and Kaolinite Minerals Using Aster Data in Biga Region, Turkey

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) covers a wide spectral region with 14 bands from visible to the thermal infrared. The different band ranges can be capable of discriminating and mapping surface rocks and predicting pathfinder minerals (i.e. alunite, kaolinite etc.) on alteration assemblages of potential targets. The aim of this study is to compare two different discrimination techniques as band ratioing (BR) and spectral indices (SI) applied on Biga – Çanakkale Area, Turkey. The ground truths were held on 19 locations to compare obtained results. The study area covers altered Miocene age acidic volcanic rocks. Alunite and kaolinite are main minerals in hydrothermal alteration zones. During the image processing, in order to reduce atmospheric effect on ASTER bands, digital numbers of each image channels are converted to relative reflectance for BR and SI techniques. In BR method, two ratios (4/5 and 7/6) were applied to discriminate alunite and kaolinite respectively. However, in SI method, 5 SWIR bands (5, 6, 7, 8, and 9) were taken into account to detect for alunite and kaolinite presence for each mineral. 19 hydrothermal sites were tested by both methods and 13 of them were identified as alunite by using SI method. On the other hand, in BR method, kaolinite and alunite were discriminated in 11 sites and 8 sites, respectively. As a result, SI and BR techniques can detect alunite and kaolinite minerals. SI method gives better results for detecting alunite than that of kaolinite. On the contrary, BR method can be more applicable to discriminate kaolinite than alunite.