Integration of concentration-area fractal model and relative absorption band depth method for mapping hydrothermal alterations using ASTER data

Abstract

Those image processing methods which use both spatial and spectral properties of remotely sensed images often increase accuracy of image classification. In this study, the concentration-area (C-A) fractal model and Relative absorption Band Depth (RBD) were applied to map hydrothermal alteration zones using ASTER data covering the northern part of Rabor area, Iran. Based on the field observations and spectral analysis of rock samples, three types of RBD were calculated and applied to map hydrothermal altered rocks (RBD6 for phyllic, RBD8 for propylitic, and RBD5 for argillic). A log-log plot of area-digital number value for RBD images showed that there are three populations in each image. Representation of these populations revealed that the third population of each RBD has enhanced separation of alteration zones. To verify the results, a spectral library of ground rock samples was built and used to compare with 10 spectra derived from each image. Spectral Angle Mapping, Spectral Feature Fitting, and Binary Encoding were applied to assess the matching of image spectra with the spectral library spectra. Results confirmed that third population of RBD6 and RBD8 contain pure pixels of phyllic and propylitic alterations, respectively whereas there are some interference between argillic and phyllic alterations in RBD5.