Identification of Alteration Zones Based on Satellite Images and Vegetation Response for Geothermal Exploration in Volcanic Fields

Abstract Several studies have also shown that direct evidence of geothermal manifestation presented by surface features, such as hot springs, steam vents, and fumaroles is often detected by remote sensing method. In this context, the appropriate remote sensing method can support the early exploration stages to assess wide coverage areas rapidly. Therefore, this study aims to characterize geothermal features based on a combination of remote sensing method and field measurements at Mts. Tangkuban Parahu, Kamojang-Guntur (West Java), and Ijen (East Java). Data were obtained from Sentinel-1A and 2B images, DEMNAS, and chlorophyll content of ferns. The selected remote sensing method was modified Segment Tracing Algorithm (mSTA), band ratio, and red edge vegetation index (REVI). mSTA was used to estimate fracture permeability, which served as hydrothermal fluid paths to the surface and changed rock minerals into soil affecting plant physiological conditions. Band ratio and REVI were used to identify alteration zones and vegetation stress, respectively. The results showed that the stressed vegetation due to soil contamination of hydrothermal fluid could serve as an indirect indicator of geothermal system. The results revealed that lineament density at selected study areas was due to the low chlorophyll content and high ground temperatures. Based on Sentinel-2B images, alteration zones were identified by R, G, and B color composite for bands ratio 4/2, 11/12, and 8, with pinkish hues. After the images enhancement process, color indicating alteration became more prominent than the original composite. The measured chlorophyll contents of the ferns confirmed that the vegetation above alteration zones was under stressed conditions, as indicated by chlorophyll content of less than 25 SPAD.