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

Wina Eka Rahmidiani (1,a), Asep Saepuloh (1,b), Estu Kriswati (2), Ridwan Rahmanto (1)

Identification of Alteration Zones Based On Satellite Images and Vegetation Response for Geothermal Exploration in Volcanic Fields
Wina Eka Rahmidiani (1,a), Asep Saepuloh (1,b), Estu Kriswati (2), Ridwan Rahmanto (1)

1) Geological Engineering, Faculty of Earth Sciences and Technology, Bandung Institute of Technology
Jalan Ganesha 10, Bandung 40132, Indonesia
*saepuloh[at]itb.ac.id
2) Research Center for Geological Disaster, National Research and Innovation Agency (BRIN)
Jalan Sangkuriang, Bandung 40135, Indonesia

Abstract

In accordance with Government Regulation No. 79 of 2014 concerning National Energy Policy, the utilization of geothermal energy is targeted up to 23% or about 7,242 MW in 2025. There are at least 62 geothermal fields with potential for exploitation on the Java Island. To fulfil this governmental target, a comprehensive preliminary survey with effective and efficient methods is necessary. Thus, remote sensing can serve as an alternative solution to support preliminary surveys. The emergence of geothermal manifestations, whether in the form of fluid or gas, can induce vegetation to be stressed or death. Direct evidence of geothermal manifestation presented by surface features such as hot springs, steam vents, and fumaroles could be detected by remote sensing methods. The appropriate remote sensing methods can support the early exploration stages to assess wide coverage area rapidly. Therefore, this study is aimed to characterize geothermal features based on a combination of remote sensing methods and field measurements at Mts. Ijen (East Java), Tangkuban Parahu (West Java), and Kamojang-Guntur (West Java). Data used in this study were Sentinel-1A and 2B images, DEMNAS, and chlorophyll content of ferns. The selected remote sensing methods are the modified Segment Tracing Algorithm (mSTA), band ratio, and red edge vegetation index (REVI). The mSTA was used to estimate fractures permeability which serve as hydrothermal fluid paths to the surface and change rock minerals into soil affecting plant physiological conditions. The band ratio and REVI methods were used to identify the alteration zone and vegetation stress, respectively. The stressed vegetation due to soil contamination of hydrothermal fluid can serve as an indirect indicator of geothermal system. The obtained results reveal that lineament density at selected study areas concordance to the chlorophyll content and ground temperatures. Based on Sentinel-2B image, the alteration zones were identified by R,G,B color composite for bands 4/2, 6/7, 5 as orange-hues and bands 4/2, 6/7, 10 as pink-hues. Following image enhancement process, colour indicating alteration became more prominent than original composite. Based on the REVI, alteration zones were identified by index less than 0,25. The measured chlorophyll contents of the ferns confirmed that the vegetation above alteration zones were under stressed condition presented by chlorophyll content less than 25 SPAD.

Keywords: Remote sensing- Sentinel-1A- DEMNAS- Sentinel-2B- Geothermal manifestation- Vegetation stress- Ijen- Tangkuban Parahu- Kamojang

Topic: Mineralogy, petrology, and volcanology