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Resistivity characterization of the Arjuno-Welirang volcanic geothermal system (Indonesia) through 3-D Magnetotelluric inverse modeling

https://doi.org/10.1016/j.jseaes.2019.01.033Get rights and content

Highlight

  • The Arjuno-Welirang conceptual model was built on the basis of 3-D MT inversion data.

  • The geological-geochemical data indicates a volcanic geothermal system in this area.

  • The upflow zone is inferred below the Mt. Welirang & the outflow zones towards W & NW.

Abstract

The Arjuno-Welirang geothermal area is located in East Java, Indonesia. In the Arjuno-Welirang geothermal area, the Arjuno, Welirang and Anjasmoro volcanic complex is dominated by quaternary volcanic rocks and a complex geological structure. Several impressive surface thermal manifestations are found in the study area, including a high temperature solfataric-fumarole (about 137 °C) on the top of Mt. Welirang and also bicarbonate hot springs with a temperature range of 39–55 °C in the northern and western parts of the study area. The occurrences of the surface manifestations are probably controlled by the NW-SE and NNE-SSW fault structures. The occurrence of the high temperature solfataric-fumarole with high magmatic gas contents indicates that the Arjuno-Welirang geothermal area can be classified as a volcanic geothermal system. To investigate the subsurface condition of the geothermal system, Magnetotelluric (MT) survey was conducted within 34 MT stations distributed in the northern and western parts of the study area. Considering the complexity of the subsurface volcanic rocks and the geological structures of the study area, one and two-dimensional MT data inversion approaches could not be applied properly. Accordingly, a 3-D inversion of the MT data will be the most representative approach in such conditions in order to investigate the reservoir structure of the volcanic geothermal system. The 3-D inversion was then applied to all MT data with a frequency range of 100–0.01 Hz. Generally, the quality of the MT data varies from good to excellent. A clear subsurface resistivity structure revealed by the 3-D inversion showed a good correlation with geological and geochemical evidences. Overall, the resistivity structure consists of a conductive (1–10 ohm m) altered rock at the upper part overlying a reservoir zone with a slightly higher resistivity range (20–60 ohm m). The lower part of the resistivity structure showed the highest resistivity value (>80 ohm m), indicating a hot rock region. The main upflow is represented by an updome structure of the base of the conductive layer (BOC) beneath the Mt Welirang summit. It is supported by the occurrence of solfataric-fumaroles on the summit of Mt Welirang. Moreover, the outflow zones of the Arjuno-Welirang geothermal system are indicated by a thickening of the low resistivity layer toward the northern and western parts of Mt. Welirang supported with the occurrence of bicarbonate hotsprings. The existence of magmatic gas components (i.e. H2S and SO2) detected in the fumarole, gives an early warning regarding the possible existence of acidic fluid. Further exploration drilling should consider this issue. A conceptual model is then developed to visualize the geothermal system of the Arjuno-Welirang prospect area by integrating the geological and geochemical data.

Introduction

Indonesia is a country blessed with huge high temperature geothermal resources. Many volcanic geothermal systems characterized by the occurrence of high temperature fumaroles, magmatic gases such as H2S and SO2 as well as solfataras on the summit of active volcanoes can be found in this country. Arjuno-Welirang is one of the volcanic geothermal systems found in Java island. The Arjuno-Welirang geothermal area is located in the East Java modern volcanic arc and situated between Tulungagung and Sidomukti (NE-SW) Faults, which are parallel to the Central Java Fault (Fig. 1a and b). A high temperature solfataric-fumarole (about 137 °C) and bicarbonate hot springs (39 to 55 °C) are found as the surface manifestations of the geothermal system in this area. Several previous studies have been conducted to investigate the possible geothermal potential. Zaenudin et al. (2010) performed a combination of 2-D Magnetotelluric (MT) inversions and 3-D gravity inversion to delineate the reservoir structure, while Suharno et al. (2010) carried out the geological and geochemical analysis to characterize the reservoir condition of the Arjuno-Welirang area.

Furthermore, in order to get more understanding about the geothermal system in the Arjuno-Welirang, the Center for Geological Resources of the Ministry of Energy and Mineral Resources (MEMR) conducted MT data acquisitions over 34 stations in 2010. MT method was chosen because it is one of the reliable geophysical methods in geothermal exploration for discovering geothermal reservoirs. The improvement of MT technology by applying 3-D inversion modeling has been achieved to provide a more realistic subsurface resistivity model of the geothermal system in the Arjuno-Welirang area. As we know that the earth has a 3-D model, so the modeling using 2-D MT inversion approach is not sufficient to represent the real subsurface condition. Possible misleading in applying 2-D inversions caused by 3-D subsurface conditions has been studied by demonstrating synthetic data (Siripunvaraporn et al., 2005, Ledo, 2006) as well as real data (Simpson and Bahr, 2005, Newman et al., 2003). In addition, the modeling by using 3-D MT inversion has been well applied in various geothermal fields (Daud et al., 2017a, Daud et al., 2017b, Daud et al., 2015b, Fahmi et al., 2015, Han et al., 2008, Newman et al., 2008, Arnason et al., 2010, Cumming and Mackie, 2010).

The application of 3-D inversion of MT data in the Arjuno-Welirang geothermal area has been conducted to delineate the subsurface resistivity distribution associated with the existence of geothermal system (Daud et al., 2015a). In addition, to understand the more comprehensive characteristics of the Arjuno-Welirang geothermal system, the 3-D MT resistivity is then integrated with new interpretation of geological and geochemical data to construct the geothermal conceptual model. The conceptual model of the Arjuno-Welirang geothermal area includes reservoir geometry and its extension, clay alteration cap, subsurface geological structure and its correlated hidrogeology (up-flow and out-flow zone), fluid type, inferred reservoir temperature and possible permeable zones. All the results will be presented in the following paragraphs.

Section snippets

Regional geology

Tectonically, Arjuno-Welirang volcanic complex is a part of Java forearc. It is located between Tulungagung and Sidomukti (NE-SW) Faults, which are parallel to the Central Java Fault (Fig. 1b). The latest GPS observation shows that today the Java block is moving eastward and becoming northeastward on its east end, implicating an anti-clockwise rotation (Koulali et al., 2016). The margin between Java and Sumba block is assumed to be an active margin with a sinistral movement due to their

Geochemical survey

The Center for Geological Resources of the Ministry of Energy and Mineral Resources (MEMR) carried out a geochemical survey and reported the result of analysis of 5 hot spring samples, namely Padusan 1, Padusan 2, Coban, Cangar 1 and Cangar 2 (Hadi et al., 2010) (Fig. 3). Padusan hot springs are located on the northwestern part of Mt. Welirang with a temperature of 50–55 °C. Meanwhile, Coban and Cangar hot springs are located on the western part of the Arjuno-Welirang volcanic complex with the

MT data acquisitions

The first MT data acquisitions were carried out over the Arjuno-Welirang geothermal prospect area in 2010 by the Center for Geological Resources of the Ministry of Energy and Mineral Resources (MEMR) (Hadi et al., 2010). It was conducted at 34 MT stations using a grid arrangement with 1 to 2 km spacing. However, due to limited budget, the first MT data acquisitions did not cover all the Arjuno-Welirang geothermal prospect area. It was only distributed on the northern part and western part of

Resistivity Cross-Sections

The 3-D inversion results of MT data show the presence of the subsurface resistivity structure which is considered as an indication of the geothermal system in this area. Two vertical cross-sections of MT Line 1 and Line 2 are shown in Fig. 9, Fig. 10, respectively. The cross-section of Line 1 cuts across the Mt. Welirang and Padusan hotspring (NW-SE direction), while the cross-section of Line 2 extends from Mt. Welirang towards Coban hotspring (W-E direction). These two lines were selected to

Conclusions

The reliability of MT 3-D inversion to observe the concealed geothermal reservoir covered by volcanic complex has been applied in the Arjuno-Welirang geothermal prospect. A clear subsurface resistivity structure can be recognized to delineate the components of the geothermal system, such as conductive clay cap, reservoir zone, indication of heat source and geological structure which control the fluid flow.

Geologically, the existence of the heat source beneath the Arjuno-Welirang volcanic

Conflict of Interest

The authors declared that there is no conflict of interest.

Acknowledgements

The authors thank to Center for Geological Resources of the Ministry of Energy and Mineral Resources (MEMR) for providing the MT data as well as management of PT NewQuest Geotechnology for their supports in providing MT data processing software.

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