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Vertical electrical sounding for revealing the groundwater resources in the geothermal spring of Jaboi volcano

Yanis Muhammad, Zaini Nasrullah, Abdullah Faisal, Isa Muhammad, Marwan Marwan, Idris Syafrizal, Paembonan Andri Yadi, Ananda Riski, Zainal Muzakir, Ghani Azman Abdul, Saputra Deni

Acta Geophysica

2024

Vol. 72, no. 3

1617--1635

Geothermal energy is considered renewable energy that is environmentally friendly and sustainable compared to the conventional energy from fossil fuels. However, uncontrolled geothermal exploitation can cause a decrease in the groundwater table and reservoir temperature, such as in the Jaboi volcano on Weh Island, where a power plant has been built to generate electrical energy with an estimated power of 50 MWe but still has not been operated. A geophysical survey is needed to determine the local hydrothermal system, including groundwater reserves under the surface which can be used to refill the hydrothermal wells during exploitation. This study measured vertical electrical sounding (VES) data at 15 points near the crater and geothermal power plant. In addition, very-low-frequency (VLF) data that pass through the crater were also collected to determine the presence of other hydrothermal resources such as fractures and faults. The results of the 1D least-square inversion show three subsurface models where groundwater resources with low resistivity (< 1.5 Qm) are found at a depth of 50-100 m. The same results are also obtained from the 2D cross-section model that impermeable resistive anomalies in alluvial and tuff rocks dominate the near-surface area. The layer after groundwater is an impermeable rock in the form of breccia. The results of 2D VES and VLF modeling also show the presence of the Ceunohot and Leumomate faults, which are beneficial as fluid access to the surface. Based on the data analysis, the combination of VES and VLF data can be used to image shallow hydrothermal systems such as groundwater resources and faults in the Jaboi volcano.

A pilot survey for mapping the fault structure around the Geuredong volcano by using high-resolution global gravity

Yanis Muhammad, Marwan, Idroes Rinaldi, Zaini Nasrullah, Paembonan Andri Yadi, Ananda Riski, Ghani Azman Abdul

Acta Geophysica

2022

Vol. 70, no 5

2057--2075

Global gravity has been used successfully for studying the tectonic mechanism around the world, and the global data can be accessed freely with different resolutions. Global Gravity Model Plus (GGM+) has the highest resolution of 200 m/px. The high-resolution data apply to regional studies and utilize more local areas, such as studying fault structures in volcanic areas, the main access point for seeping fluids to the surface. We used GGM +data to assess the geological structure and faults of the Geuredong volcano located in the central of Aceh, Indonesia. The volcanic mountain is estimated to have an energy of 55 Mwe. We validated the GGM+data with TOPEX/Poseidon data with the lower resolution of 1.83 km/px and showed the same response. The GGM+data with higher resolution describe the regional fault system in Geuredong volcano. Moreover, the global gravity data were also validated by ground survey data measured over 20 km2 with a distance between 250 and 300 m, the validation results between GGM+and ground gravity survey show the same response between the two gravity data with an RMS error of 15.07%. The residual anomaly from GGM+can map several local and regional faults in the Geuredong volcano area. The horizontal transformation specifies fractures and strike-slip faults, the critical mechanisms for the formation of surface manifestations. The 3D modeling with Occam’s algorithm and Singular Value Decomposition (SVD) shows the fault is at a 1–2 km depth indicated by a low density of 2.4 g/cm3 . The cross section results from the inversion model also show the same response as the Geuredong geothermal conceptual model. Based on data processing, the GGM+data can study the fault structure in volcanic areas, especially in high terrain conditions such as tropical countries with inaccessible mobility and developing countries that are financially limited to gravity ground surveys.