Investigation of the main reservoir of the Tulehu geothermal system (Indonesia) using 3-D inversion of MT data.

• A new hypothesis of the Tulehu geothermal conceptual model is reconstructed by reducing the existing data gap from many previous studies, including additional MT data over Mt. Eriwakang and its surrounding. • Graben-like structures are identified based on geological remote sensing interpretation, which is then confirmed by field observation. • Thermoluminescence (TL) dating result indicates that Mt. Eriwakang volcanic product (413±6 kA), located in the middle of the graben structure, still has potential as a heat source. • Three-dimensional (3-D) inversion results from both existing and additional MT data strengthen the indication of the heat source possibility by showing an updome-shaped resistivity pattern beneath Mt. Eriwakang. • The other updome-shaped patterns can also be recognized at the Salahutu complex and beneath the fossil sinter cone, however, both patterns indicate possible geothermal activity in the past and recent past, respectively. Tulehu geothermal prospect area is located in Maluku Province, Indonesia. The existence of the geothermal system in this area is recognized by the appearance of hot springs on the western side of Mt Eriwakang and the northern side of Mt. Huwe. Most of these hot springs occur along inferred geological structures. The geological conditions of this area are dominated by Quaternary volcanic rocks, limestone, alluvium, and metamorphic rocks. Several authors have conducted several studies to propose various geothermal conceptual models, from structurally controlled to volcanic-hosted geothermal systems. However, the geoscientific data supporting the hypotheses are not comprehensive yet. There are still gaps in the data coverage as well as in the interpretation of geological, geochemical, and geophysical data, so a comprehensive study should be done before deciding on further stages. Therefore, this paper proposes a new conceptual model of the Tulehu geothermal area by reducing the existing data gap. Interpretation of remote sensing data, geological field observations, age dating analysis, reanalysis of geochemical data, and additional MT data covering Mt. Eriwakang are conducted to reconstruct the conceptual model. Graben-like structures are identified based on the geological data where the product of Mt. Eriwakang is in the middle. Based on the age dating data, the product of Mt. Eriwakang erupted in the Quaternary time (413±6 kA), so it is still quite potential as a heat source. This indication is also supported by the trend of Cl, HCO 3 , and SiO 2 , which describe the possible flow from Mt. Eriwakang to the surrounding hot springs. Based on the Na-K-Mg and Silica geothermometers, the reservoir temperature is estimated at 210-240 °C. A three-dimensional (3-D) MT inversion was applied to all the existing and new MT data to obtain a 3-D subsurface resistivity model. The oceanic bathymetry data was also incorporated to mitigate the possible coastal effect from the surrounding seawater to the MT data. The new 3-D MT inversion results show a good agreement with the geological and geochemical indications. The updome-shaped pattern of the subsurface resistivity distribution beneath Mt. Eriwakang supports the geological and geochemical data analysis results concerning the indications of the prospects area around Mt. Eriwakang. These interpretations can strengthen the recommendation to focus on the area around Mt. Eriwakang for further development. [Display omitted] [ABSTRACT FROM AUTHOR]