magnetotellurics; geothermal exploration; electrical resistivity; Sumatra; Indonesia
The Sipoholon geothermal system is located in the vicinity the Tarutung basin, which is part of the Sumatra fault system, Indonesia. In the Sipoholon area, 18 hot springs indicate interconnected (hot saline) fluids in the subsurface. The hot saline fluids in the rock pore space increase the bulk electrical conductivity. Hence, an electrical conductivity model can be analysed based on the dependence of resistivity on porosity, fluid conductivity and temperature.
Here, I present a magnetotelluric (MT) study which provides the electrical conductivity distribution at depth of the Sipoholon geothermal system. The main objective of this study is to develop a conceptual model for the Sipoholon geothermal system based on the conductivity distribution at depth. Together with additional geological, geochemical, and seismological studies, the MT results infer a comprehensive understanding about the Sipoholon geothermal system, particularly about the reservoir and the heat source.
The MT data reveal a deep reaching conductive anomaly (< 10 Ωm) approximately 6 km to the east of the Tarutung basin. This conductor is associated spatially with the Panabungan normal fault zone and the Panabungan hot spring. At shallow depths, i.e. from surface to 3 km depth, the conductivity anomaly could be caused by ascending magmatic and/or meteoric water from a deeper zone. At depths of 1 km until 2 km, the vertical conductor shows a resistivity of 2 Ωm, which is most likely the location of a possible reservoir.
Below the reservoir, the deep reaching conductive anomaly is interpreted as circulating hot fluids which heat up the fluids in the reservoir. It is likely that the heat is caused by magmatic activity related to the subduction process. This interpretation is consistent with geochemical studies which indicate magmatic origin of fluids sampled from the Panabungan hot spring.
About 2 km to the south-west of the Tarutung basin, a shallow conductive body is revealed at depths of 0.5-2 km beneath cold springs close to the inactive Martimbang volcano. This conductor is interpreted as Andesitic material which was altered by high temperatures in the past due to the magma intrusion of the Martimbang. Geochemical and geophysical studies, including MT, do not support a hypothesis that the heat source of the Sipoholon geothermal system is related to the inactive Martimbang volcano.
A shallow conductive anomaly is also revealed beneath the Tarutung basin. This shallow conductor is interpreted as unconsolidated sediments of volcanic origin (Quaternary Toba Tuff). In the westernmost part of the study area, a very resistive region corresponds to Permian granites. This resistor is interpreted as the western border of the Sipoholon geothermal system.
In summary, the resistivity models from this study display the previously unknown fluid distribution of the Sipoholon non-volcanic geothermal system. This interpretation is consistent with the high porosity of surface rocks and the distribution of low Vp and high Vp/Vs observed around the Panabungan hot spring. Combined with other geoscience disciplines, MT is a very powerful geothermal exploration tool because of its sensitivity to image hot saline fluids as zones of high conductivity.
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