Article révisé par les pairs
Résumé : Papandayan (2665 m asl) is an Indonesian stratovolcano located at 50 km from Bandung in west Java and characterized by an intense hydrothermal activity. An advanced alteration takes place where acid fluids interact with rocks, weakening the edifice, so that even minor explosive eruptions threaten the stability of its flanks. The purpose of the current study is to delineate the geometry of the acid hydrothermal plume using Electrical Resistivity Tomography (ERT). We used self-potential, pH measurements in water (in situ) and of soil samples, SO2 and CO2 soil concentration mappings to better understand the resistivity structure. Measurements have been performed inside the 1772 crater with a maximal depth of investigation of about 250 m for electrical resistivity tomography. At low pH, the mobility of H+ (or H3O+) ions represents a dominant contribution to the electrical conductivity leading to an unusually high conductivity of pore water. For Papandayan spring water, the theoretical electrical conductivity calculated for chemical composition and pH yield indeed exceptionally high values in the range 20 − 25 S m−1. The surface conductivity of the altered unconsolidated samples determined from a recent study (< 0.04 S m−1) is negligible in comparison with the conductivity of the pore water. The theoretical conductivity values are then compared with the results of our geophysical survey. Our 3-D inversion of electrical resistivity data identifies the entire crater of Papandayan volcano as a relatively conductive medium (> 0.005 S m−1) with an extremely high bulk conductivity within the central part of the crater (∼ 2 S m−1). The main degassing zones in the crater, Kawah Emas, Manuk and Kawah Baru, are all connected by conduits to this common reservoir at a depth of 100 m. Because the location of this good conductor coincides with elevated ground temperature, main fumaroles, and with detectable SO2 degassing, we interpret it as an acid hydrothermal plume. Low pH impacts also the self-potential distribution: a clear correlation is observed between the pH values measured in soil samples and the self-potential. The main degassing area is associated with a negative anomaly of self-potential likely produced by the electro-kinetic effect due to upwelling fluid flow in acid conditions. It follows from our results that the assessment of the pH conditions is necessary for the interpretation of electrical resistivity structures and self-potential distribution on hydrothermal systems where acid conditions and acidity variations can be expected due to chemical reactions between volcanic gases and groundwater.

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