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Authors: Gresse, Marceau* 
Vandemeulebrouck, Jean* 
Byrdina, Svetlana* 
Chiodini, Giovanni* 
Revil, André* 
Johnson, Timothy C.* 
Ricci, Tullio* 
Vilardo, Giuseppe* 
Mangiacapra, Annarita* 
Lebourg, Thomas* 
Grangeon, Jacques* 
Bascou, Pascale* 
Metral, Laurent* 
Title: Three-Dimensional Electrical Resistivity Tomography of the Solfatara Crater (Italy): Implication for the Multiphase Flow Structure of the Shallow Hydrothermal System
Issue Date: 27-Nov-2017
Series/Report no.: /122 (2017)
DOI: 10.1002/2017JB014389
Keywords: 3-D gas-dominated reservoir feeds the Bocca Grande fumarole at 164 degrees Celsius through a ~10 m thick
The Fangaia mud pool and Pisciarelli fumarole are both conductive liquid-dominated plume
Subject Classification04.08. Volcanology 
Abstract: The Solfatara volcano is the main degassing area of the Campi Flegrei caldera, characterized by 60 years of unrest. Assessing such renewal activity is a challenging task because hydrothermal interactions with magmatic gases remain poorly understood. In this study, we decipher the complex structure of the shallow Solfatara hydrothermal system by performing the first 3-D, high-resolution, electrical resistivity tomography of the volcano. The 3-D resistivity model was obtained from the inversion of 43,432 resistance measurements performed on an area of ~0.68 km2. The proposed interpretation of the multiphase hydrothermal structures is based on the resistivity model, a high-resolution infrared surface temperature image, and 1,136 soil CO2 flux measurements. In addition, we realized 27 soil cation exchange capacity and pH measurements demonstrating a negligible contribution of surface conductivity to the shallow bulk electrical conductivity. Hence, we show that the resistivity changes are mainly controlled by fluid content and temperature. The high-resolution tomograms identify for the first time the structure of the gas-dominated reservoir at 60mdepth that feeds the Bocca Grande fumarole through a ~10mthick channel. In addition, the resistivity model reveals a channel-like conductive structure where the liquid produced by steam condensation around the main fumaroles flows down to the Fangaia area within a buried fault. The model delineates the emplacement of the main geological structures: Mount Olibano, Solfatara cryptodome, and tephra deposits. It also reveals the anatomy of the hydrothermal system, especially two liquid-dominated plumes, the Fangaia mud pool and the Pisciarelli fumarole, respectively.
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