Three‐Dimensional Electrical Resistivity Tomography of the Solfatara Crater (Italy): Implication for the Multiphase Flow Structure of the Shallow Hydrothermal System

• Published in: Journal of geophysical research. Solid earth 2017-11, Vol.122 (11), p.8749-8768
• Main 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
• Format: Article
• Language: English
• Subjects: Area; Buried structures; Calderas; Campi Flegrei caldera; Capacity; Carbon dioxide; Carbon dioxide flux; Cation exchange; Cation exchanging; Cations; Condensation; Conductivity; Degassing; Earth Sciences; Electrical conductivity; Electrical resistivity; electrical resistivity tomography; Exchange capacity; Flow structures; fumarole; Fumaroles; Gases; Geologic depressions; Geological structures; Geophysics; High resolution; hydrothermal system; Image resolution; Infrared imagery; Interactions; Mud; Multiphase flow; pH effects; Plumes; Renewal; Resolution; Sciences of the Universe; Soil; Soil temperature; Soils; Solfatara volcano; Steam electric power generation; Surface temperature; Tephra; Three dimensional models; Tomography; Volcanoes
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB014389

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 60 m depth that feeds the Bocca Grande fumarole through a ~10 m thick 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. Key Points A 3‐D gas‐dominated reservoir feeds the Bocca Grande fumarole at 164 degrees Celsius through a ~10 m thick conduit The Fangaia mud pool and Pisciarelli fumarole are both conductive liquid‐dominated plume A buried fault drives the condensate water from the main fumaroles toward the Fangaia mud pool