The Anatomy of a Fumarole inferred from a 3-D High-Resolution Electrical Resistivity Image of Solfatara Hydrothermal System (Phlegrean Fields, Italy)

Abstract:

Solfatara, the most active crater in the Phlegrean Fields volcanic complex, shows since ten years a remarkable renewal of activity characterized by an increase of CO₂ total degassing from 1500 up to 3000 tons/day, associated with a large ground uplift (Chiodini et al., 2015). In order to precisely image the structure of the shallow hydrothermal system, we performed an extended electrical DC resistivity survey at Solfatara, with about 40 2-D profiles of length up to 1 km, as well as soil temperature and CO2 flux measurements over the area. We then realized a 3-D inversion from the ~40 000 resistivity data points, using E4D code (Johnson et al., 2010).

At large scale, results clearly delineate two contrasted structures:

- A very conductive body (resistivity < 5 Ohm.m) located beneath the Fangaia mud pools, and likely associated to a mineralized liquid rich plume.

- An elongated more resistive body (20-30 Ohm.m) connected to the main fumarolic area and interpreted as the gas reservoir feeding the fumaroles.

At smaller scale, our resistivity model originally highlights the 3-D anatomy of a fumarole and the interactions between condensate layers and gas chimneys. This high-resolution image of the shallow hydrothermal structure is a new step for the modeling of this system.