Structure et Dynamique des systèmes hydrothermaux

Hydrothermal systems are found at most active volcanoes and their understanding is important for following reasons :

1. Variations in hydrothermal circulation reflect the temporal evolution of the volcanic activity like the reactivation of the magmatic processes or preparation of the volcanic crisis ;
2. Hydrothermal systems may interact with magma ascent leading to explosive hydro-magmatic eruptions ;
3. Their emplacement and geometry are controlled by principal geological structures, therefore the map of the hydrothermal system gives us an image of the buried ancient crater rims, faults or permeable zones ;
4. Long term hydrothermal circulation can contribute to destabilization of the edifice by weakening the stiffness of the rocks.

We study hydrothermal systems by geophysical methods like the electric self-potential and resistivity methods as well as the measurements of the ground temperature distribution or analysis of ambient seismic noise. Self-potential method allows ground flow systems to be detected and their geometries to be constrained (Figure 1). Knowledge of the ground temperature distribution allows us to couple the models of mass and heat transfer (Figure 2).

 
Figure 1. Fluid circulation at a shallow depth below Solfatara crater of Phlegreen Fields. 2-D Electrical resistivity cross-section along two intersecting profiles compared to a) Google Image draped over a digital elevation model of the area b) a ground temperature distribution. Resistive zones like those below the Bocca Grande fumarole correspond to an upward flow of the carbon dioxide/vapor mixture (dashed line), conductive zone below the Fangaia mud pool indicates the presence of the liquid water (solid line).

Figure 2. Influence of the regional topography on the geometry of the hydrothermal body. We build our study on field data of self-potential (a), spring temperature, and on the results of direct numerical simulation coupling mass and heat transfer inside the volcanic edifice (b). Our numerical results show a significant derivation of hydrothermal system due to gradient of regional topography. Byrdina et al., Earth Planet. Sci. Lett., 2013