Towards Monitoring Volcanic Hydrothermal Alteration Using Geophysical Approaches

Most volcanoes on Earth host a volcano-hydrothermal system. Lying between the surface and magma reservoirs, they exert a dramatic influence on volcano dynamics. Understanding their behavior is however challenging because of the complex interplay between gas, liquid, and rocks. Volcanic gas can, for example, be completely scrubbed through interactions with groundwater whereas the kinetics of these reactions are controlled by thermodynamic conditions that are poorly constrained. While fluid circulation gives rise to a range of geophysical signals such as ground vibrations, self-potential, or variable resistivity observable at the surface, their complex dynamics complicate the isolation of pre-eruptive signals and the interpretation of the observed volcanic activity. Indeed, some volcanoes remain on alert for months or years without experiencing any eruption. Such situations severely affect the credibility of the agencies in charge of monitoring volcano activities.In this contribution, we will focus on multi-disciplinary efforts to better characterise the time evolution of these complicated systems. Our ultimate goal is to deconvolve the contribution of dynamic processes occurring in such systems (temperature, gas saturation, alteration, precipitation) to possibly facilitate real-time monitoring efforts. Our examples come from different parts of the world, in both hemispheres.