Imaging the shallow hydrothermal system of Lastarria volcano in Chile using electromagnetic techniques

Lastarria Volcano is characterized by persistent fumarolic activity and substantial sulfur gas emissions, ranking among the most significant active volcanoes in the Central Volcanic Zone of the Andes. This study presents the results of a transient electromagnetic (TEM) survey carried out over one of the main fumarolic fields of the Lastarria volcano in Chile, aimed at imaging the subsurface resistivity structure. We applied an innovative 3D inversion approach, focusing on resistivity variations at shallow depths (to \(\sim\) 250 m) in the vicinity of the fumarolic field emitting hot sulfur-rich gasses. In addition, electromagnetic induction (EMI) mapping was conducted to investigate the very shallow conductive manifestations within the fumarolic area. The inversion results reveal two significant features: (1) a thin, shallow double conductive layer beneath the fumarolic field, extending to \(\sim\) 30–60 m depth, and (2) a deeper conductor at \(\sim\) 200 m depth. Modeling studies confirm the robustness of these conductive structures and validate the inversion models. At shallow depths, we observe strong agreement between the surface conductivity distribution derived from EMI mapping and the TEM results in their spatial context. We interpret the shallow double conductor as evidence of fluid circulation and steam condensation, the primary processes driving fumarolic activity, which is dominated by both gas and liquid phases. The deeper conductive zone, below \(\sim\) 250 m, is interpreted as the main hydrothermal system, consistent with independent magnetotelluric measurements. The geophysical results provide clear subsurface imaging based on resistivity, offering new insights into the behavior of fumarolic activity and its connection to underlying hydrothermal systems. Such near-surface geophysical methods have not previously been applied at these depths at Lastarria. These findings contribute to a better understanding of magmatic–hydrothermal processes not only at Lastarria but also at other volcanoes with similar geological characteristics.