The combined approach to Ground-based and Satellite Monitoring techniques applied on a close conduit volcano (La Fossa cone, Vulcano, Aeolian Islands, Italy).

Here we present methodologies and practices resulting from the integration, comparison, and validation of thermal monitoring data collected on the close conduit volcano (on the Island of Vulcano, Aeolian Archipelago, Italy). The last unrest phases of La Fossa volcano, manifest since September 2021, allowed us to closely follow the time variations of ground surface temperatures, through the permanent monitoring network, operating since 1991. Our ground control data were used to verify the systematic quantification of time and space variation of thermal anomalies retrieved by the Visible Infrared Imaging Radiometer Suite (VIIRS). The ground-based permanent monitoring network generally provides almost continuous time coverage and higher accuracy, but with limited aerial coverage. On the other hand, satellite data are a powerful tool to study surface thermal anomalies, providing a useful way to monitor the thermal evolution of restless volcanoes by remote platforms. The main applications of thermal remote sensing studies are related mostly to active lava flows and explosive eruptions, whereas the first attempts to remotely track the thermal evolution of quiescent volcanoes are generally lacking ground control reference data. For this reason, those former applications may result in more qualitative than quantitative surface effects of endogenous processes, sometimes including the underestimation of biases of external origin.

This near real-time evaluation of the thermal behavior of the close conduit volcano (La Fossa crater) has been based on three series of long-term monitoring data, independently acquired: the time series of ground temperatures measured within the High-Temperature Fumaroles (a); the depth variation of the convective front rising below the diffuse degassing area (b) and the thermal variations tracked from January 2021 onwards, by the VIIRS images (c). The contact sensors registered the intensity and duration of some periodical modulations of surface temperatures and highlighted other intervals of times when the dynamic equilibrium, usually represented by the gentle emission of fluids in the condition of stationary convection, was critically altered. The radiant flux, retrieved by the nighttime images of VIIRS, confirmed the wider extension of the altered thermal state of the La fossa crater, out of the High-temperature fumaroles, registered from June 2021, up to date.

The complementary nature of the two techniques (direct and remote thermal sensing) has been confirmed by the correlation among the time-series of thermal data. Moreover, the radiant heat remotely sensed by the VIRSS, resulted more closely related to the mild thermal anomaly of the steam-heated ground. Many papers have already presented other monitoring data supporting the same evidence of this volcanic unrest, defined by a significant, and long-lasting phase of pressure buildup, driven by the enhanced supply of magmatic gases in the hydrothermal system (e.g. Diliberto 2021; Inguaggiato et al., 2022a, 2022b; Federico et al., 2023).