Drilling a volcano: scientific experiment at Alban Hills, Italy

Abstract

Only a few deep boreholes have been drilled for scientific purposes on active volcanoes in the whole world. Indeed, data collected from deep wells are fundamental to better model geophysical processes. Within the Italian research project INGV-DPC-V3.1 (funded by the Italian Civil Protection Department), we planned to drill a 400m hole with the main goal to define the orientation and magnitude of present stress field in the shallow crust in the Alban Hills. The Alban Hills are considered a quiescent volcanic district, belonging to the Quaternary volcanic belt of the Tyrrhenian coast. They are located in a densely populated area close to Rome, then an eruption would be a real risk, also considering the type of their past activity. Alban Hills have been fully studied by means of surface or very shallow observations and indirect methods: now we are going to start the first scientific program to investigate them directly at depth. We will perform some hydrofracturing tests at different depth in the drilling located in a key area, to compute, for the first time beneath a volcano, the absolute values of stress principal axes and reconstruct the stress path along depth. Analysis on core samples will allow to better understand the geomechanical characters of volcanic rocks and their underlying sedimentary basement. Coupling these studies with structural, geochronological and palaeomagnetic investigations will constrain the recent volcano-tectonic processes. The comparison of new data with the available stratigraphic logs will give insights on the occurrence of tectonic movements. Analysis of the anisotropy of the magnetic susceptibility could provide information on the Middle Pleistocene strain to be compared with present-day data. These results will be integrated with new geodetic and seismological data obtained by other research units and will be used for physical and numerical modeling to understand the behavior of the whole volcanic complex. This experiment represent a first step to test methods and instruments for future applications also in other sites. At the end of the project the borehole will be used to host instruments, e.g. a seismometer that will provide good quality seismic data in such a noisy area where earthquakes are characterized by swarms of small magnitude and shallow depth.