Physical properties of tuffs from a scientific borehole at Alban hills volcanic district (central Italy)

Recent seismic swarms and hydrothermal activity indicate that the Quaternary volcanic complex of Alban hills may pose a threat to the city of Rome. A 350m scientific borehole was drilled for the first time in Italian volcanic areas to understand the inner structure. Wire-line logs were run in the borehole in order to characterize the physical properties of the volcanic rocks and their variations with depth. In particular, we ran a detailed sonic log to measure P-wave velocities from the well-head down to 110m. To further investigate velocity changes, we carried out laboratory measurements on selected core samples representative of the main volcanic units. We have studied two pyroclastic units that are the most representative of the whole volcanic succession: a coarse-grained, extremely lithified facies, containing abundant mm-to-cm lava clasts and crystals and a fine-grained, matrix- supported pyroclastic deposit, with rare lithic lava clasts and sparse pumice. Elastic wave velocities reveal significant difference (from 2.6 to 4.2 km/s) and strong variation of anisotropy (from 3 to 25 percent) due to different presence of clasts and to the degree of lithification. In order to understand the variation of rock properties with depth, we also report elastic wave velocities and fluid permeability measured at effective pressures from 5 to 80 MPa, during both increasing and decreasing pressure cycles.