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

Abstract

Recent seismic swarms and hydrothermal activity suggest that the Quaternary volcanic complex of the Alban Hills may pose a threat to the city of Rome. A 350m scientific borehole was therefore drilled into this volcanic area to elucidate its inner structure for the first time. Wire-line logs were run in the borehole in order to characterize the physical properties of the rocks and their variations with depth. In particular, a detailed sonic log was run to measure the P-wave velocity from the well-head down to 110 m. To further investigate velocity changes, we carried out laboratory measurements of P and S elastic wave velocities and fluid permeability at effective pressures up to 70 MPa during both increasing and decreasing pressure cycles on selected core samples representative of the main volcanic units. Specifically, we studied samples from two pyroclastic units representative of two classes of volcanic deposits that are representative of the whole succession: (i) a coarse-grained, well-lithified facies (Pozzolane Rosse unit), containing abundant mm-to-cm lava clasts and crystals; and (ii) a fine-grained, matrix-supported pyroclastic deposit (Tufo Pisolitico di Trigoria unit), with rare lithic lava clasts and sparse pumice. Elastic wave velocities reveal significant differences between units and indicate how, within the same lithology, the different degree of lithification and presence of clasts can affect significantly physical property values. The mean laboratory value of the Pwave velocity for Pozzolane Rosse and Tufo Pisolitico di Trigoria units is respectively of 3.75 and 3.2 km/s at an effective pressure equivalent to that at the depth at which the sonic velocity was measured. Under increasing effective pressure a profound influence on the transport properties is observed. Permeability ranges from the order of 10−18 m2 for the Pozzolane Rosse unit to the order of 10−15 m2 for the Tufo Pisolitico di Trigoria unit, in good agreement with the shallow aquifer circulating in the shallower units.