How a borehole can help volcanology: the scientific drilling in the Colli Albani volcanic area (Italy)

Abstract

In the framework of a small-scale drilling project at the Colli Albani volcanic district (Central Italy), a 350m deep borehole was drilled for 1) a better understanding of the shallow crust structure under the volcanic complex, which is considered now quiescent, by characterization of litho-stratigraphic units, 2) the definition of present-day stress field and 3) good seismic recordings in a urbanized area, by a broad-band seismometer installed at 200m depth. We describe the investigations carried out at the drill site, first results of laboratory analysis and other ongoing studies. We want to highlight the amount of data coming from a relatively small hole, the wide range of different disciplines involved in the studies that can be done and the general contribution to the volcano-tectonics, with final application to hazard evaluations. The borehole is located in an area of the volcanic complex where most of the seismic swarms occurred and where an uplift was recognized by geodetic investigations and satellite images analysis. In addition, gas concentrations (mainly CO2 and H2S) in the aquifers are high, local tectonics is peculiar and seems quite different from the regional trend. There is, however, still a lack of active stress data. The borehole was drilled wire-line with continuous coring and a very good core recovery in the volcanic units (tuffs and lavas) and in the underlying sedimentary basement of Plio-Pleistocene sands and clays. A detailed stratigraphic log was built, also thanks to biostratigraphic and petrographic analysis. A blow–out from the deeper sandy unit occurred while a hydraulic fracturing test was about to start and did not allow to continue the tests. Fluid and gas were sampled providing information about deep fluid flow and the likely connection to a deep-seated magma chamber. The following wire-line logs were performed: gamma-ray, resistivity, sonic, caliper, borehole televiewer (BHTV) and magnetic susceptibility. The data were compared with laboratory investigations (e.g. mechanical, magnetic analysis) to define the rock parameters, useful for a better comprehension of the different units behavior under different P and T conditions and their seismic response. Caliper and BHTV provided data that helps to interpret the main tectonic characters and the present-day stress field.