Geophysical downhole logging analysis within the shallow-depth ICDP STAR drilling project (central Italy)

Abstract. The International Continental Scientific Drilling Program (ICDP) STAR (A Strainmeter Array Along the Alto Tiberina Fault System) drilling project aims to study the seismic and aseismic fault slip behavior of the active low-angle Alto Tiberina normal fault (ATF) in the northern Apennines, central Italy, by drilling and instrumenting six shallow boreholes (maximum depth 160 m) with seismometers and strainmeters. During the STAR fieldwork, a geophysical downhole logging campaign was carried out to define the optimal target depth for instrument deployment and formation rock characterization. In particular, the main objectives of this study were to define in situ physical properties of the rocks and the tectonic discontinuity geometry along the boreholes. The downhole logging data provide new findings and knowledge, especially with regards to physical properties such as resistivity and gamma-ray and wave velocity. The collected parameters were compared to the results of literature data collected in similar lithologies, as well as with the results of logging performed in deeper wells drilled for commercial purposes. The physical properties of the Mesozoic–early Tertiary calcareous formations show low gamma-ray values and high compressional (Vp) and shear wave (Vs) velocities (up to 5.3 and 2.9 km s−1, respectively), whereas the overlying clay-rich Late Tertiary formations exhibit high gamma-ray and low resistivity values as well as relatively low Vp and Vs values (up to 3.5 and 2.0 km s−1, respectively). The results obtained from the analysis of the orientations of the tectonic structures, measured along the six boreholes, show good agreement with the orientations of the present-day extensional stress field, which is NE–SW-oriented. Our study allowed us to bridge the gap between the physical properties obtained from literature data and those obtained from the deep well measurements, representing a possible case history for future projects. These new outcomes represent an almost unexplored window of data and will contribute to the advancement of knowledge of the physical properties of the rocks at shallow depths, which are typically overlooked.