ICDP STAR drilling project in Italy: preliminary analysis of geophysical downhole logging data

Abstract

The ICDP STAR drilling project (Strain Meter Array) is a joint research project among different institutions that aims to study the fault slip behaviour of the low angle Alto Tiberina Fault (ATF) in the Northern Apennines, Italy. The ATF is an active low-angle normal fault (mean dip 20°) whose activity and mechanics is still debated. Therefore, STAR drilled and instrumented six shallow boreholes, providing an excellent opportunity to study creep at local scale and over periods of minutes to months, poorly constrained by other geophysical instruments.Two drilling campaigns were made in Fall 2021 and in Summer 2022, drilling a total of six 80-160 m deep vertical boreholes. Each borehole was instrumented with seismometers (three-component (3C) borehole geophones) and strainmeters (Gladwin Tensor Strainmeters, GTSM).  Strainmeters are the only instruments capable of measuring small creep events, as has been demonstrated near other creeping faults, such as the creeping section of the strike-slip San Andreas fault near Parkfield. Two boreholes drilled the Mesozoic-Paleogene, Umbria-Marche carbonate succession (Maiolica, Marne a Fucoidi, Scaglia Bianca, Scaglia Rossa and Scaglia Variegata formations). The other four boreholes encountered the Neogene marls and turbidite sandstones (Schlier and Marnoso-Arenacea formations). Upon completion of the drilling operations, a suite of downhole logging measurements was performed in each borehole, comprising: total gamma ray, full wave sonic, electrical conductivity and temperature, caliper, resistivity, optical and acoustic borehole images. The sondes recorded data only in the deepest portion (open section) of the wellbore, except for the total gamma ray that run also in the cased section. The objective was to record the physical properties of the rocks in situ, and to reconstruct the spatial distribution and characteristics of the fractures (i.e. partially open, closed, thickness) and their connection with the geological structures mapped on the surface.  Here we present a preliminary analysis of the logging data. In the carbonate units (i.e. Maiolica and Scaglia Rossa) the gamma ray shows low and flat curve (less than 30 cps) and P-wave velocity about 3 km/s. In the sandstone-marly units (Marnoso-Arenacea), encountered in three boreholes, the gamma ray records high values (about 80-100 cps) correlated mainly to marly intervals and P-wave velocity of 3-3.5 km/s. The hemipelagic marls of the Schlier Formation are characterized by high gamma ray (mean value of 80 cps) and by an average P-wave velocity of 3.5 km/s. These values will be compared with the results of laboratory analysis of samples, collected in similar lithologies, as well as with the results of logging performed in deeper wells drilled for commercial purposes. Through this comparison, we will evaluate the effect of depth (i.e. pressure) on the main physical properties of sedimentary rocks. The physical properties, in combination with the orientation and geometry of the discontinuities (fractures, veins, bedding), acquired by downhole logging, will contribute in building-up improved 3-D geological models of the ATF.