Drilling the Ivrea-Verbano zonE project: DT-1b borehole geophysics

The ICDP Drilling the Ivrea-Verbano zonE project (DIVE), aims at unravelling long-standing fundamental questions on the nature of the continental lower crust, its lithological correlation with geophysical anomalies and the characteristics of the underlying physical and chemical rock properties. Borehole geophysics provides an excellent opportunity and is the method of choice to explore the origin of geophysical anomalies, observed within the Ivrea-Verbano Zone, and their link to lithologies of the lower crust across several spatial scales. The first borehole of the DIVE project DT-1b in Ornavasso, Val’d Ossola (Italy), drilled into the Massone antiform, has been completed in December 2022 at a depth of 578.5 m. Geophysical borehole experiments comprising a suite of downhole logging and vertical seismic profiling (VSP) measurements have been conducted in two stages. A select set of first-look logs were collected to a depth of 315 m and a full suite at the end of drilling. The drilled rock types are metapelite, metapsammite, schist, gneiss, amphibolite, and, in places, migmatite and pegmatite. Several fractures are encountered within the drilled rock mass, exhibiting a NW-SE orientation and a variation of dip angles as identified by acoustic televiewer data. The acoustic televiewer data also show a couple of breakout zones, which may allow to constrain the current stress field orientation after carefully analyzing the impact of the topography. Preliminary results of the wireline logs suggest that they broadly correlate with the different rock types. Notably, the amphibolites as well as some of the more pelitic migmatites and gneiss exhibit locally high values of magnetic susceptibility of the order of 1000 10^-6 SI. These values are confirmed by magnetic susceptibility measurements performed on drilled cores on-site with a self-built manual core scanner. Such high values have been reported in previous studies for amphibolites and mafic granulites in the Ivrea-Verbano zone and are most likely related to pyrrhotite and magnetite. Apart from the amphibolites, for most of the other rock types encountered, a weak correlation between magnetic susceptibility and the natural gamma radiation can be observed. The sonic P-wave velocities and preliminary P-wave velocity estimates of the VSP data are generally consistent. The average P-wave velocity estimate from the VSP is 5.3 km/s and slightly lower than the average estimate of 5.6 km/s obtained from the sonic logs. An integrated analysis of the complete set of the borehole geophysical data is currently undertaken to classify the rock mass with respect to their geophysical responses and to systematically delineate the underlying major factors of influence governing these responses.