Resisting the unknown: Enhancing resistivity imaging of the crust through a multidisciplinary approach from µ- to km-scale at the DIVE DT-1B drill site

Geophysical methods based on the conductivity of electrical currents through the subsurface (IP, ERT, magnetotellurics) are commonly used to identify mineral deposits or aquifers, for which large conductivity contrasts exist between host rock and target. These methods may also provide insights about tectonic processes, such as when rock masses contain partial melt, or are affected by active deformation. However, further advances in electrical imaging of rocks at depth are hindered by the lack of understanding of the relative contributions of paragenesis, fabric, and active processes to electrical conductivity. The ICDP project DIVE (Drilling the Ivrea-Verbano ZonE) provides a unique opportunity to evaluate these parameters by combining samples and measurements from up to ~580 m depth with a wide range of geophysical surface surveys.

The DT‑1B drill core comprises lower crustal rocks consisting mostly of metapelite and amphibolite with embedded pegmatitic lenses. We took 25 samples from these main lithologies that cover the major variations in fabric (e.g., foliation strength, continuity and style, grain size), as well as intervals rich in micro fractures, hydrous alteration, or highly conductive phases (graphite, sulfide). We focus on two main research questions: (1) How do aspects of rock fabrics such as style and strength of foliation or mineral content and the connectivity of conductive phases affect the electrical properties, and (2) can these micro-scale, fabric-induced electrical properties be extrapolated to larger scales?

Fabric analysis and quantification of mineralogy are carried out at thin-section scale by optical and electron microscopy (EDX, EBSD). Computed tomography (CT) performed on small cylinders drilled from the same samples allows the microstructural data to be extended into the third dimension. The CT ably reveals the elongation and alignment of sulfides, the style, and continuity of the foliation which is defined by biotite, and in places graphite- or sulfide-decorated fracture systems. Measurement of electrical properties of the same cylinders under various fluid saturation conditions and a wide frequency range completes the comprehensive database, enabling us to detect and model electrical pathways in the lower crust.

To scale from the micro to the macro scale, these results will be compared to the data from electrical surveys carried out around the DT-1B drill hole. Results will expand the applicability of resistivity imaging for a wide range of future, structural applications.