Seismic imaging and petrophysical classification of the Balmuccia peridotite and surrounding upper crust

An interdisciplinary approach is needed to fully understand the formation and evolution of the Earth’s crust. Different disciplines (geology, geophysics, geochemistry, etc.) provide complementary information that cover different depth and time scales. By combining this information, a better understanding of crustal processes becomes possible. Seismic methods, for example, are utilized to obtain structural images of the present state of the crust and derive its petrophysical properties. This knowledge can then be used as constraints in geodynamic modelling.

In this presentation, we will focus on the Ivrea Verbano Zone (IVZ) where lower crustal rocks and mantle peridotites are exposed at surface. The IVZ is the subject of recent studies and drilling projects to gain a better understanding of the crustal evolution and crust-mantle transition. In preparation of a proposed drilling campaign (ICDP-DIVE), seismic exploration surveys (fixed-spread and roll-along) were carried out across the Balmuccia peridotite body and the Insubric Zone. However, the seismic data show strong first break onsets of P- and S-waves and converted waves which interfere with signals from the peridotite body. Hence, we combined different seismic processing techniques to derive a structural image of the Balmuccia peridotite and its surroundings: Conventional seismic reflection imaging shows a rather diffusive image of the subsurface. The results can be improved by applying coherency-based Prestack Depth Migration and stacking which reveals reflective structures at the borders of intrusive bodies. Tomographic imaging of the fixed-spread data set mapped the 3D structure of an asymmetric high-velocity body that extends down to 3 km depth and is limited in the West by the Insubric Zone. The smooth 3D velocity model is supplemented by a high-resolution image of the near-surface structure that was obtained by inverting the travel times from the roll-along data set. By performing a machine-learning based cluster analysis the near-surface structure is subdivided into distinct model regions with well-defined seismic properties enabling now petrophysical interpretation.

To support the preparation of the proposed drilling campaign (ICDP-DIVE), our results together with the results from recent studies can now be implemented in a joined geomodel of the Balmuccia peridotite.