• active-source seismics
• Drilling the Ivrea-Verbano zonE
• Geophysical Site Characterization
• Travel-time Tomography

Travel time tomography has been utilized to delineate the boundary between crystalline
metamorphic lower crustal rocks and quaternary sediments of a site. The area investigated is
approximately 50x50m within the Ivrea Verbano Zone (IVZ). The IVZ is a region of
significant geological interest due to deep crustal segments that have been uplifted to shallow
depths. DIVE (Drilling the Ivrea-Verbano zonE) is an international scientific collaboration,
funded by the ICDP, which aims to unravel fundamental inquiries of the IVZ. This is done
through drilling 3 drillholes, one of which is DT-1B located in Ornavasso Italy. In order to
characterise the DT-1B drill site, geophysical investigations were conducted prior to the start
of drilling activities including Ground Penetrating Radar (GPR), Electrical Resistivity
Tomography (ERT), and active-source seismic surveys. The seismic data was acquired as a
tightly spaced 3-D rolling spread with the source lines offset from the geophone lines. This
source-receiver geometry is not optimal for 2-D seismic refraction profiling, and a 3-D
refraction travel-time tomography scheme was subsequently used to derive a subset of 2.5-D
profiles that span the drilling site. An Eikonal solver was used to generate velocity models
and produce theoretical travel-time curves. The starting velocity model utilized velocities
derived from first-break analysis and subsurface geometry assumed from the ERT and GPR
data results. An RMS error of 4.97 ms between the observed and calculated travel-times is
achieved after 12 iterations. The final velocity models compare well with the ERT and GPR
profiles and the 3D-velocity cube serves as a good starting model for improved seismic
reflection imaging.