3D geological model of a shear zone conditioned on geophysical data and geological observations
Creators: name: Hediger, Roman
Publisher: name: ETH Zurich
Year: 2020
Subjects: 3D Shear Zone Model
Bedretto Underground Laboratory for Geoenergies
Bedretto Tunnel
Ground penetrating radar (GPR)

description: A considerable amount of research is currently conducted in Switzerland in the field of geothermal
energy. That is because the Swiss Energy Strategy 2050 aims to increase the percentage of them in energy
production to 7% of projected energy demand. One of these places where research is done is the Bedretto
Underground Laboratory for Geoenergies (BULG) in Ticino. The BULG is a research facility in the
Bedretto Tunnel surrounded by Rotondo-granite, starting at 2 km (Tunnel Meter TM2000) into the 5 km
long tunnel. During three different drillings (boreholes CB1-CB3, lengths: 300m, 220m, 190m) in the
southwestern wall, enhanced water outflow from the boreholes was noted, when intersecting a shear zone
called Characterization Borehole Shear Zone (CBSZ). This indicates that CBSZ has a high hydraulic
permeability. A permeable structure in the subsurface is of high interest concerning geothermal reservoir
research. However, before this zone can be examined more precisely, its course in the underground
must be determined. In this thesis, firstly, a model of CBSZ is developed; secondly, the intersection
of this shear zone with the tunnel is deduced, and thirdly the history of its origin is investigated. The
modeling tool used is called Gridfit and approximates a best-fitting surface between all inserted data
points. Therefore, optical and acoustic borehole logging data sets and ground-penetrating radar images
of all CB boreholes, as well as geological drill core descriptions, are available. From the logging data,
the orientation of CBSZ in the boreholes is determined. Whereas from the GPR images, the course of the
shear zone further away from the CB’s, is deduced. A combination of these two data sets unites regionally
exact logging measurements as well as slightly less accurate GPR data sets of the shear zones’ course
in the granite. This data are sequentially transferred to Gridfit, which constructs a more accurate model
with each additional information. The resulting model has a slightly curved fracture surface towards the
tunnel and is now compared with known fractures from the tunnel. Thus it follows that CBSZ is most
likely the structure TM1986 from the Bedretto Tunnel. From the history of its origin, it can be concluded
that the shear zone was initially formed under ductile conditions with temperatures over 250-350 C
prevailing. These are conditions that are reached in depths of more than 10-15 km. Afterwards, there
was an uplift of the Rotondo-granite and an associated cooling of the rock. Later the shear zone was
reactivated again but deformed brittle due to the relatively low temperatures. This brittle deformation is
very likely to be the origin of the high porosity that CBSZ has today. An important application of these
findings is that it is now possible to predict how deep further drillings have to be planned to encounter
CBSZ. Furthermore, based on them, more research around this shear zone can now be initiated.