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| Titel |
Monitoring the Excavation Damaged Zone in Opalinus clay by three dimensional reconstruction of the electrical resistivity in the Mont Terri gallery G-04 |
| VerfasserIn |
N. Lesparre, A. Adler, F. Nicollin, D. Gibert, C. Nussbaum |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062760
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| Zusammenfassung |
| The characteristics of opalinus clay have been studied in the last years for its capacity to
retain radionuclide transport as a low permeable rock. This formation presents thereby
suitable properties for hosting repository sites of radioactive waste. The Mont Terri
underground rock laboratory (Switzerland) has been excavated in opalinus clay layer in order
to develop experiences improving the knowledge on the physico-chemical properties of the
rock.
The study of electrical properties furnishes information on the rock structure, its anisotropy
and the changes of these properties with time (Nicollin et al., 2010 ; Thovert et al., 2011).
Here the three dimensional reconstruction of the electrical resistivity aims at monitoring the
temporal evolution of the excavation damaged zone. Three rings of electrodes have been
set-up around the gallery and voltage is measured between two electrodes while a current is
injected between two others (Gibert et al., 2006). Measurements have been achieved from
July 2004 until April 2008 before, during and after the excavation of the gallery
04.
In this study we develop a computational approach to reconstruct three dimensional images of
the resistivity in the vicinity of the electrodes. A finite element model is used to
represent the complex geometry of the gallery. The measurements inferred from a
given resistivity distribution are estimated using the software EIDORS (Adler and
Lionheart, 2006), this constitutes the forward problem. The reconstruction of the
media resistivity is then implemented by fitting the estimated to the measured data,
via the resolution of an inverse problem. The parameters of this inverse problem
are defined by mapping the forward problem elements into a coarser mesh. This
allows to reduce drastically the number of unknowns and so increases the robustness
of the inversion. The inversion is executed with the conjugate gradient method
regularised by an analysis of the Jacobian singular values. The results show an evolution
of the resistivity with time as the gallery excavation is carried on. They indicate
physically plausible resistivity patterns, corresponding to known changes in the rock
structure.
Adler, A. & Lionheart, W.R.B., 2006. Uses and abuses of EIDORS: An extensible
software base for EIT, Physiol. Meas., 27, S25–S42.
Gibert, D., Nicollin, F., Kergosien, B., Bossart, P., Nussbaum, C., Grislin-Mouëzy, A.,
Conil, F. & Hoteit, N., 2006. Electrical tomography monitoring of the excavation damaged
zone of the Gallery 04 in the Mont Terri rock laboratory: Field experiments, modelling, and
relationship with structural geology, Applied clay science, 33, 21–34.
Nicollin, F., Gibert, D., Lesparre, N. & Nussbaum, C., 2010. Anisotropy of electrical
conductivity of the excavation damaged zone in the Mont Terri Underground Rock
Laboratory, Geophysical Journal International, 181, 303–320.
Thovert, J.F., Mourzenko, V.V., Adler, P.M., Nussbaum, C. & Pinettes, P., 2011. Faults
and fractures in the Gallery 04 of the Mont Terri rock laboratory: Characterization, simulation
and application, Engineering Geology, 117, 39–51. |
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