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| Titel |
Radon as a tool to monitor transient permeability changes: on-line measurements in a tri-axial cell |
| VerfasserIn |
Frédéric Girault, Alexandre Schubnel, Eric Pili |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093849
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| Publikation (Nr.) |
 EGU/EGU2014-8977.pdf |
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| Zusammenfassung |
| Radon-222 is a radioactive gas of half-life of 3.8 days which is naturally produced in the
Earth’s crust. This informs generally about the transfers in the subsurface and is considered as
a potential earthquake precursor. In this study, deformation experiments are performed
in the laboratory on various granites, for which the radon source term is known
(effective radium concentration). Radon concentration is continuously measured on-line
under isotropic stress and deformation using a tri-axial cell, together with acoustic
emissions, seismic velocity, deformation rates and geometry. Pore fluid pressure is fixed
and maintains a flush of inert gas through the sample, which allows radon to be
expelled from the porous network of the sample. This also leads to permeability
determinations. Radon release shows a large sensitivity to various intrinsic and
external parameters. While radon release is stable under constant isotropic conditions,
it increases with confining pressure, because radon is more concentrated as the
permeability decreases. However, above a given confining pressure threshold, radon release
decreases while permeability continues to decrease, mainly due to the closure of cracks.
Therefore, any change of permeability in the sample, even relatively small, is marked
by a consecutive change of the radon release. At the macroscopic rupture of the
sample, significant transient radon release is observed simultaneously to the rupture,
or some time after. This transient radon peak results from isolated microporosity
being newly connected to the permeable network rather than new crack surface
creation per se. This hypothesis is sometimes proposed to account for radon anomalies
observed before and after large earthquakes. As our transient radon signals are
observed just after or near the time the rupture occurs, we might tentatively raise
questions concerning some precursory observations. All these effects are now being
quantified. This study and the developed procedures open interesting perspectives for the
understanding of the relation between deformation and radon release from crustal rocks. |
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