Abstract
Seismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.
Highlights
In the development and usage of underground infrastructure, claystones are challenging, but they have relevant properties which make them useful for a wide range of geoengineering applications
5.1 Parameterization The grid size for the traveltime tomography was defined by a spacing of 2 m in x, y-direction
For the shaly facies, but only ∼ 30◦ for the carbonate-rich sandy facies, which only covers for the fast direction
Summary
In the development and usage of underground infrastructure, claystones are challenging, but they have relevant properties which make them useful for a wide range of geoengineering applications. Their extremely low permeability has enabled the generation of Editorial handling: Michael Schnellmann. A detailed characterization of clay formations is of high importance in many applications. Mechanical and hydraulic properties, especially potential heterogeneities, have significant impact on the assessment of short- and long-term behaviour of these formations under varying environmental conditions. A complex anisotropy (structural anisotropy) results from interferences of different components of the rock structure (e.g. bedding planes, clay mineral-rich layers, sandy lenses, and cemented matrix). An example are laminated materials like Opalinus Clay (Siegesmund et al 2014)
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