High-resolution mini-seismic methods applied in the Mont Terri rock laboratory (Switzerland)

Abstract

We present several mini-seismic methods developed and applied in recent years in the Mont Terri rock laboratory. All these applications aimed at correlating and interpreting seismically derived parameters with relevant rock-mechanical parameters and findings. The complexity of the local site setting always required very high spatial and parameter resolution. Both, seismic P-and S-wave velocities and dynamic elastic parameters, such as the dynamic Poisson's ratio t dyn and the Young's modulus E dyn , are used to characterise the Opalinus Clay under real in situ conditions. We were able to establish a correlation between static and dynamic elastic Young's moduli. We describe the extremely large, small-scale variability of seismic parameters normal and parallel to the bedding plane orientation and address the question of fracture detection. We also present examples of the characterization of excavation-damaged zones with seismic parameters, including extent as well as degree of damage, and compare these to geological and structural mapping. The evolution of borehole-disturbed zones (BdZ) was deduced from repeating high-resolution borehole measurements. Finally, we quantify seismic anisotropy at dimensions between several cm and tens of m.

Fault sealing and caprock integrity for CO<sub>2</sub> storage: an in-situ injection experiment

“…Values of V P we observed in situ are clearly smaller than what has been previously measured by other researchers on drill cores in the laboratory (e.g. Bossart et al (2017), 2220-3020 m/s normal to bedding, 3170-3650 m/s parallel to bedding), but exhibits a similar degree of anisotropy, with A = 0.26, which is close to A = 0.3 estimated after Bossart et al (2017). Lower values in absolute V P compared to ultrasonic measurements has been expected in accordance with the Kramers-Kronig dispersion relation (e.g.…”

“…It is an overconsolidated clay, which has reached an estimated maximum burial depth of around 1350 m at the current laboratory area, with an overconsolidation ratio of almost 5, assuming a current average overload of 280 m (Hostettler et al 2017). The formation is 131 m thick (Bossart et al, 2017) at the MTRL. The Opalinus Clay and the adjacent formations form an anticline, crosscut by the tunnels of the lab.…”

Mont Terri rock laboratory, 20 years of research: introduction, site characteristics and overview of experiments

“…The in-situ derived gas-entry pressure of the shaly facies originates from k Miehe et al (2010). The seismic P-wave velocity values in this table were derived from drillcore measurements in the laboratory; in-situ derived P-wave velocities are provided by Schuster et al (2017) providing diffusion parameters such as diffusion coefficients parallel and normal to bedding, effective porosities, and retardation parameters for sorbing radionuclides (Leupin et al 2017a). As indicated in Table 2, these diffusion parameters can vary considerably and have to be evaluated separately for every species.…”

Understanding the evolution of nuclear waste repositories by performing appropriate experiments – selected investigations at Mont Terri rock laboratory

“…Therefore, when shotcrete is used as a support system for the construction of a repository for radioactive waste or for tunnelling in claystone, it has to be used directly after the excavation. The advantage in this instance is that the extension of a zone with an increase in permeability reaches only about 1.5 m away from the tunnel wall (Shao et al, 2017a). In fact, the fracture network in the EDZ will seal in time due to swelling processes of the fracture surfaces, and thus reduce the permeability (Bossart et al, 2002;Zhang, 2011).…”

“…Figure 9. Permeability distribution around the FE-tunnel in the section without shotcrete (a, after Shao et al, 2017a) and comparison between measured and modelled temperature and pore water pressure in the boreholes during the heating phase (b). Figure 10.…”