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 υdyn and the Young’s modulus Edyn, 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.
Highlights
The term ‘‘high-resolution mini-seismic methods’’ (HRMSM) comprises all seismic, sonic, and ultrasonic in situ methods that the German Federal Institute for Geosciences and Natural Resources (BGR) has developed and continues to develop for underground investigations
The Opalinus Clay at the Mont Terri rock laboratory can be differentiated into three facies, all of which possess different petrophysical properties (Bock 2002; Bossart and Thury 2008)
MSM can resolve the heterogeneity of the Opalinus Clay in the cm range, supporting structural interpretation
Summary
The Opalinus Clay at the Mont Terri rock laboratory can be differentiated into three facies (shaly, sandy, and carbonate-rich sandy), all of which possess different petrophysical properties (Bock 2002; Bossart and Thury 2008). Since 1996, BGR has carried out MSM in the Mont Terri rock laboratory in close cooperation with experiment partners. Other aspects appropriate for study using our MSM methods and concepts, such as detection and characterization of fault structures, seismic long-term monitoring to characterise evolution of the EDZ (generation and selfsealing), and evolution of backfill material (sand-bentonite), are presented in other papers in this issue (e.g., Jaeggi et al 2017; Wieczorek et al 2017). Combining results from MSM with those derived under laboratory conditions (mostly static tests) on cores can improve our understanding and interpretation of rock-mechanical modelling. Bossart et al (2017a), this issue) present an overview of the experiments and sites where the MSM have been applied
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