Nagra Station Characterization

In cooperation with the National Cooperative for the Disposal of Radioactive Waste (Nagra), the Swiss Seismological Service has recently completed the installation of ten new seismological observation stations, three of them including a co-located borehole sensor. The ultimate goal of the project is to densify the existing Swiss Digital Seismic Network (SDSNet) in Northern Switzerland, in order to increase the sensitivity to very-low magnitude events and to improve the accuracy of future location solutions. This is strategic for unbiased monitoring of micro-seismicity at the places of proposed nuclear waste repositories.

At each site, to further improve the quality and usability of the recordings, a full seismic characterization of the area surrounding the installation area was performed. The investigation consisted of a preliminary geological and geotechnical study, followed by an accurate seismic site response analysis by means of state-of-art geophysical techniques. For the borehole stations, in particular, the characterization was performed by combining different types of active seismic methods (refraction tomography, surface wave analysis, Vertical Seismic Profiling, VSP) with ambient vibration based approaches (wavelet decomposition, H/V spectral ratio, polarization analysis, three component f-k analysis).

Results converged to the definition a unique summary velocity profile for the site, later used for the computation of the analytical SH-wave amplification function.