Analysis of seismic anisotropy derived from sparse three-component receivers at the Forsmark spent nuclear fuel repository site, central Sweden
(English)Article in journal (Refereed) Submitted
The Forsmark area, located within the Fennoscandian Shield, has been chosen forstoring Sweden's spent nuclear fuel. The bedrock in the area belongs to thePaleoproterozoic Svecokarelian orogen and consists of low-temperature, fracturedcrystalline bedrock which has since been deformed both in the ductile and brittleregimes. Three major sub-vertical deformation zones that strike in the WNW to NWdirections surround a so-called tectonic lens (less deformed rock). These deformationzones have structurally anisotropy that extends into the tectonics lens. Here we presentan analysis of large offset 3-component seismic data that were acquired along with aconventional reflection seismic survey. Clear P-wave and S-wave arrivals are observedat offsets up to 4 km. We observe large time delays in the shear wave arrival timesbetween the transverse and radial components, with the transverse generally havingearlier arrival times. There is no little offset dependence in the time delays and noazimuthal variations can be documented. A 1D P-wave velocity model based on firstarrival picks shows a rapid increase in velocity in the uppermost 100 m. We suggest that most of the observed shear wave splitting occurs in these uppermost 100 m due to ahigh density of sub-horizontal fractures. To complement our analysis, we used theconventional Differential Effective Medium (DEM) theory to predict seismic velocitiesand anisotropy considering several different fracture aspect ratios (α). In order togenerate the observed time delays between the shear wave components, a large aspectratio is needed for reasonable porosities.
IdentifiersURN: urn:nbn:se:uu:diva-251620OAI: oai:DiVA.org:uu-251620DiVA: diva2:806846