A new approach in describing and interpreting of the properties of the fractures and fracture zones in radioactive waste disposal environment

This report describes a new approach in describing and interpreting properties of fractures and fracture zones within a limited bedrock volume at OLkiluoto study site. The study has been done in co-operation with the Finnish Radiation and Nuclear Safety Authority. The dimensions of the bedrock volume under investigation was chosen to 500 x 500 x 200 m, including parts of boreholes OL-KR1, OL-KR4, OL-KR7 and OL-KR10. The starting point of this so called focussed modelling is that previous modelling results as well as data outside the limited volume will not be taken into account in interpretation and only the most informative and compact data sets are considered. In analysing the fracture zones, the most important data sets were fracture frequency (calculated from the detailed fracture database or from drill core examination), hydraulic conductivity, seismic P-wave velocity and electric resistivity of bedrock. For these parameters, certain 'alarm limits' have been defined and used in fracture zone interpretation. The observations have also been checked visually from drill cores. Furthermore, correlation of some data sets and results of interpretations have been examined, and attention is also paid in presentation techniques of fractures and fracture zones. In conventional bedrock modelling, fracture zones are conceptualised as plane-like geometrical units with spatially uniform properties. In highly metamorphosed and deformed surroundings, however, the structures may be faulted and formed by complicated networks of fractures, making plane-like conceptualisation difficult or even impossible. In this study the fracture zones are not brought forward as continuous, planar planes. Instead, attention is paid to describing fracturing and estimating possible crosshole connections on the basis of observed fracture properties, borehole radar and seismic VSP results. Bedrock within the modelled volume is typically slightly fractured, and fractures are generally gently dipping. On the basis of this examination, a total of 23 fracture zone sections were detected from the boreholes. Correlation of fracture zones between the boreholes proved in most cases to be impossible. However, some possible gently dipping connections could be located. (orig.)