HR: 14:20h
AN: NS53A-02    [Abstracts]
TI: Reflection seismic imaging of a hydraulically conductive fracture zone in a high noise area, Forsmark, Sweden
AU: * Juhlin, C
EM: Christopher.Juhlin@geo.uu.se
AF: Uppsala University, Dept. of Earth sciences Villavagen 16, Uppsala, 75236, Sweden
AU: Stephens, M B
EM: Michael.Stephens@sgu.se
AF: Geological Survey of Sweden, Box 670, Uppsala, 75128, Sweden
AU: Cosma, C
EM: calin.cosma@vibrometric.com
AF: Vibrometric Oy, Jaakonkatu 2, Vantaa, 01620, Finland
AB: High resolution reflection seismic methods have proven to be useful tools for locating fracture zones in crystalline rock. Siting of potential high-level nuclear waste repositories is a particularly important application of these methods. By using small explosive sources (15-75 grams), high resolution images of the sub-surface have been obtained in the depth range 100 m to 2 km in Sweden, Canada and elsewhere. Although ambient noise conditions in areas such as the Fennoscandian and Canadian shields are generally low, industrial noise can be high in some areas, particularly at potential sites suitable for repositories, since these are often close to existing infrastructure. In addition, the presence of this infrastructure limits the choice of sources available to the geophysicist. Forsmark, located about 140 km north of Stockholm, is one such potential site where reflection seismics have been carried out. Existing infrastructure includes nuclear reactors for power generation and a low- level waste repository. In the vicinity of the reactors, it was not possible to use an explosive source due to permitting restrictions. Instead, a VIBSIST system consisting of a tractor mounted hydraulic hammer was used in the vicinity of the reactors. By repeatedly hitting the pavement, without breaking it, at predefined sweeps and then stacking the signals, shot records comparable to explosive data could be generated. These shot records were then processed using standard methods to produce stacked sections along 3 profiles within the reactor area. Clear reflections are seen in the uppermost 600 m along 3 of these profiles. Correlation of crossing profiles shows that the strongest reflection (B8) is generated by a gently east-southeast dipping interface. Prior to construction of the reactors, several boreholes were drilled to investigate the bedrock in the area. One of these boreholes was located close to where two of the profiles cross. Projection of the B8 reflection into the borehole shows that the reflection corresponds to a hydraulically conductive fracture zone that was intersected at about 320 m depth.
DE: 3025 Marine seismics (0935, 7294)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly