HR: 14:50h
AN: NS53A-04    [Abstracts]
TI: Seismic Characterisation of Fractured Hard Rock, Drawing from Experience at the Olkiluoto Nuclear Waste Repository Site, Finland
AU: * Cosma, C
EM: calin.cosma@vibrometric.com
AF: Vibrometric, #119, 372 Richmond St West, Toronto, ON M5V1X6, Canada
AU: Enescu, N
EM: nicoleta.enescu@vibrometric.com
AF: Vibrometric, #119, 372 Richmond St West, Toronto, ON M5V1X6, Canada
AB: The characterization of a deep-rock nuclear waste repository site requires an understanding of the structural geometry of the rockmass over a range of scales, from fractures identified in boreholes, to lower resolution, but wider coverage surface geological and geophysical data. Where complex fracture networks exist the inclusion of interpretations derived from VSP data, having an intermediate scale, leads to a robust structural model. VSP produces higher frequencies (up 200 Hz typically and up to 500 Hz occasionally) than surface seismic reflection and has the capability of estimating the 3D orientation of reflectors, including the sub-vertical. Seismic reflectors are typically assessed up to a 300-400 m lateral distance from the borehole and to depths exceeding 2000 m. The Olkiluoto rock mass consists of migmatised banded and foliated gneisses, cut by granite, amphibolite and metadiabase veins. The rock mass has undergone a polyphasic deformation, faulted and brecciated zones being related to the transition between the ductile and brittle deformation phases. VSP investigations were performed to determine the orientation and spatial continuity information of fracture zones and hydraulic conductive zones observed in boreholes. Fracture zone orientations were determined for typical trace lengths of 200 - 1000 metres, and a minimum thickness (set cut off limit) of two metres. Specialist VSP processing techniques, including 3- component Image Point Transform (IPT) (Cosma, 1995), polarization analysis and dip determination, were used to identify and map reflectors in 3D space. A verification procedure has been set to identify seismic reflectors in boreholes and attempt to correlate the geometrical and physical properties of the seismic reflecting features. Seismic features were found to be associated, not only with pronounced fracture zones, but also with lithological contacts. Consequently, reflectors without borehole control data may not directly be used as fracture zone indicators. However, once an identification procedure has been set, VSP has been found a useful tool for both structural and lithological modeling of crystalline rock. From a subset of 29 most proeminent features analyzed in a comprehensive interpretation exercise, 15 (50%) were clearly identified as fracturing-related and 14 (50%) were identified as combination of rock type/shearing and fracturing. Of the entire subset 24 (83%) were unambiguously explained. Without a firm explanation remained only one event and four of features although explained, had partly unconfirmed orientations. Boreholes are intersecting sub-horizontal features at optimal angles, making them better and more reliably represented in the site model. Sub-vertical events are somewhat more difficult to associate with the borehole data due to their less likely intersections with the boreholes, typically not being encountered in several boreholes.
DE: 0910 Data processing
DE: 0915 Downhole methods
DE: 0994 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly