Fig. 4.11. Natural tracer in porewaters in the Opalinus Clay and surrounding formations. Best fit simulations of data for pure diffusion and constant concentrations in Keuper and Malm. Groundwater =groundwater compositions in under and overlying formations (Nagra, 2002b).

4.2.3. Crystalline rock environments

4.2.3.1. Lithology and structure

Determining the structure and lithology of crystalline rocks is more likely to be based on information obtained from boreholes, as seismic studies reveal less information of use to a site characterisation programme in such rocks. As indicated above, boreholes in crystalline rocks, at least in those where such rocks are present at the surface, are likely to be of smaller diameter than those in sediments and to be similar to the types of boreholes that would be used in mineral assessment programmes. Examples of

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investigations in such rocks are provided by the many characterisation programmes carried out by SKB in Sweden (for summary and listing of reports see Milnes, 2002),

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Posiva in Finland (e.g., Anttila et al., 1999 a,b; McEwen and Aikas, 2000), AECL in Canada (e.g., AECL, 1994), Nagra in Switzerland (e.g., Peters et al., 1986; Pearson et al., 1991) and JAEA in Japan (e.g., JNC, 2000).

The most important structures in crystalline rocks are the fracture zones, which are ubiquitous in all rocks and in all geological settings (sedimentary rocks also contain fracture zones and fractures, although in low permeability sediments at depth their significance is likely to be less than in crystalline rocks – see the comments on fractures in the Opalinus Clay above). It is easier, conceptually, to divide up the rock mass into different scales of fracture zones and to distinguish these from the rock lying between these zones, which will also be fractured but have a lower fracture density, than to consider the rock mass as containing a complete spectrum of fractures or discontinuities from very small to very large. Due mainly to their greater fracture densities, these fracture zones are, in many cases, more permeable than the rock lying between them and have a major control on the flow of groundwater. They are also likely to provide local areas of potential weakness in the rock mass, which is important from the geomechanical point of view when constructing underground openings and also from the point of view of long-term stability, when considering future movements in the rock mass due to seismic activity, loading by icesheets, etc. Fracture zones on the local scale, i.e., perhaps less than 1–2 km in length, need to be avoided when locating a repository and larger fracture zones need to be avoided when defining investigation areas. At an even smaller scale, small-scale fracture zones and larger single fractures need to be avoided when planning the layout of a repository.

Examples of the influence of fracture zones on the definition of investigation areas, on the control the groundwater flow, on the location of a repository and on its layout are provided by numerous reports from SKB, Posiva, Nagra, Nirex and AECL (e.g., Nagra, 1994; SKB, 1999; Posiva, 2003a; AECL, 1994). Figure 4.12 shows the ‘‘rock block’’ at Olkiluoto, Finland, defined by regional-scale fracture zones, inside which it is proposed to develop a repository for SF. Within such a rock block, there are localscale fracture zones which control the location and layout of the repository, as can be seen in Fig. 4.13; Figure 4.14 illustrates the influence of fracture zones on the location of the investigation boreholes. These can, in turn, be placed in different classes depending on the level of knowledge of their location, orientation and extent. Information on the rock lying between these fracture zones is provided again from boreholes and from interor cross-borehole geophysical and hydrogeological techniques.

Three-dimensional seismic surveys are less commonly carried out in areas of crystalline rocks than they are in sediments, although more conventional 2D seismic, VSP, horizontal seismic profiling (HSP) and cross-hole seismic surveys have been used in several radioactive waste disposal site characterisation programmes in such rocks, e.g., in Finland, Sweden and Canada (e.g., Enescu et al., 2003; Cosma et al., 2003). In fact, in the recent investigations in Sweden by SKB, 2D seismics has proved to be extremely useful in detecting fracture zones (e.g., SKB, 2005). It is generally less obvious in this type of rock what the reflectors might be and hence, when reflectors are detected, it can be more difficult to relate them to structures such as fracture zones, rather than to the bedding planes and to changes in lithology which are picked out in sedimentary rocks. In addition, the areas covered by such surveys are normally considerably smaller than in

characterisation and selection Site

Fig. 4.12. Major fracture zones defining the ‘‘rock block’’ in which it is planned to develop a repository at Olkiluoto, Finland (Posiva, 2003a).

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Fig. 4.13. Possible location and layout of a repository at Olkiluoto, Finland, at 400 m depth as determined by the position, orientation and expected properties of local-scale fracture zones (marked as R number) (from Posiva, 2003a).

Fig. 4.14. Boreholes KR1 to KR28 drilled in the central investigation area at Olkiluoto, Finland. Many of the boreholes have been located and oriented to intersect specific fracture zones. Grid is at 500 m (from Posiva, 2003a).

sedimentary environments; for example HSP surveys have been carried out up to several hundred metres from boreholes at Olkiluoto.

Geophysical techniques other than seismics are also generally of more use in crystalline rocks, at least where they are relatively close to the surface. Where crystalline rocks are overlain by thicker sedimentary sequences, possibly less use can be made of the majority of airborne and surface geophysical techniques for detecting fracture zones, and such techniques are perhaps of more use in defining broader features, such as the form of the basement-cover surface. The use of 3D seismic and cross-borehole techniques can,