Introduction Deep underground repositories

Ž . Journal of Applied Geophysics 43 2000 227–237 www.elsevier.nlrlocaterjappgeo Borehole radar measurements aid structure geological interpretations S. Wanstedt , S. Carlsten, S. Tiren ¨ ´ GEOSIGMA, Box 894, S-751 08 Uppsala, Sweden Received 2 October 1998; received in revised form 25 February 1999; accepted 20 April 1999 Abstract Successful site characterization for a repository of nuclear waste or underground construction in general provides basic data concerning engineering aspects of repository design with impact on both the efficiency of the repository in isolating Ž . waste and in constructing the repository. Three-dimensional 3D visualization of data is an essential step in the development of descriptive hydrologic and rock-mechanical models of fractured rock systems. Modeling of fractures and fracture zones in 3D is usually based on the correlation of fracturing in drillcores and on outcrops. Difficulties with this procedure arise when the vertical or horizontal separation between fractures is large. Directional radar surveys help decrease the uncertainty in correlation. At great depths, such as is the case when investigating potential nuclear waste repositories, the errors present as borehole deviations, and dip determinations of structures as well as the varying characteristics of geological features, may make interpretations virtually impossible. Tomographic radar measurements help improve the 3D modeling because zones with anomalous properties can be traced across the investigated plane or volume. This leads to a further decrease in uncertainty and eventually to better models. The comparison of directional reflection surveys and tomography shows that the accuracy of single-hole surveys is reasonably good. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Borehole investigations; Directional borehole radar; Tomography; Structural geology

1. Introduction

The objectives of a site investigation for a tunnel are to provide information for the assess- ment of the technical and economical advan- tages of alternative schemes, the selection of the most suitable alternative, the preparation of an adequate and economical design, and to allow difficulties that may arise during construction, modification or repair to be foreseen and pro- Corresponding author. Tel.: q46-18-65-08-00; fax: q46-18-12-13-02; e-mail: swgeosigma.se Ž . vided for West et al., 1981 . Potential nuclear waste repositories comprise an especially com- plicated kind of site investigation because they will be located at a depth of about 500 m below the ground surface. For obvious reasons, investigations com- mence at the surface with whatever information is available there. In every exploration opera- tion, the investigator eventually finds the situa- tion where he needs detailed information about features at depth. The only way to obtain this information, without excavating, is to drill a hole through the rock volume to be investigated. 0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 9 2 6 - 9 8 5 1 9 9 0 0 0 6 1 - 0 Drilling, regardless of the method employed, involves a sizeable expense in the evaluation of a site. Hence, it is important to extract as much information as possible from the boreholes. This paper deals with the difficulties in ex- trapolating geologic surface and subsurface in- formation when investigating large, deep vol- umes of rock and how such difficulties can be resolved, or at least decreased, with borehole radar measurements.

2. Deep underground repositories

The investigations prior to the siting of a repository for radioactive waste involve isolat- ing or detecting a volume of rock that fulfils some basic requirements. According to the Ž . Swedish Nuclear Waste Management SKB , the main objectives of the initial site investiga- Ž . tion are SKB, 1992 : Ø To define the site-specific position of a rock volume for a repository; Ø To plan the surface and subsurface facilities; Ø To provide a basis for a preliminary site- specific performance assessment; and Ø To provide information to ensure safe and efficient underground activities. The Swedish bedrock is usually highly com- petent with very low porosity. Hence, the frac- tures and fracture zones are fundamental con- cepts in the site characterization as the struc- tures control the groundwater transport and the mechanical stability. In other words, most of the work is concerned with determining the geome- try of these structures. A Swedish repository will be located in Pre- cambrian crystalline rocks. It will be a multi- barrier system intended to minimize the proba- bility of radioactivity escaping to the surface. At a depth of approximately 500 m, the storage will be protected from natural and human dis- turbance, supposedly maintaining favorable conditions for isolating waste without further human aid. The waste is to be enclosed in steel-coated copper canisters deposited in verti- cal holes about 1.5 m in diameter. A final full-scale repository might contain as many as 4500 canisters and thereby covering an area of about 1 km 2 . Fig. 1 shows a hypothetical reposi- Ž . tory about 10 of full-size placed at 500 m ¨ depth below the island Aspo. The three-dimen- ¨ Ž . sional 3D fracture zone model in the figure, which governs the layout of the facility, was developed from surface-based investigations of ¨ Ž . the Aspo Tiren et al., 1996 . A fracture has to ¨ ´ fulfil certain requirements to be included in the model, such as being possible to extrapolate and identify in more than one location, borehole or at the surface. The extrapolation of features was essentially performed with the directional bore- hole radar.

3. Extrapolation of geological data