REFERENCE

It is clearly understood that nuclear wastes don’t go away, that the time required for its isolation, and thus for assessing the safety, is extremely long and that the organic environment has to be protected at all times. From all thinkable alternatives, the geologic disposal has been proven to be the most suitable to provide safe isolation of the long-lived nuclear wastes.

In Finland, the site selection research programme has been conducted from 1983 until 2000. The Olkiluoto site has been selected for deep geological disposal of spent nuclear fuel in 2001. Extensive geophysical data and imaging have been used to produce and refine a 3D model of the site. An underground research, testing and demonstration facility, ONKALO, was built at Olkiluoto for detailed characterisation of the planned nuclear waste repository host rock.

Hardrock investigations target characterisation of crystalline bedrock features that may have any orientation and may display discontinuous and diffuse boundaries. The investigations for high level nuclear waste repositories must be focused on deep and relatively small volumes of rock: target investigation depths are larger than typical depth of a repository (500 m – 1000 m) and resolution of determinations needs to be better than the typical transverse size of relevant site features (1-2 m). Seismics is the only geophysical method to accommodate both requirements.

The Millennium uranium deposit is located within the Athabasca Basin, in northern Saskatchewan, Canada. In 2007, as part of a prefeasibility study for potential mine development, a seismic program consisting of a 3D surface survey, vertical seismic profiling, moving source profiling, and side-scan surveys was undertaken to map the complex geology. The geometry and resolution of these different seismic surveys allowed for direct imaging of the geologic targets of interest, regardless of orientation and size. After integration with drill-defined geology, the program successfully imaged the location and character of the unconformity, the post-Athabasca structural setting at camp and deposit scales, and the alteration around the deposit. This information increased the understanding of geotechnical aspects of the geology hosting the deposit, and is currently being used to help minimize risk and costs associated with mine development. Seismic surveys are now viewed as an integral part of risk reduction associated with mining in the Athabasca Basin. (Wood et. al., Geophysics, vol. 77, 2012)

Reflection seismic has been used for the mapping of a coal deposit and associated structural setting. An area of approximately 4500m x 5000m was covered by the survey using roll-along geophone arrays and a mechanical VIBSIST 1000 source.

The purpose of the work has been to acquire and process multi-lines 2D seismic data, which would allow the mapping of the coal seam, believed to be at a depth of 200m-500m, the evaluation of the thickness of the coal seam, and the identification of normal faults with a vertical displacement larger than 5m-10m.

To obtain a better understanding of the properties of the disturbed zone and its dependence on the method of excavation, ANDRA (France), UK Nirex (UK) and SKB (Sweden) performed a joint study of excavation zone effects. The project, named ZEDEX (Zone of Excavation Disturbance EXperiment), is expected to contribute to the basis for selecting or optimising the construction method or combination of methods for a deep radioactive waste repository and subsequent sealing.

The fieldwork for the ZEDEX project was carried out at SKBs Hard Rock Laboratory (HRL) at Äspö, Sweden, in two parallel tunnels, one excavated by Drill & Blast and the other by Tunnel Boring Machine. The ZEDEX site at Äspö is located at 420 m depth below ground surface, in granite.

The objectives of the study have been the following:

1. To estimate the transverse extent, the seismic and elastic properties of the near field disturbed zone and of the transition zone from near field to far field,
2. To determine the relation between the seismic properties and the local stress,
3. To determine the seismic response of the rock features displaying outstanding rock mechanic and hydraulic properties, induced by the excavation of the drifts.

The CO2SINK project has been the first onshore European CO2 injection experiment. The seismic characterization of CO2 storage at Ketzin consisted of several investigation and monitoring elements covering different experimental scales mutually validating and potentially complementing each other. The objective has been to cover the kilometer scale encompassing the region where the CO2 can migrate, while resolving at a meter scale the potential heterogeneities of the aquifer in the vicinity of the injection site. The crosshole measurements were meant to cover the smallest scale of all measurements performed, between wells approximately 100 m apart. Three wells were drilled, one serving as the CO2 injection well and the other two as observation wells. Crosshole tomographic surveys were repeated between the observation wells located at 50m and 120m from the injection well in order to provide a high-resolution model for the reservoir near the injection and to observe the change of seismic velocities between the observation wells. The baseline measurements were done in May 2008, followed by three time-lapse repeats in July and August 2008 and in July 2009.

RD&D activities for structure characterization ahead and around tunnels are ongoing, with focus on the safety assessment for engineering and mining applications. With hard rock nuclear waste repositories, brittle deformation zones and large fractures are considered to pose a potential risk for the mechanical integrity of the spent fuel disposal canisters. These are to be avoided in positioning of the deposition holes that will host the canisters, and they need to be identified during construction of the deposition tunnels and ultimately the deposition holes. Results from two high resolution seismic surveys carried out in ONKALO (Finland) and Äspö HRL (Sweden) are presented here, providing continuity information for several large fractures identified through geological mapping to cut the tunnels and/or boreholes. These were recognizable in transmission and reflection images produced from measured seismic data sets. We show that reflection seismic surveys are relevant to the detailed characterization of crystalline bedrock. Relatively small-scale features, even single fractures, can demonstrably be detected. On the other hand, the detection of some distinctive features, even large-scale, can be uncertain if the survey layout is spatially constrained. Combinations of borehole and tunnel measurements using measuring arrays with diverse orientations helped reducing the location ambiguities and should be used in the future wherever possible.

Structural and lithological delineation is seen as an emerging mining application of seismic imaging. The general benefits expected from applying delineation techniques include: reduced economic risk, shorter project time-lines, and more accurate resource evaluations. The structural aspect has been documented relatively well in the past decade and consists mainly of mapping faults, fracture zones, and dissolution features, bearing an impact on geotechnical models and designs. Mineral resource delineation by seismic techniques is a comparatively recent addition to the seismic application menu and is seen as a promising technique for the coming decades.