3D Tunnel Seismics

Detection and characterization of fractures around tunnels and galleries

Rock characterization ahead and around tunnels, with focus on safety assessment, is instrumental for rock engineering and mining operations.

Posiva Oy is constructing a deep rock storage for final disposal of spent nuclear fuel at Olkiluoto, Finland. As the excavation progresses, R&D activities are conducted in the ONKALO facility, focusing on predictions of the bedrock and groundwater conditions of the site, during construction and after completion.

In Sweden, SKB carries out R&D work for detailed rock characterization at the Äspö Hard Rock Laboratory (HRL), the final repository for spent nuclear fuel being actually planned to be built at Forsmark.

Knowledge about existing network of fractures is important for the safety and feasibility of a final spent nuclear fuel repository.

Methodological advances and procedures developed in this environment can be of interest for other tunneling and mining applications.

Investigations at the Äspö Hard Rock Laboratory, Sweden

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Investigations at ONKALO, underground rock characterization facility, Finland

EAGE E-lecture: High Resolution 3D Tunnel Seismic Reflection at Olkiluoto, Finland by Calin Cosma

Very High Resolution EDZ characterization at ONKALO, Finland

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With detailed 3D tunnel investigations novel acquisition tools, measuring routines, processing techniques and interpretation approaches needed to be developed and put to test:

  • Highly accurate, time efficient and repeatable tunnel-wall and borehole seismic sources
  • Very diverse frequency, multi-component receivers (10 Hz - 40000 Hz) and rock coupling methods
  • Processing & Imaging techniques that work in a truly 3D environment, with target features displaying very diverse orientation and character
  • Interpretation tools able to verify and reciprocally validate results of various geoscientific disciplines.

The studies at ONKALO and the Äspö Hard Rock Laboratory (HRL) were primarily meant to:

  • Ensure that the bedrock is suitable for the final disposal. The desired result has been a model of rock features characterized by orientation, size and extent.
  • Fractures producing large seismic responses can generally be associated with high hydraulic conductivity but do not necessarily have large transverse dimensions. Within these studies, very small-scale features, even single fractures, could demonstrably be detected.
  • Brittle deformation zones and large fractures are generally heterogeneous in their detailed structure, making their characterization a challenging task. Bedrock characterizations comprised geology, geophysics, hydrogeology and geochemistry, in addition to seismics.