Cross-scale rockmass characterization by high resolution seismic imaging around tunnels

Abstract

Reflection seismic surveys are relevant to the detailed characterisation of crystalline bedrock. Relatively small-scale features, even single fractures, can demonstrably be detected. Integrated analysis and modelling of geological, hydrogeological and geophysical data is instrumental in the interpretation of reflection seismic data. This requires the inference of the nature of the reflectors, e.g. lithological contacts or deformation zones and resolving the orientation ambiguity of the reflectors. Brittle deformation zones and large fractures may be heterogeneous in their detailed structure, making their characterization a challenging task. Several examples of structure characterization ahead and around tunnels are presented, with focus on the safety assessment for engineering and mining applications. Seismic imaging techniques adapted for tunnels and galleries are presented with examples from high resolution seismic surveys carried out at ONKALO (Finland) and Äspö HRL (Sweden). Studies performed over a span of several years demonstrate that large fractures identified initially through geological mapping, can also be found, positioned and characterized by seismics at the small scale of tunnels and/or boreholes. We show that reflection seismic surveys are relevant to the detailed characterization of crystalline bedrock. 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 reduce the location ambiguities and are recommended to be used in mining and tunneling rock characterization surveys wherever possible.