Seismic Tomography and Monitoring in Underground Structures: Developments in the Freiberg Reiche Zeche Underground Lab (Freiberg, Germany) and Their Application in Underground Construction (SOUND)

Abstract

The construction of large tunnels and underground infrastructures faces increasingly large dimensions and complex geological conditions. Under these conditions, exploration techniques are needed which enable for a detection of potentially hazardous structures during construction. Seismic sensors, integrated into rock anchors, and small seismic signal sources using defined pneumatic impulses or sweep signals generated by magnetostrictive actuators are the components of an exploration system which can be easily integrated into different types of underground excavation work and which can also be deployed for the long-term monitoring of already existing tunnels or caverns. However, for a continuous acquisition of seismic signals during tunnel excavation, the strong and broadband signal generated by a tunnel boring machine (TBM) may be used as a continuously operating source. Within the collaborative project SOUND, the seismic equipment at the Underground Lab of the Reiche Zeche Research Mine in Freiberg (Germany) has been used for a tomographic monitoring study during the excavation of an inclined gallery. A synthetic, but realistic seismic data set was simulated using a randomly heterogeneous velocity model which can be regarded as a realistic prototype of the velocity distribution in the real Gneiss block. The simulated acquisition geometry has been derived from the actual source and receiver point distribution in the Underground Laboratory. It can be shown that the analysis of the modelled seismic data by full waveform inversion (FWI) was able to reveal the lateral heterogeneity of the velocity model with significantly higher resolution compared to traveltime tomography of the direct P-wave arrivals. The analysis of field data from the Underground Laboratory has shown that there are complex interactions in close vicinity to the receiver location, and before FWI can be applied to this real data set, source and receiver dependant signatures need to be removed by inversion and deconvolution. A further field experiment, performed during gallery excavation in the Underground Laboratory, has shown that the setup of seismic receivers in rock anchors and a sparse array of adaptive vibro-sources is able to detect subtle changes in seismic wave propagation related to stress changes due to the excavation of an inclined gallery. After the deployment in the Underground Laboratory, a field survey was carried out on a tunnel construction site. A broadband seismic data set, using the tunnel boring machine could be acquired providing a basis for high resolution imaging of structures ahead of the construction site and geotechnical characterization of the imaged volume.