Tomography of the subsurface - from sounding to real time monitoring
The intention pursued in this project is to investigate the geological-geotechnical setting in the near field of underground constructions by using seismic tomography. The findings shall lead to an improved risk assessment and reduced costs during the constructional phase of underground development projects as well as enhanced long-term security after constructions have been brought into service.
Worldwide the development of the underground increases, leading continuously to enlarged construction projects, often in complex geological environments. A widely known example is the Gotthard Base Tunnel (GBT) that is currently drilled though the Alps and will soon be the world‘s longest tunnel with a route length of 57 km. The GBT will constitute an important link in the Trans-European Transport Network, bringing the North and the South of Europe closer together. However, the Alps constitute a highly complex geological environment with steeply dipping lithological units that underwent multiple faulting and folding resulting in many planning challenges. Just as the GBT, many other large-scale projects are currently planned or under development at geologically complex sites. To evaluate the geological-geotechnical conditions fully in advance is not possible at these sites. Thus, the GFZ investigates the possibilities of seismic exploration of the tunnel near-field and in front of the tunnel head face.
In the framework of this research topic the Integrated Seismic Imaging System (ISIS) was developed at the GFZ. The modular structure of ISIS enables us to optimally adapt the system to the specifics of different underground excavations, especially tunnelling. So far the system has been applied to tunnel construction sites in Germany, France, Scotland and Switzerland.
During the next field survey with ISIS at a construction site with TBM excavation - that will take place during 2011 - the noise of the cutting wheel will be continuously measured over a time period of several weeks. An optimized seismic broad band acquisition as well as monitoring and the further inversion of this dataset shall lead to a fast high-resolution characterization of the tunnel surroundings. These steps will be checked against the interpretation methods used so far. The latter, however, are also brought forward so that the geological-geotechnical interpretation of the seismic results can be automated as much as possible. Thus, information needed in real- time can be prompted to the engineers on-site much faster.
The research of our partners at the Karlsruhe Institute of Technology (KIT) deals mostly with the modelling and evaluation of seismic datasets to investigate the excavation damage zone (EDZ). This zone of disruption around a tunnel develops as a consequence of the excavation process and is of major importance to tunnel stability calculations.
New concepts for monitoring and mapping the EDZ with seismic methods are developed. This requires the construction of realistic models and the simulation of the wave field propagation in the respective models. In this sense Fig. 2a shows a model comprising an EDZ around a tunnel and a lithological boundary dipping in an acute angle to the tunnel axis.