For a safe and efficient construction of underground structures (tunnels, galleries, caverns, etc.), it is essential to be provided with a most detailed exploration of the subsurface. Direct observations from boreholes can be ideally complemented with geophysical investigations. Seismic measurements have proven to be particularly appropriate. They are being applied successfully from the surface for shallow structures.
ISIS (Integrated Seismic Imaging System) is a method which enables the preliminary exploration of the rock mass in a tunnel. The technical innovation lies particularly in the fact that with this procedure, sounding explosions are no longer necessary and the system can be employed while actual tunnel construction work is running. Herewith, tunnel excavation will be safer, faster and more efficient.
In spite of all progress to date, tunnel construction is still a technical and economical risky undertaking. The biggest imponderability is that an exact pre-exploration of the rock mass in which the tunnel boring machine has to drill is essential. To investigate the rock material in front of the tunnel boring machine, seismic procedures are usually applied: a small explosive charge is ignited and the propagation of the explosion’s sonic waves are evaluated. For this purpose, the TBM has to stop operation and construction work is interrupted.
And this is where the ISIS comes to use. The ISIS allows for the forecasting of the geological characteristics of the rock mass in the surroundings of the tunnel excavation in advance and without disturbing the drilling procedures to any great extent. This is similar to ultrasound in medicine.
The idea is to use the tunnel anchor to install a measuring system for seismic three-component receivers with their antenna in such a way that a high-resolution seismic image of the rock mass during excavation is possible. Small earth microphones (geophones) serve as receivers, which are implanted in the pinnacles of the rock anchor. Herewith the different seismic waves can be supersensitively compiled. The data gives information on changes in the rock mass and eventually on water-bearing stratum.
The anchors are cemented in metre-deep boreholes. They can be fixed radial to the tunnel or in the direction of tunnelling. The seismic impulses are triggered with a pneumatic hammer or an electromagnetic vibration source, whereby the impulses radiate in the specified direction and can be repeated in intervals of seconds. And all this can take place during the ongoing tunnelling procedure.
The components of the Integrated Seismic Imaging System (ISIS) for exploration ahead of a construction are composed of:
• Seismic sources: a pneumatic hammer to generate a strong and repetitive impact signal and a magnetostrictive vibration source to generate a repetitive high resolution sweep signal.
• Seismic receivers: three-component geophones are fit into the tips of anchor rods.
• Processing and interpretation software: the ISIS software provides tools for seismic data processing, and for the integrated imaging and interpretation of the observed data and other geological and geotechnical parameters.
The seismic sources and receivers are constructed and manufactured by the geotechnical development workshop of the GFZ Potsdam. Visit www.gfz-potsdam.de
Since March 2000, the system has been used within several campaigns during the construction of the Gotthard base tunnel in Switzerland. It has also been applied in the tunnelling of the Glendoe hydropower tunnel at the Lough Ness in Scotland.
The development of the ISIS components and their application to measurements is supported by the following partners: Amberg Technologies, Christian-Albrechts-Universität Kiel, FH Brandenburg, Freie Universität Berlin, Herrenknecht, Institut für Geowissenschaftliche Gemeinschaftsaufgaben Hannover (GGA), and TU Bergakademie Freiberg. Visit www.amberg.ch, www.uni-kiel.de, www.fh-brandenburg.de, www.fu-berlin.de, www.herrenknecht.com, www.gga-hannover.de and http://tu-freiberg.de 25/08.
