EAGE/DGG Workshop on Mining and Tunneling Geophysics

The Tunnel Seismic While Drilling (TSWD) method has been developed and applied at several tunnel sites to predict the geological situation ahead of the tunnel face during mechanical tunnel driving.

When tunneling with a Tunnel Boring Machine, the vibrations of the drilling head, resulting from the cutting process, offer to be employed as a seismic source signal, ensuring a continuous seismic monitoring without hindering the drilling and driving operations.

With the appropriate signal processing the continuous monitoring data can be converted to conventional seismic traces from which relevant fault zones within a geophysical forecast window of up to 100 m ahead of the current tunnel face can be predicted.

Since the implemented instrumentation, data transfer and logistics guarantee processing on a daily basis, significant geological structures can be observed over long distances.

The TSWD-method gives excellent continuous seismic data, from which deeply incised valleys, karst cavities, fault zones and other unexpected degradations of rock quality can be predicted. Wider fault zones over a thickness of 10 m can be successfully resolved, smaller fault zones are largely detected, depending on seismic impedance contrast and the position relating to the tunnel axis.

To image the rocks and fracture zones between the tunnel and the surface, a surface-tunnelsurface seismic experiment was conducted at the Äspö Hard Rock Laboratory (HRL) in southeastern Sweden. Seismic data were acquired using 333 vertical geophones on the tunnel side, and a three-component MEMS-based seismic landstreamer. Surface part was covered with 75 wireless seismic recorders, enabling simultaneous seismic wavefield sampling on all tunnel and surface seismic receivers. This acquisition geometry enabled excellent data coverage for first arrival tomography, with the velocity model showing good match with known fracture zones intersected in the tunnel and indicating new ones previously unknown. A more focused analysis was conducted along a major fracture zone consisting of three subsets with different hydraulic conductivity intersected in one portion of the tunnel. Here, Vp/Vs, Poisson’s ratio and P- and S-wave seismic quality factors were calculated, with low values of all parameters obtained for the none or low hydraulically conductive fracture sets, while highly hydraulically conductive fracture set shows significantly higher values of all parameters. The studies shown indicate the used acquisition geometry and data handling potential for seismic fracture characterization and show its applicability for different purposes, particularly mining and other subsurface studies.

Synthetic Aperture Radar (SAR) satellites acquire images of the Earth’s surface by emitting radar signal that are captured again by the satellite. By processing these signals, the evolution of the surface deformation caused by underground construction operations (e.g. tunneling and other mining operations) can be monitored worldwide.

This technology allows for the monitoring of all ranges of motion (millimetric, centimetric and metric magnitudes) over extremely large areas (e.g. 50 km × 50 km) with a high density of measurement points.

The main benefits of this monitoring technology are: (1) it provides millimetric precision in the measurements, (2) it is very suitable for restricted sites since in situ measurement are not necessary, (3) all key elements of the mine site can be monitored at the same time: open pit slopes, mine heaps, access roads, etc. and (4) it feeds geomechanical models improving their forecasting capabilities.

Furthermore, historical analyses (using satellite data archived since 1992) provide a unique opportunity for studying the surface ground motion occurred in the past. This information is highly interesting in the planning phase of any construction operation and for providing (through the analysis of time series) changes in the deformation rates of the area of interest. In order to illustrate the applicability of this technology, different case studies related with tunnelling (applied to linear infrastructures) and mining activities are presented.