Characterisation of rock structures ahead of tunnel face feasibility study

A relatively unexplored aspect of geophysical data is the potential to obtain structural information ahead of a tunnel face. However, it is crucial for the safety of underground works. Several methods can be mentioned: GPR (Ground Penetrating Radar) provides high resolution images, but conductive materials as shale absorb the electromagnetic wave [8], resistivity methods [3] are now widely applied to engineering problems but they cannot provide azimutal locations. Seismie methods provide images with a smaller resolution but often provide useful information [4]. In this paper, we present results of an experimental study in a mine at a 300m depth. The method proposed is similar to Vertical Seismie Profiling (VSP) which has been widely used in the field of the borehole seismic. The major difference between these two methods is in the configuration of the survey line. In contrast with VSP, the borehole is horizontal and shotpoints are located in the tunnel face. A reflection tomography algorithm is then used because it can provide both velocity and reflector location.