Characterization of fractures by GPR in a mining environment

Ground penetrating radar (GPR) is a suitable tool for the detection and location of fractures in resistive rocks, for example, limestone, salt and granite. It is commonly used in quarries and mine excavations (Dubois 1995; Grasmueck 1996; Dérobert & Abraham 2000; Halleux et al. 2000). The determination of the fracture (opening and filling material) cannot be directly deduced from the radar profiles. It requires a more detailed analysis of the radar signals. Recently, radar signal analysis has become an innovative research topic for the characterization of interfaces and layers (Fechner & Yaramanci 1996; Al-Qadi et al. 2000; Olhoeft 2000; Zeng et al. 1995). In order to characterize thin layers (thinner than the resolution), an inversion method based on the frequency content analysis of the radar reflection was developed (Grégoire, 2001a,b). This method is based on a comparison between the real reflection coefficient and a synthetic reflection coefficient. The reflection coefficients corresponding to field data are calculated using a reference signal. In this paper, some examples are given of the efficiency of GPR for the location of fractures of various scales (millimetric scale up to decimetric scale) in a limestone quarry and in an operating potash mine (K+S Group in Germany). In this potash mine, GPR investigations are regularly carried out to detect new fractures or the further extension of existing fractures by the exploitation itself (excavation of galleries and drifts, the use of explosives, etc.). Their location is of great importance for the safety of the miners and of the equipment. Safety measures, such as the installation of anchors, can be taken in time. The inversion method was tested by experiments with controlled fracture openings using a 1 GHz radar antenna. Then it was applied to radar data registered in the potash mine to estimate the openings of open fractures present in the roof of the gallery.