Acoustic monitoring of underground instabilities in an old limestone quarry

The presence of abandoned and unstable underground cavities can give rise to a potential risk of surface collapse, particularly alarming when the presence of buildings is attested on the surface. The INERIS (Institut National de l'Environ-nement Industriel et des Risques, France) has carried out many studies, in recent years, investigating the effectiveness of several methodologies, devoted to monitor the evolution of the damaging mechanisms, reducing the risks of surface instability. Among the investigated methodologies, the acoustic monitoring has appeared a suitable technique to the detection of the damaging processes. During 2012 the Brasserie quarry, an old limestone mine located in the South-East of Paris (France) was instrumented with microphones to detect the acoustic waves generated by the collapse of blocks from the roof and the walls of the cavity. A series of tests were carried out with the aim of determining the propagation characteristics and the attenuation of the acoustic waves inside the complex geometry of the quarry. Preliminary data processing concerned the classification of the recorded signals: the analysis of the signal-to-noise ratio (SNR) evidenced that the quarry can be subdivided into three main areas on the base of different attenuation attitude. The presence of critical areas and paths influencing the wave propagation also emerged. The research is still in progress. Specific analyses will be carried out to deepen the aspects related to the source localisation, simulating the acoustic behaviour of the quarry to obtain maps of sound attenuation and reverberation times.