Detection and characterisation of underground cavities using high resolution seismic reflection (HRSR)

Old underground mining works are subjected to risks of collapse of which the assessment is a complex process. In the case of inaccessible underground mines, the difficult task for the risk assessment is to classify the already collapsed zones (no further risk) from still stable structures which may collapse in the future. The knowledge of the characteristics of the underground cavities and hosting rock mass can provide relevant information. However, accurate underground cavities detection and characterisation based on geophysical techniques are still a scientific challenge in the subsoil prospecting domain. Among these techniques, the high resolution seismic reflection is the most successful in the frame of underground cavities detection. The study presented in this paper, is a part of an extended research program aiming at prediction and controlling ground motions induced by underground cavities. The objective is to identify the most appropriate geophysical technique to localize and characterize the underground cavities at variable depths (several meters to 300 m). Thus, to meet these goals we have investigated the high resolution seismic reflection technique (HRSR). The selected test site is located close to Gréasque municipality in south France (figure 1). In this region, the underground works undertaken during the last two centuries consist of two mines: coal mine and cement stone quarries1 (marl limestone). The geological setting is characterized mostly by coal layers and massive limestone formations of the upper cretaceous with alternated coal strata of variable thickness dipping westwards.