Association of different geophysical methods for detection of old quarries

The towns growing extension leads to build in some area where, in the past, were underground quarries working. These quarries are closed today. Bordeaux and its area are in this case. The north side of Garonne, Entre-Deux-Mers and the both sides of Dordogne were exploited trom the XVIIth century and had been abandoned in XIXth century. These old quarries, are often unknown, induce many risks by their instability because of pillars that are breaking down. During last years, some galleries had collapsed causing many disorders to habitations and roads They are subject to an inventory and watching by the Bureau des Carrières of Gironde Departemental Council, since 1978. Without records or acces, the indirect methods are the only solution to detect them. As a rule, many geophysical methods are usable: gravimetry, refraction et reflection seismic technics, resistivity and conductivity methods, ground penetrating radar .... But the choice of some methods depends of economical and practical attentions. Among those, environmental geology is an essential one. The detectability of old quarries will use physical properties contrast between rocks and air-tor water). The Calcaire à Asteries layer was used to build Bordeaux and more of near towns and villages. It is a biocalcarenite and calcirudite (packstone) within the lower Oligocene (stampien). lts thicknessis about 5 to 20 meters. This layer shows a resistivity of 150 -300 ohm.rn, 3-6 mS/m. The mean velocity is about 1500-2000 mis. This layer overlies the lower Oligocene, composed of marls, molasses and clays. These two layers are variable horizontaly and verticaly also. This variability must be considered for the choice and interpretation of geophysical survey.