Geophysical surveying for cave detection below a concrete slab in a tunnel railway

In 1998 the Spanish National Railway Company (RENFE) proposed a geophysical research in the tunnel located near Gobantes in the Madrid – Malaga railway line. This tunnel has a total length of 1690 meters, and a concrete ring covers it with a mean thickness of 60 cm. The tunnel structure is designed to support dissolution caves in the nearest 10 meters gypsum rocks up to seven meters of size. The main objective of this work is to detect and locate natural caves in the tunnel closeness by means of geophysical surveying. Classical resistivity and electromagnetic methods were rejected, due to the shield effect derived from metallic core of the tunnel’s ring. Seismic method was discarded too because of the high P-waves propagation velocity of the concrete slab. Finally we selected two geophysical methods due to the length of the tunnel and the short period of time to take the measurements because of the train traffic: gravimetry for an extensive application and GPR to carry out detail profiles in the critical zones.