On request of the Italian National Electrical Agency, the Company IDROGEO carried out a G.P.R. survey inside an old water-supply tunnel 14 km long belonging to an hydroelectric power plant located in the North East of Italy. The aim of the survey was the geo-structural investigation of the rock formations surrounding the tunnel with particular interest in the mapping of cavities and fractures associated to the water occurrences and circulation. A detailed investigation was also requested to detect the presence of voids at the concrete-rock interface. The tunnel crosses different rock formations belonging to the Alpine sequence with the presence of evaporitic formations affected by strong tectonic deformations. More than 7,000 meters of G.P.R. profiles were recorded by using a GSSI SIR 10 equipped with 100 and 500 MHz antennas with simultaneous data recording on two channels. The survey at 500 MHz .was aimed at the precise determination of the concrete thickness and at the detection of the voids at the concrete-rock interface, whereas the use of 100 MHz transducers permitted the detection of larger unconformities and cavities up to a distance of 15-20 metres. The identified structural elements were divided into 5 groups: - lack of contact and delaminations at the concrete-rock interface - geostructural elements - open fractures - voids and unconformities - honeycomb alterations The survey also permitted the location of some old artifacts whose position and nature were uncertain.


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