Several zones of the northern part of Barcelona City have old water mining galleries located in the upper part of the granite formation. These galleries are now out of use and their localization are unkown. This fact involves some problemes when foundations are built near or over them. Usually geothecnical or geophysical prospecting are only carried out to detect these underground galleries when foundation has been already built and, therefore, geophysical prospecting is more difficult and hard to carry out. The presence of parts of the totally of the building foundation makes very difficult to apply some of the geophysical prospecting techniques like some electromagnetic ones. GPR prospecting surveys are one of the few techniques that are little influenced by near undergound objects, only being affected by down metallic objects or water cables. In this work we present an example of a georradar survey made in Barcelona when building foundation was already made.