Prior to the excavation of a 4 km subsea tunnel in the Precambrian Iddefjord Granite of southeast Norway, major fracture zones were located by use of aerial photographs and seismic techniques. When encountered during tunnel construction the majority were found to be of low transmissivity and to be filled with secondary clay minerals. Water inflows to the tunnel tended to occur through lesser, simpler fractures not detected by the preliminary investigations. This finding is supported by experiences from other tunnels and implies that the identification of major fracture zones by geophysical and remote sensing techniques may not always be a satisfactory method of locating groundwater resources in hard rock aquifers.

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