Low-angel fracture zones in rapakivi granite at Hästholmen, southern Finland

Geological and geophysical borehole data from the site investigation programme for spent nuclear fuel in Finland are integrated to analyse the main engineering geological structures of the rock mass. At the Loviisa nuclear power plant site at Hästholmen, there are 100–150-m-long altered sections with anomalies in geophysical logs along the holes drilled in the orthogonally fractured rapakivi granite. In detail, the high water conductivity is concentrated in narrow sections as observed in the tube wave logs and hydraulic measurements. The TV image locates sporadic subhorizontal fractures that are most probably the main hydraulic conduits in the rock mass. The mean length of the measured fracture traces is 8 m. However, geophysical cross-hole investigations and seismic surveys indicate the continuity of a few fracture patterns between shallow boreholes drilled with spacing of the order of 100 m. Furthermore, the continuity of five gently dipping fractured structures that contain a few sections of broken rock between the 1-km-deep holes drilled approximately 0.5 km apart is suggested according to similarities in the acoustic character in the full waveform sonic borehole logs.