Structural interpretation of the Hastholmen rapakivi granite

Geological and geophysical borehole data from the site investigation program for nuclear waste disposal in Finland are integrated to analyze the main engineering geological structures of the rock mass. At the Loviisa nuclear power plant site at Hastholmen, there are 100 - 150 m long altered borehole sections with long anomalies in geophysical logs along the nine deep holes drilled in the rapakivi granite. More 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 only sporadic rusty fractures that are most probably the main hydraulic conduits. The continuity of four near-horizontal zones between the l-km deep holes drilled approximately 0.5 km apart is suggested according to similarities in the acoustic character in the full waveform sonic logs. Altogether, the present bedrock model contains 28 hydrological fracture zones based on direct observations and interpretation of geophysical data.