Geological and geophysical investigations for the ROGFAST project

NGU Rapport 2006.076

In cooperation with "Statens vegvesen Region vest" and "Statens vegvesen Vegdirektoratet", the Geological survey of Norway (NGU) has performed research where new methods in tunnel planning were tested in the ROGFAST project.

Analysis of the new airborne magnetic data, petrophysical data, digital topography/ bathymetry and geological follow up work have revealed new information about the bedrock geology in the Boknafjord area. Results from digital structural analysis have already been used in the planning of seismic follow up studies. Magnetic dislocations coincide with lineaments from bathymetric studies, and seem to give information on the most pronounced faults in the area. The depth extent of magnetic bodies at the seabottom between Randaberg and Kvitsøy are calculated from aeromagnetic data.  Geological follow up studies discovered outcropping rock of the Karmøy ophiolite complex at the islands between Randaberg and Kvitsøy. North of Kvitsøy lack of exposures and magnetic signature made it difficult to reveal new information. A new geological map with profiles is produced introducing a thrust plane at a few hundred metre depth between Randaberg and Kvitsøy. A major NE-SW trending fault zone is proposed just north of Kvitsøy. To avoid unexpected problems during the construction work, a vertical drillhole from the island Alstein, and a long drillhole along the tunnel trace from an island north of Kvitsøy are highly recommended.