Detection and Characterization of Fracture Zones in Bedrock - Possibilities and Limitations

In Norway, resistivity measurements have already been tested in marine environments in order to detect subsea fracture zones. However, most of these data have been processed without taking into account the special conditions the presence of seawater creates. More recent studies worldwide have also applied ERT in marine conditions, but under more favorable conditions nevertheless since they dealt with brackish water of considerably higher resistivity than pure seawater. This study summarizes our efforts to establish basic rules when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock, a problem engineers in Norway usually come up against in tunnel construction sites. The scope for this study is related to the construction of a sub-sea tunnels and the potential application of ERT to detect fractured zones as part of the geotechnical study. Our results indicate that ERT surveys for fracture zone detection in Norwegian marine environments can be promising under certain conditions but at the same time ambiguous since they suffer from reduced resolution and major artificial effects. Based on the modeling results, we were able to improve interpretations of ERT measurements made across the straits at Kvitsøy and plan further investigations in southern Norway.