Action map for tunnel planning, Oslofjord - Telemark region: Mapping deeply weathered weakness zones
Joints and fractures were weathered during a sub-tropical climate regime and thus may contain smectite and kaolinite. The presence of such minerals prohibits groundwater flow in fracture and fault zones. The clay-bearing zones may cause mechanical problems during both tunnel construction and later operation. Due to the chemical alteration of magnetic minerals during weathering, weak zones are characterized by negative magnetic anomalies.
NGU has been acquiring airborne data in the Oslofjord area from early eighties until 2011. Some of the airborne surveys were performed by NGU itself and some of them were contracted to surveying companies e.g. Fugro, SGU. These airborne data were presented in various reports and stored as separate grids and databases for individual surveys. Later, it seemed necessary to compile this data together and present them as a single grid data.