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. Such zones are also generally marked by topographic depressions. The recognition of this relationship has led to a method involving the combined analysis of magnetic and topographic data to predict zones of deep weathering referred as AMAGER method. The first version of "Aktsomhetskart for Tunnelplanlegging, Østlandet" was published in 2006. A second version was published in 2012. In this and version 3 and 4 published in 2016, we have used AGC (Automatic Gain Correction) to the magnetic data to improve signals from areas having low/medium magnetic anomalies and the area has been expanded westwards to Telemark. The present report describes the production of the last maps.