Aeromagnetic mapping of deep-weathered fracture zones in the Oslo Region – a new tool for improved planning of tunnels
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pp. 253-267

A new method referred to as the AMAGER Method (AeroMAgnetic and GEomorphological Relations) has been developed to map the occurrence of deep clay alteration in the bedrock of the Oslo Region. Rock instability and water leakage have caused significant problems during tunnel construction in this particular area of southern Norway. Structural weakness zones contain clay minerals, including smectite and kaolinite, and are to a large extent the result of chemical weathering of pre-existing fracture zones during sub-tropical conditions in the late Triassic, Jurassic and Early Cretaceous. The weathering penetrated deeper into the fracture zones and was preserved below Late Jurassic and Cretaceous sediments. Subsequent exhumation removed most of the deep weathering, however clay zones occurring at depths of 200-300 metres were preserved. Deep weathering produces negative magnetic anomalies because ferromagnetic minerals such as magnetite are altered to less magnetic hematite and ironhydroxide minerals. In situ measurements at four different locations show that the magnetic susceptibility in the weathered zones is reduced by 35-93 % compared to fresh rocks. We have developed a filtering technique to enhance the magnetic signal from the weathered zones. Coinciding negative lows in the high-pass filtered topography/bathymetry and magnetic data are used as indications of deep weathering. The resulting signal is classified as ‘probable’ or ‘possible’ depending on the signal to noise ratio. The new Amager method has been able to detect more than 90 % of the known fracture zones inside the Lieråsen and Romeriksporten railway tunnels and the Hvaler road tunnel. Modelling of the observed magnetic field suggests that some of the low-magnetic zones continue to a depth of c. 300 metres below the surface. Engineering geologists have consequently been presented with a new tool that can facilitate the mapping of potential clay-bearing weakness zones in tunnel planning. We will, however, recommend that an experienced geologist or geophysicist is at hand to ensure that conditions necessary for using the method are satisfied before the results are applied in planning of new tunnels.

O. Olesen, J.F., Dehls, J. Ebbing, O. Kihle & E. Lundin, Geological Survey of Norway, Leiv Eiriksson vei 39, N-7491 Trondheim, Norway;
H. Henriksen, Sogn og Fjordane University College, Faculty of Engineering and Science, P.O. Box 133, 6851 Sogndal, Norway.
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