Aeromagnetic surveys have in the past mainly been used for mapping depths to magnetic basement and igneous units in sedimentary basins. NGU has since 1994 acquired high-resolution aeromagnetic surveys and has also revealed the existence of significant magnetic anomalies arising from sedimentary layers. We have recognized that susceptibility measurements on core samples, hand specimens and in situ on bedrock exposures are essential for the interpretation of these anomalies. Petrophysical data (magnetic susceptibility and remanence) of 40.000 rock samples from the Norwegian mainland and offshore wells and drill holes have been acquired in order to constrain the interpretation of the aeromagnetic data.
Sub-cropping Late Paleozoic to Tertiary sedimentary units along the Trøndelag-Nordland coast produce a very distinct anomaly pattern. The asymmetry of the anomalies, with a steep gradient and a negative anomaly to the east and a more gentle gradient to the west, relate the anomalies to a strata gently dipping westward. The susceptibility measurements on Sintef’s cores indicate that these coast-parallel anomalies are caused by 1) alternating beds of sandstone and claystone/siltstone/mudstone [mean suscept. 0.00013 and 0.00025 SI], 2) siderite-cemented sedimentary rocks [mean suscept. 0.00135 SI], and 3) sedimentary units containing detrital Fe-Ti-oxides [suscept. 0.00100-0.01000 SI]. Negative anomalies are caused by low-magnetic gypsum, anhydrite, salt or coal [mean suscept. 0.00007 SI]. Recent aeromagnetic surveys in the Barents Sea have also revealed distinct negative magnetic anomalies clearly associated with salt diapirs.
Buried Quaternary channels incised into the underlying strata are often observed to generate magnetic anomalies in the North Sea. Susceptibility data obtained from deep groundwater drilling in Denmark have revealed that analogous anomalies are caused by gravel and boulders consisting of typical basement rocks from the Fennoscandian shield. The average susceptibility of the infill is 0.00036 SI while the underlying Miocene shales are 0.00007 SI. The small contrast is sufficient to produce magnetic anomalies in the order of 1-2 nT.
Remapping of the oceanic crust has also improved our under-standing of the Tertiary opening of the North Atlantic. Tectonic reconstruction using the new aeromagnetic datasets has shown that the opening of the NE Atlantic between the Jan Mayen and Senja fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies and without jumps in spreading axis Transfer zones have previously been associated with oceanic fracture zones along the Mid-Norwegian and East-Greenland margins. Transfer zones are important entry points for sedimentary drainage systems, a relationship that has also been suggested for the transport of Cretaceous sands to the mid-Norwegian margin. Our new interpretation can consequently have implications for evaluating the petroleum potential in the area.