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Magnetics of North Germany

This project looks at the spatial resolution and accuracy of regional anomaly data. These have increased considerably in recent decades, particularly with respect to aeromagnetic surveys. Based on two case studies, we have compared older measurements from aerial surveys carried out by Prakla-Seismos between 1965 and 1971, with a modern aerial survey carried out by Sander Geophysics in 2005 to look at information content in the short-wave range.

Findings

  • Short-wavelength anomalies (smaller than 5 km) exist in both datasets. In addition to more accurate positioning, the resolution of the anomalies is also better in the modern data.
  • Short-wavelength anomalies are associated with stratigraphic boundaries and where formations crop out on the surface. This correlates with well-known geology (e.g. outcrops of base Upper Cretaceous/Jurassic).
  • Field samples and samples from cores confirm the details in the literature: the total magnetisation in the upper Lower Cretaceous can be as high as 0.15 A/m. The remnant magnetisation of the rocks from the lower Upper Cretaceous (so-called 'Trümmererze') is roughly parallel to the present-day geomagnetic field.
  • 3D models of a salt dome as well as of a Jurassic outcrop provide good explanations for the observed anomalies in both surveys, after introducing a thin, higher magnetised horizon in the upper Lower Cretaceous, and/or higher magnetisation in the Upper Jurassic.
  • The stratigraphic interpretation of shallow lithologies can be improved by taking magnetic anomalies into consideration.
Findings from geomagnetic investigations. Upper left: strength of the induced and remnant magnetisation of 30 field specimens (Q: Königsberg Ratio). Upper right: strength of the induced and remnant magnetisation of 15 core samples. Lower left: direction of remnant magnetisation in the field specimens. Lower right: Direction of total magnetisation of the field specimens, calculated as a total vector of the induced and remnant magnetisation. The figure also shows the statistical values after Fisher (1953) (precision parameter k and semi-aperture angle alpha-95 of the 95% confidence interval cone).
Comparative magnetic 3D-forward modelling in a region with iron ore deposits. The long-wavelength anomalies (a) can be explained by known intrusions at depths of 6 km and 8 km (not depicted). The magnetisation in the Jurassic has to be increased to explain the short-wavelenth anomalies (b, c). In some regions, the limited resolution of the Prakla data becomes clear, which is due to data acquisition at larger altitudes and the larger gaps between flight lines (middle part of c).

Project-relevant literature:

  • On the use of small amplitude magnetic anomalies for the improvement of geological models: case studies from Northern Germany. - Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 167(2-3): 131-148; DOI: 10.1127/zdgg/2016/0064.
    2016, SKIBA, P., GABRIEL, G., KRAWCZYK, C.M., KÖNIG, M., BÜCKER, C. & ROLF, C.

Project Management

Dr. Gerald Gabriel

Prof. Dr. Charlotte Krawczyk (now at GFZ)

Project Scientist

Peter Skiba

Duration

2010-2011

Funding

Dea Deutsche Erdöl AG

Partner

Dea Deutsche Erdöl AG
(Dr. C. Bücker, Dr. M. König)