Original paper

On the use of small amplitude magnetic anomalies for the improvement of geological models: case studies from Northern Germany

Skiba, Peter; Gabriel, Gerald; Krawczyk, Charlotte M.; König, Matthias; Bücker, Christian; Rolf, Christian

Abstract

Aeromagnetic data from reconnaissance surveys are routinely used for geological mapping and modelling, although high-resolution aeromagnetic data are only rarely acquired. In this study, with investigation areas located in NW Germany, two datasets of different resolution are analysed in comparison. The first dataset was acquired in the 1960s at 700 m altitude whereas the second dataset was acquired in 2004/2005 at 320 m terrain clearance. From a qualitative and quantitative analysis it can be concluded that both datasets contain similar information about local geological structures, i. e. even in the short-wavelength range. These local anomalies can contribute to the improvement of local stratigraphic and 3D structural models. The modern dataset reveals a better quality, especially in the short-wavelength range. The presented study is based on numerical data analyses techniques, rock magnetic analyses, and 3D forward modelling. The qualitative analysis of both datasets reveals the same overall pattern of small-sized anomalies with wavelengths below 10 km and amplitudes below 20 nT spread over large parts of the Lower Saxony Basin. They can be associated with known faults as well as subcrops and outcrops of Cretaceous and Jurassic layers. Quantitative analyses in terms of filter techniques enhance these local anomalies, but are partly affected by the limited quality of the older data. Two local sites in the Lower Saxony Basin, that are characterised by short-wavelength anomalies of small amplitudes, are chosen for more sophisticated quantitative studies by means of 3D forward modelling. The first case study results in a 3D model of a salt dome. The observed magnetic anomalies require modifications of the initial stratigraphic model. Rock magnetic investigations of field and core samples help to improve the model. They indicate a weak but measurable magnetisation of Cretaceous and Jurassic rocks in general and slightly increased values in iron-bearing formations (so-called "Trümmererze"). The second case study is located in the vicinity of the Weser Uplands in the southern part of Lower Saxony, hence in an area that was affected by basin inversion. From 3D modelling it is found that thin, magnetised Jurassic layers must be assumed to fit the observed magnetic anomalies.

Keywords

magnetic anomaliesgeological model3d magnetic modellinglower saxony basintrümmererze