Analysis of modelled satellite magnetic data to explore lithospheric magnetization
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The increasing availability of magnetic satellite measurements in addition to aeromagnetic data, requires new tools and concepts for interpretation and modelling. Realistic forward modelling requires a precise definition of the spherical geometry of the magnetic sources. At satellite height, only long wavelengths of the magnetic anomalies are reliably measured and the spherical shape of the Earth has to be taken into account. For an improved modelling approach, a magnetic field calculation software was developed, which utilize spherical prisms, tesseroids, as magnetic sources. Induced magnetizations are then derived from the products of the local geomagnetic fields for the chosen main field model and the corresponding tesseroid susceptibilities. Remanent magnetization vectors can be directly set. As an extension to magnetic tesseroids software, an accurate finite difference approach was developed to calculate the magnetic field gradients at satellite height in local coordinates from magnetic field grids. To enable a combination of airborne and satellite-derived data a staged inversion method of spherical satellite-measured grids together with the short-wavelength airborne data was developed. The method uses magnetic field gradients for a large-scale inversion to diminish far-field effects from sources outside of the region, and edge effects due to spherical geometry. The airborne dataset of Fennoscandia was inverted to obtain a high-resolution tesseroid model of the shallow crustal layers. Magnetic susceptibilities obtained for these shallower layers via satellite gradient inversion are used as a-priori values for the airborne data inversion. The here developed tools and methods can be applied to reliably construct comprehensive lithospheric magnetization models from large scale magnetic studies that combine satellite and airborne data with different wavelength content. Application of this method provided new insights to the large-scale structures in Fennoscandia. The negative anomaly over southern Finland was found to be caused by a crust with a very low magnetic susceptibility. Major positive anomalies over Kiruna, Oslo and the Baltic Sea are attributed to the Transscandinavian Igneous Belt.