ALBERT

Notes: Magsat data have been used to design scalar magnetic anomaly maps in an area that covers about half of Europe on both sides of the north-south directed European Geotraverse (EGT). The two maps refer to different altitudes and are intended to place recent compilations of aeromagnetic surveys along the EGT (Wonik et al. 1992) into global perspective.The presentation of the magnetic anomalies and their compatibility with the aeromagnetic data required (i) a suitable reference field for subtraction from the measured total field, which was derived from the field model M102089 by Cain, and (ii) a proper function of time reduction to the epoch 1980.0, which has been obtained from the actual satellite data. Because of the considerable noise in the data only those anomalies have been chosen for mapping, which show up in all Magsat tracks independent of altitude and epoch of measurement. The anomaly maps are based upon the less disturbed dawn track data, but a few dusk tracks have also been used as tie lines.The anomalies are interpreted in terms of a magnetization model, which divides the Earth's lithosphere in Europe into blocks of extension 1°N-S by 2°E-W and attributes to each block the ‘Depth Integrated Magnetization' of the magnetized layer, i.e. the magnetization integrated over the vertical extension of the block. Each block is represented by a dipole parallel to the inducing core field, and the calculated model field is adjusted to the anomaly maps by trial and error, starting with an initial coarse distribution that seems plausible from the anomalies and from local geomagnetic surveys. For a realistic model field in the EGT area a rather large portion of the Earth's magnetized lithosphere outside the area under investigation must be included in the calculation.In order to check, whether the features of the magnetization model are necessary for the interpretation of the field anomalies, they are compared with the distribution of magnetization that is obtained by the application of linear inverse theory.Prominent features of the model of crustal magnetization are the contrast between high values over the East European Platform and low values in the younger parts of Europe, maxima at the iron ore districts of Kiruna and Kursk, a broad minimum centred at St Petersburg, and only small differences between the Mediterranean Sea and the adjacent continental areas. There is a fairly good agreement of these anomaly maps with the corresponding maps presented by Cain et al. (1989a).

Notes: The Fourier transform of a square-shaped section of a magnetic survey, digitized in a square grid, forms a rectangular matrix of coefficients which can be condensed to a series of average amplitudes dependent only on their frequency and no longer on the direction of the respective partial waves. These average amplitudes together represent a spectrum which–when plotted in a semilogarithmic coordinate system (log amplitude versus frequency)–often shows straight segments which decrease with increasing frequency. By continuing the given field downwards these straight segments become horizontal at a certain depth, the so-called “white depth”. This white depth may be used as a first estimate for the depth of magnetic sources producing the respective part of the field. It is shown that the sources which correspond to such use of the white depth can be expected to be “randomly distributed with some positive autocorrelation”.As an example for such a depth estimation the interpretation of the aeromagnetic survey of NW-Germany by a relief in 8–16 km depth is given. The relief divides the subsurface in an upper nonmagnetic layer and a lower layer with magnetization M= 2 Am−1.