Lateral variations of the mid-mantle conductance beneath Europe

Europe is a region with the largest density of geomagnetic observatories and several authors have used these data to estimate local geomagnetic response functions for various period ranges, typically of the width of 1.5 to 2.5 decades. By collecting the local response functions from 35 European observatories, and by their precise selection and subsequent combination, the independent regional geomagnetic induction data set could be extended to a period range of 4.5 decades. The initial local responses that were estimated by two magnetovariation methods, with two different external source fields employed, have been further supplemented by continental and global 11-year data, providing thus a data set extending over a period range from the harmonics of the daily variations up to 11 years. The combined responses have been inverted individually for each observatory by two techniques, by an Occam procedure and a stochastic 1D inversion for spherically symmetric Earth. The integrated mantle conductance has revealed rather regular lateral changes that have been used to design a mantle conductance image down to a depth of about 770 km. The presented conductance image can be correlated with major European tectonic units like the Baltic Shield and the Trans-European Suture Zone. To examine possible distortions to the inferred mantle conductance models due to large-scale near-surface heterogeneities, specifically those caused by the oceans, seas and large sedimentary basins, a spherical forward modeling was carried out for a radially symmetric conductor coated by an inhomogeneous thin shell with the variable surface conductance. The model responses for the 35 observatory positions were inverted in the same way as previously the experimental data. The results for 28 observatories have shown that the depth down to a pre-defined conductance level could be retrieved with a high accuracy of a few percents, but for seven southernmost observatories the recovery error increased up to 9%. With these seven observatories removed from the analysis, the effect of the seas and oceans on the upper and mid-mantle conductance estimates beneath Europe can be considered negligible.