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Source effect impact on the magnetotelluric transfer functions

It is known that the deviation from the plane wave assumption (the so-called source effect) has an impact on the results of vertical transfer function (VTF) estimation, especially for long periods. We observe the so-called seasonal effect, i.e. the VTF calculated from the data measured in the summer months is different from the VTF estimated from the winter months data. This is related to the length of the day, as in the diurnal data the effect of deviation from the plane wave is greater. In the present work, its potential effect on the estimation of the impedance tensor for magnetotelluric soundings is investigated. A unique, very long series of magnetotelluric recordings at the Belsk Magnetic Observatory was used for the analysis. The results showed that we do not observe the summer-winter seasonal changes as it is in the case of VTF. Small differences can be noticed as a result of TF estimation separately for diurnal and nocturnal data. The analysis of prediction errors confirms this finding and proves that the daily data are more distorted (distant with the plane wave). More detailed analyzes were performed by making a precise selection of data, dividing them into those that fulfill and those that do not fulfill the assumptions of the plane wave. The results show that the impedances estimated from data separated in this way may differ by several percent.