Source effect impact on the magnetotelluric transfer functions
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Abstract
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.
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