Waldemar Jóźwiak

This study analyzes the instability of interstation magnetic transfer function estimation, known as the horizontal magnetic tensor (HMT), with a particular focus on how this variability affects the effectiveness of reference methods. HMT describes the relationship between horizontal magnetic field variations recorded at two observation sites and is increasingly used to investigate regional conductivity structures and to support reference processing techniques in magnetotellurics. Despite its widespread application, relatively little attention has been paid to its temporal stability and the implications of possible source-related variability. Previous research has shown that deviations from the ideal plane wave, manifested as the presence of an external vertical magnetic field component, exhibit seasonal characteristics influenced by day length and regional variations, especially for periods exceeding 1400 s. This phenomenon is attributed to variations in the source’s characteristics or the presence of an additional, unstable wave source. Notably, this additional field may also include a horizontal component, which could influence HMT estimation and consequently affect the effectiveness of reference methods. This study analyzed measurement series from observatories in Belsk (Poland) and Niemegk (Germany) over 22 years to determine whether the degree of deviation from the spatially uniform plane wave could have a local character. The results showed that, unlike tippers, HMT exhibits seasonal variability across the entire analyzed frequency range, indicating the variable nature of the source. Winter estimates differ significantly from summer ones, possibly due to shorter daylight duration. The impact of HMT variability on the effectiveness of reference methods was then analyzed. The results indicate that seasonality does not affect the performance of the classic single-reference method. However, in the case of the multi-reference method proposed by Egbert, HMT variability influences the results, potentially rendering the method ineffective.

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.

This study’s main objective is to better define and understand results for the most commonly used inversion algorithms in magnetotelluric data interpretation as part of geological exploration of the region of the Dolsk fault and the Odra fault. The data obtained from the eastern part of Fore-Sudetic Monocline measurements were used to describe the boundaries of lithospheric blocks (terranes) and recognize their origin. The magnetotelluric (MT) soundings were carried out to achieve this goal. There were conducted 51 soundings on five quasiparallel profiles. That allows constructing a quasiregular mesh in the area of the Fore-Sudetic Monocline. This arrangement of the measuring grid allowed reducing the influence of the largest sources of disturbances on MT data. 1D and 2D models were created by using the inverse algorithms. The models were prepared for each profile separately. Further, parallel (ModEM) 3D inversion codes were applied. The area where the investigation was done involves the region of the Dolsk fault and the Odra fault. These zones are essential geologic borders of a regional nature, and they pull apart the crust blocks with different origins. It was vitally needed to correctly identify the crust and upper mantle structure around a part of the Fore-Sudetic Monocline. The paper shows how these key features of the geological structures are revealed using 1D, 2D, and 3D algorithms.