Variability of horizontal magnetic tensor estimation and its impact on reference methods in magnetotellurics

Abstract 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.