Deep resistivity structure of the Trans-European Suture Zone in Central Poland

The deep resistivity structure was estimated along a 400-km profile in central Poland crossing the Malopolska Massif (MM), the Lysogory Unit (LU), the Trans-European Suture Zone (TESZ) and ending at the East European Craton (EEC). Magnetotelluric transfer functions, corresponding to 20 sites, were supplemented by magnetovariational responses obtained at the geomagnetic observatories situated at the same tectonic units. Such a combination made it possible to extend the initial period range, which is from fractions of a second to several hours, up to months in order to reliably cover crustal and upper-mantle depths. The geoelectrical structures, revealed using 2-D inversions, do not contradict the known features of the lithosphere structure determined using seismic and gravity data along the profile.
The subsurface conductance, varying from approximately 10 Siemens at the inner part of the EEC to about 600 Siemens in the TESZ, is produced by sediments, the deep part of which contains conductive, highly mineralized water. The existence of two crustal conductive faults at the southwest and northeast of the TESZ were established mainly by the use of induction arrows. It was also revealed that rather high mantle conductivity beneath the MM, LU and TESZ at depths of about 150–200 km contrasts with the resistive upper mantle of the EEC. This can be interpreted as the decrease of asthenosphere conductance and/or as its submersion beneath the EEC. Generally, the results confirm the idea that the TESZ forms not only specific seismic boundaries but also causes peculiar conductivity anomalies in the crust and upper mantle.     

Approximate depth averages of electrical conductivity from surface magnetotelluric data

This paper presents a simple non-linear method of magnetotelluric inversion that accounts for the computation of depth averages of the electrical conductivity profile of the Earth. The method is not exact but it still preserves the non-linear character of the magnetotelluric inverse problem. The basic formula for the averages is derived from the well-known conductance equation, but instead of following the tradition of solving directly for conductivity, a solution is sought in terras of spatial averages of the conductivity distribution. Formulas for the variance and the resolution are then readily derived. In terms of Backus-Gilbert theory for linear appraisal, it is possible to inspect the classical trade-off curves between variance and resolution, but instead of resorting to linearized iterative methods the curves can be computed analytically. The stability of the averages naturally depends on their variance but this can be controlled at will. In general, the better the resolution the worse the variance. For the case of optimal resolution and worst variance, the formula for the averages reduces to the well-known Niblett-Bostick transformation. This explains why the transformation is unstable for noisy data. In this respect, the computation of averages leads naturally to a stable version of the Niblett-Bostick transformation. The performance of the method is illustrated with numerical experiments and applications to field data. These validate the formula as an approximate but useful tool for making inferences about the deep conductivity profile of the Earth, using no information or assumption other than the surface geophysical measurements.