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  • Author or Editor: L. Szarka x
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Starting from the magnetic vector potential and the electric scalar potential a compact derivation is given to determine the one-dimensional subsurface resistivity function in different ways, leading to the basic relationships of the classical magnetotelluric and the magnetovariational methods. With measurements on the horizontal ground surface Schmucker's C-response and the surface impedance can be determined by using two different combinations of the electromagnetic field components. With measurements at more than one height (depth) levels (assuming the knowledge of horizontal and/or vertical gradients) some further theoretical possibilities emerge. The novelty of the paper lies first of all in the very compact presentation of the one-dimensional case, introducing a local distortion due to a small-size inhomogeneity.

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It is demonstrated mathematically that the length of the geomagnetic induction vector in the so-called S-interval at any observation site on the surface gives the horizontal derivative of the natural logarithm of the depth below the given site, and not simply the depth as it had been assumed by Ádám and Koppán (2004).

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This paper presents the general features of parameter sensitivity maps and illustrates their practical use. We define parameter sensitivity maps as geoelectric responses in the measuring plane (at the surface) due to an elementary cube within the subsurface at three different depths. Responses of the three component as well as the total response of electrical dipoles formed on opposite cube surfaces are shown. In this paper we present such maps for 14 various linear electrode arrays. This paper is followed by an accompanying one about nonlinear and focussed arrays. Parameter sensitivity maps in these two papers provide a compact characterization of all known geoelectric arrays. We give several examples how to use them in planning and interpretation of field measurements.

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In this paper we present, at first time in a geophysical journal, parameter sensitivity maps of nonlinear and focussed electrode arrays. We present them as anomalies due to electric dipoles forming on opposing surfaces of an elementary cube within the subsurface at three different depths, and not only the total effect of the dipole, but also of its components are shown. Parameter sensitivity maps of non-linear arrays, compared to those of linear arrays, have in general 1. more equal sensitivity values in x and y directions, 2. more chances for antisymmetry axes, 3. smoother lateral distribution of sensitivity values. We recommend a systematic use of parameter sensitivity maps in geoelectric prospecting, both in planning and interpretation of field measurements.

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Detailed magnetotelluric soundings along the Hungarian section of the CEL-7 seismic profile (SW Hungary, where a series of very deep 3D sedimentary basins is known from various geophysical-geological investigations) enabled us to produce magnetotellurics-based estimations for the topography of the high-resistivity basement. Both TM and TE modes were used for 1D inversion, and the resulting depth values were compared to the depths, taken from the “Pre-Tertiary Basement Contour Map of the Carpathian Basin” by Kilényi and Šefara (1989), called as K-S depths.

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In order to investigate imaging properties of various magnetotelluric interpretation parameters over complicated models, we carried out numerical model calculations, where the models contained a 3D near-surface (``shallow") part and a 3D or 2D deep part. Various alternatively defined magnetotelluric responses, all of them based on rotational invariants of the magnetotelluric impedance tensor were considered. Then we calculated correlation coefficients between all these MT responses, and the characteristic geometrical parameters of the subsurface models, considered as a composition of “shallow+deep” elements.A systematic behaviour, similar to that had been observed in 1D situation was found: det(Re Z), Re det(Z) based apparent resistivity has the largest depth of  investigation and the best lateral resolution. Furthermore, besides the phase, the Re det(Z) (a twin-parameter of the phase) seems to give the most direct response about deep structures. In presence of 3D near-surface inhomogeneities the most surprising result is that there are narrow period windows, where the deep model can be directly seen in the Re det(Z) and in the phase responses.

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This paper investigates the effect of electrode positioning errors on the inverted pseudosection. Instead of random spacing errors (as usually assumed in geoelectrics) we exactly measured this effect among field conditions. In the field, in spite of the greatest possible care, the electrode positions contain some inaccuracy: either in case of dense undergrowth, or varied topography, or very rocky field. In all these cases, it is not possible to put the electrodes in their theoretical position. As a consequence, the position data will contain some error. The inaccuracies were exactly determined by using a laser distance meter. The geometrical data from real field conditions and by using Wenner- α , Wenner- β , pole-dipole and pole-pole arrays were then considered over homogeneous half space.As we have found, the positioning errors can be regarded as insignificant, even in case of relatively uncomfortable field conditions. However, in case of very rocky surface the distortions are more significant, but it is still possible to make some corrections: either by neglecting a few electrode positions with the greatest positioning error, or to minimize the inline errors, even on the price that offline deviations are high.

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In the contact zone of three tectonic units (Pannonian Basin, Eastern Alps and Dinarides), in a complicated - basin and range - geological situation magnetotelluric deep soundings were carried out along a 140 km long profile\linebreak (CELEBRATION-007) with a site distance of 2 km. In this area deep fractures of the Basin run together in NE-SW direction. In the paper various magnetotelluric images completed with gravity and magnetics are provided. In the traditional magnetotelluric approach, the structural indication of the TM and TE mode magnetotelluric sounding curves is clearly separated. The TM mode curves well express the resistive basement structure, already known from dense boreholes and detailed seismic exploration. The TE mode curves on the other hand (together with the induction vectors of very low values) definitely show the conductive root of the deep fractures, where the ductile materials are assumed to be raised into a very shallow depth of about of 8 km. The high heat flow of the area (about 100 mW/m\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $^2$ \end{document}), which explains the shallowness of the conductive asthenosphere is also well indicated. The asthenosphere has more Alpine character in the NW part of the profile (its depth is about 80 km) and it is at smaller (about 50 km) depth in the SE part  of the profile, due to the higher heat flow near the extensional Drava Basin. The induction vectors are also separated into two characteristic regions, according to their general direction, influenced by both local and remote effects. A strong correlation is shown between magnetotelluric and gravity inversion results. A joint interpretation of magnetotelluric, gravity, magnetic results provide a quite comprehensive interpretation about the deep geological structures in SW-Hungary.

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