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(ReZ), 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.
Authors:A. Ádám, Attila Novák, L. Szarka, and V. Wesztergom
Firstly the authors give an overview on the geological, geophysical and tectonical features of the Diósjenő dislocation belt (or zone, according to some authors) around the river Ipoly near the Hungarian-Slovak border among great structural units: Vepor, Gemericum and formations of the Mid Hungarian Mts. The longest magnetic anomaly of the Pannonian Basin lies in this belt. It is assumed that it is due to ultrabasic magmatite of greenschist facies. The near-surface geoelectric soundings did not find any conductivity increase near Diósjenő (western part of the zone), but there are graphitic micaschists in the boreholes around Szécsény. There is some earthquake activitiy in the region with hypothetical depth of 7-8 km. Two deep magnetotelluric (MT) profiles cross the dislocation zone. The resistivity distribution from the surface to the conductive asthenosphere along these profiles was obtained by using instruments, operating in two different period ranges. After processing the measured data by 1D/2D inversion, it became obvious that the dislocation zone includes electrically conducting roots at a depth of 7-11 km. This result hints at the presence of fluid in the broken rocks having increased porosity in the dislocation zone. Another component that can increase the conductivity could be the graphite (carbon) originating from the Paleozoic crystalline rocks of the Gemericum (or Vepor). The ductile phase (fluid/graphite) observed by high conductivity in the centre of the dislocation zone can play an important role in the generation of the earthquakes according to the most recent statements of the international literature.
Every geophysical exploration method has non-uniqueness/ambiguity due to which interpretation is often misleading. If two or more data sets that deal with different physical properties are interpreted jointly then the combined study reduces the ambiguities in either method significantly. For layered structures magnetotelluric and seismic method provide information about the conductivities, velocities and thicknesses of the layers, where the thicknesses are the common parameters of seismic and magnetotelluric models. Assuming that the conductivity and velocity discontinuities are identical, the joint inversion of the two methods can improve the reliability of the estimation of thicknesses. In this paper on synthetic data will be shown, that the joint inversion yields better layer parameters, than the individual magnetotelluric or seismic inversions. The joint inversion decrease the effect of the equivalence, which at the magnetotelluric inversion often leads to misconclusion. As inversion technique the simulated annealing and the linearized inversion will be used. In the present work, attempt is made to study the seismic refraction and magnetotelluric data to delineate reliable subsurface information.
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.
Authors:A. Ádám, F. Kohlbeck, A. Novák, and L. Szarka
In the continuation of the CELEBRATION-007 deep seismic profile from Hungary to Austria a series of deep magnetotelluric soundings has been carried out, using the instruments (from GFZ MT instrument pool). In spite of the high noise level, the relatively good imaging of the structure of the sedimentary Graz Basin and possibly fluid filled conducting fractures in the resistive rock matrix of the Eastern Alps have been indicated. They might be potential source of geodynamics (earthquakes?).
In the area of the typical 3D Békés basin ("graben") high anisotropy (difference) appears in the extreme values of the magnetotelluric impedance phases in the period interval corresponding to the indication of the conductive asthenosphere. As proved by the forward 2D/3D modelling, this anisotropy can only be explained by a strong 2D effect. This fact may conform the validity of the results of the 2D inversion of PGT1 magnetotelluric data indicating the upwelling of the asthenosphere below the Békés basin.