A. Ádám

Estimation of the electric resistivity distribution (EURHOM) in the european lithosphere in the frame of the eurisgic WP2 project

In connection with the EURISGIC WP2 project the authors present those procedures which have been used to construct a map in cells on the electrical resistivity distribution in Europe at least till to the asthenosphere. The data are based on the deep magnetotelluric soundings published in the international literature. This map is the basis of the calculation of the induction risk endangering the electric network and communication systems.

Are there two types of conductivity anomaly (CA) caused by fluid in the crust

Abstract The different effects (temperature, fluids, tectonics) that play a role in the generation of the crustal conductivity anomalies (CA) are taken into account to be able to distinguish between typical conducting zones. In addition to a general depth distribution of the CA-s described by Adams empirical relation (first type CA), the data show that along some linear deep fractures in East-Central Europe the CA-s are at shallower depths probably due to an increase of the crack porosity in these zones (second type CA).

Magnetotellurics (MT) in mountains-noise, topographic and crustal inhomogeneity effects

Abstract In a mountainous region the aim of magnetotelluric sounding is to detect deep fractures, indicated by electric conducting zones in the crust. Nevertheless, some factors, such as the EM noise of different character, the rugged topography, the electric inhomogeneities, are increased in a mountainous region, and to determine or overcome them special measuring and data processing techniques should be applied. This paper reviews these factors or effects illustrating them with examples from the Eastern Alps and the Hungarian Central Mountains.

A new approach to the electrical conductivity anomalies in the Drauzug-Bakony geological unit

Abstract A major electrical conductivity anomaly has been detected in Transdanubia (West Hungary) in the eastern part of the Drauzug-Bakony geological unit (DBU), a collision zone of the Alpine orogeny. Assuming the source of the conductivity anomaly to be a characteristic formation of the whole DBU, long period magnetotelluric (MT) measurements were made in two regions of the western part of the DBU, in the Gail valley and in the Karawanken. The general features of this formation were studied also in order to elucidate the cause of the Transdanubian conductivity anomaly. The conductivity anomaly detected by long period MT in the Western DBU lies much deeper (12–17 km) than in Transdanubia (5–9 km). As the anomaly in both regions is associated with a well-known tectonic zone, its origin was initially explained by ionic conduction, arising from the presence of pore fluids. The very low resistivity values, the great anisotropy, the effect of charges on the H -polarized MT curves, and the anomalous magnetic field variations also indicated an increase of electronic conduction, possibly caused by graphitic rocks or ores nearer to the surface. In the Gail valley, audiomagnetotelluric measurements made in 1986 have led to the discovery of these standing blocks of graphitic shales lifted up gradually to the surface by tectonic forces from below the dolomites of the Carnic Alps. It is suggested that the same plate collision arranged the Paleozoic graphitic shales to narrow dikes in the Gail valley as well as in Transdanubia along the Balaton line and zones parallel to it.

Electromagnetic Induction Profile (PREPAN95) from the East European Platform (EEP) to the Pannonian Basin

Complex electromagnetic measurements along a profile crossing different great tectonic units in East-Central Europe (East European Platform, Paleozoic Europe and Carpatho-Pannonian region) were carried out by Hungarian, Polish, Slovak and Ukrainian institutes. Beside recent MT sounding curves, geomagnetic induction vectors, polar diagrams, apparent resistivity and phase pseudosections, a short description of earlier magnetotelluric (MT) and magnetovariational (MV) experiments is also given. For interpretation of these data 1D and 2D inversions were used after separation of the quasi E and B polarized curves. The Carpathian conductor clearly appears in the magnetovariational profile. MT soundings indicate that the mantle conductive basement — presumabely the asthenosphere — steeply deepens from the Neogene Pannonian Basin characterized by high heat flow towards the much colder EEP through the Paleozoic area.

Anomalous directional behaviour of induction arrows above elongated conductive structures and its possible causes

On some well documented field examples, it is found that, straddled between oppositely directed induction arrows which signify the existence of a conductive structure, a narrow belt exists over which real induction arrows align with the strike of the conductor. This distinctive behaviour either appears to signify a weak three-dimensional character of the elongated structure, marking gradients in the conductivity or current distribution along the length of the conductive body, or simply represents control of currents flowing in another conductor positioned at right angles to the mapped conductor. The lateral extent of this anomalous zone correlates well with the width of the conductive body. Incorporation of this feature in the interpretation of the half-width of the anomaly provides realistic constraints on the likely depth of the conductive body.

Geoelectrical Structure of the Earth’s Mantle in Pannonian Basin

Data from four geomagnetic observatories: Nagycenk, Tihany, Hurbanovo and Wien-Kobenzl together with magnetotelluric data obtained at 4 points situated not far from these observatories were analyzed to estimate the geoelectrical mantle structure in the Pannonian Basin. Response functions were estimated in the period range from seconds to several hours by the MT method and from 3 days to 1 year by using Dst variations. The published responses obtained from Sq variations were added to remove gaps between MT and MV curves. These deep soundings have confirmed that the upper mantle conductance below the Pannonian Basin is at least twice higher than below the Canadian and Ukrainian shields. The asthenosphere was detected with a conductance of about 4 kS at 80–280 km depths. A conductive zone (20 kS) was required by inversions at the depth of about 500 km. The mid mantle conductive zone was estimated to 550 kS at 850 km depth.

Distortions of the electromagnetic field by shallow basins and by resistive outcrops

Abstract The interpretation of magnetotelluric (MT) measurements carried out on shallow (several hundred metres deep) basins and on the surrounding highly resistive rock outcrops can be difficult due to different forms of distortion, mainly 3D effects. A good example for study of this problem is the Mor Graben, which is the transition zone between the Bakony and Vertes Mountains (W-Hungary), where more than 20 MT soundings have been made with stations spaced about 2 km apart to determine the structure of a conductivity anomaly at a depth of about 3–4 km. The statistical treatment of the different distortions due to varying sediment cover (the S -effect or static shift) and to the steep wall of the resistive basement outcrops (edge effect), etc., enabled the estimation of the actual parameters of the conductive formation. The interpretation of the field data is supported by numerical modelling.

Natural and man-made EM variations in the Komló coalfield

Abstract The strongly tectonized structure of the Komlo coalfield (Southern Hungary) was studied by magnetotelluric (MT) deep soundings. An experiment was also made to use the man-made electromagnetic noise very frequently appearing on MT records to obtain some information concerning the coalfields structure. Comparison of the two different data sets highlights the limits of the application of EM noise in geophysical exploration. A long-term MT study between 1982 and 1985 showed some MT resistivity variations probably associated with mining at the place of the greatest soil subsidence.

Tensor decomposition with static shift correction of the deep magnetotelluric (MT) soundings to improve the asthenospheric depth values in the great Hungarian Plain

Abstract26 deep — broadband — magnetotelluric soundings (MTS) have been carried out in the Eastern part of Hungary in the Great Hungarian Plain. These MT soundings aimed first of all at the determination of the structure of the conducting asthenosphere besides the study of the surface sedimentary cover. As the greater part of the MTS curves are distorted by the basement relief their magnetotelluric structural anisotropy could generaly be high.Groom-Baily (GB) decomposition has been applied to reduce the 3D near-surface effect to decrease so the structural anisotropy. It resulted in the decrease of the MT anisotropy from 1.5 to 1.1 in a statistical sense. The GB distortion parameters — shear and twist — are in average between 7° and 11° (absolute values). The deviation between the asthenospheric map calculated on the basis of the original ID asthenospheric depths and that obtained after decomposition is relatively small. Besides the decomposition the static shift should have been taken into account because the GB decomposition did not do it. As conclusion the paper gives a corrected asthenospheric map — as a present state of this kind of investigations — for the Great Hungarian Plain with average depth of about 60 km in agreement with the early value of the author from 1963. In addition Bahr’s decomposition parameters have been computed to check the GB strikes in selected sites.