Ernő Prácser

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.

A correction to Bahr's "phase deviation" method for tensor decomposition

For models having moderate departures from the basic distortion model (from the so called “principal superim-position” model, that is where local three-dimensional and regional two-dimensional structures are superimposed) a special tensor decomposition method (the so called phase deviation method) was suggested by Bahr (1991). As far as we know, this technique has never got a wide field application in the field. In a careful examination of the suggested solution, an error was observed in the original derivation of the formulas. In this paper Bahr’s (1991) solution is corrected. Using the new equations, more understandable and interpretable results are obtained, as it is illustrated on synthetic examples.

Characterisation of a landslide by its fracture system using Electric Resistivity Tomography and Pressure Probe methods

We have studied a slowly moving loess landslide along the River Danube in South Hungary. In contrast with other efforts, we aimed to determine its fracture system. Due to the homogeneous composition of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. Beside of the well-known Electrical Resistivity Tomography (ERT) the so-called Pressure Probe (PreP) method was applied to characterise the supposedly dense fracture system. This method was developed to detect and characterise mechanically weak zones, which may not visible from the surface, and may occur e.g. due to landslides. Fracture zones had been especially well localised by the ERT, enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zones. PreP was able to give a very detailed image about the surface projection of the fractures. Both methods proved to be good to characterise the fracture system of such a landslide area. Geophysical predictions have been verified also in reality: the mass movements occurred about 1½ years after the measurements. Therefore, to provide early risk warnings and to avoid damage to constructions or endangering human life, the application of the ERT and PreP methods is highly recommended.

Slingram Measurements in the Mecsek Mountains, Hungary

The so-called Slingram measurements form a family of frequency-domain electromagnetic methods, when the transmitter is a vertical magnetic dipole, and the vertical component of the magnetic field is measured, at some distance from the transmitter. The measured value is usually the imaginary (out-of-phase) part of the vertical magnetic field component. Such an equipment was manufactured at the University of Neuchâtel (Switzerland) and supplied to the Geodetic and Geophysical Research Institute in frame of a Swiss-Hungarian co-operation, supported by the Swiss National Research Foundation. As a part of test measurements, Slingram profilings and soundings were carried out for exploration of karstic water reservoirs connected to tectonically weak zones in the Mecsek Mountains, close to Pécs (Hungary). One-dimensional inversion and comparison with other geo-electromagnetic techniques as radio-magnetotellurics (RMT) and radiofrequency-electromagnetics (RF-EM) resulted in good and interpretable results.

Prediction of the Process of a Slowly Moving Loess Landslide by Electrical Resistivity Tomography

A slowly moving loess landslide along the River Danube in South Hungary was studied using electrical resistivity tomography (ERT). The aim of the research was to determine the fracture system of the study site. It seems to be the only possibility to get information about the landslide and its further evolution due to the homogeneous composition of the loess. The mass movement was expected to occur in the direction of the identified crack openings. The applicability of the ERT technique for such a supposedly dense fracture system was studied by numerical modelling and the results have been verified in the field. It was shown that it is especially important to carry out the field measurements following dry periods; otherwise the interpretation may become extremely difficult if not impossible. The dip of the fractures could not be observed and they could not be explored deeply. It was possible to map their surface projection to get the desired information about the structure of the landslide. Fracture zones could be especially well localized enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zone. Although the area outside of the one that already subsided is not endangered yet, the area which has already started to move is going to break into two. Parts of the about 5 m wide blocks at the front of the landslide may fall or slide down anytime. The area below the buildings was assumed to move as one unit. Most of our predictions have been verified by the mass movements which occurred about one and half years after the measurements. The ERT method proved to be a good tool to characterize the fracture system of such a landslide area, enabling the prediction of future rupture surfaces and also delineation of the endangered area. Its use is therefore highly recommended to monitor landslides.

Field Applicability of the g11n Configuration

Firstly a short introduction is given about the theory of the g11n configurations which belong to the group of the geoelectrical quasi null arrays. On the basis of our numerical investigations the application of these arrays seem to be rather perspective. Their field application is however difficult because of the complexity of the inversion of the data of these arrays. These arrays have very large geometric factor, they may change their sign and they can produce very sharp changes. Using an own developed code we inverted field data. The very first results show the field applicability of these configurations.

The First Inversion Results with the g11n Configuration

Geoelectrical null and quasi arrays have been studied since very long time, but they have not been introduced in the practice. Its reason is that their application was rather difficult and that the inversion of the data of the arrays whose application is simpler has not been solved. We studied the theory of these arrays and developed a code to invert their data. In the presentation a short introduction will be given in their theory and the very first field experiments will be shown.

Indication of meta-anthracite by magnetotellurics in the Kőszeg-Rechnitz Penninic window: a test area

One of the Penninic Nappes is the Kőszeg-Rechnitz (K-R) tectonic window at the Eastern end of the Eastern Alps. It has a complicated metamorphic history from the Jurassic time. The organic material of the Penninic Ocean was transformed to electrically conductive meta-anthracite. Its amount in the chalcophyllite is estimated by geochemists to 0.2 per cent.Taking this conducting structure as a test area pilot deep magnetotelluric (MT) soundings have been carried out and we determined the structure of the conductivity anomaly due to 0.2 per cent meta anthracite in the K-R window and its surroundingsthe different kinds of MT distortions as lateral (side) effect of the conductor appearing in the crust and mantlethe most probable depth of the conductive asthenosphere at the border of the Pannonian Basin (having extreme shallow asthenosphere). The obtained ∼140 km depth is in correlation with value of the asthenospheric map based mainly on seismic data.