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Central European Geology
Authors: Tamás Bodoky, Márta Kis, István Kummer, Károly Posgay, László Sõrés, and György Don

. Nemesi G. Varga 1990 Crustal Conductivity Anomalies in the Pannonian Basin Acta Geod. Geoph, Mont. Hung. 25

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A network of fluxgate magnetometers was operated over Garhwal Lesser Himalaya Seismic Belt (GLHSB) in Uttarkashi area during 1987–1990. This study delineated a narrow conductive zone south of MCT and named as Garhwal Lesser Himalaya Conductivity Anomaly (GLHCA). Pseudo-sections of | Z / H | for a traverse across GLHCA implicate that the top of the anomaly is within 10 km depth and mostly of 2D nature. The tectonic formation associated with the anomaly is found to be polarization sensitive and the polarization characteristics show that the anomaly is not parallel to Main Central Thrust (MCT) but inclined by about 10°–30° in anti clockwise direction. The frequency and polarization characteristics of the induction response of GLHCA are discussed in this paper.

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From the very beginning of the magnetotelluric (MT) studies two notions have been coupled to each other: the graphite as one of the causes of strong conductivity anomalies and  tectonics. The graphitic formations of very low resistivity (< 10-1 Wm) are accumulated in the shear zones, thrust sheets, detachment horizon etc.\ and by this way they indicate the tectonics/paleotectonics which may not be indicated by other geophysical method so definitely. The author firstly surveys the manifestation of this phenomenon in case histories of the literature, then illustrates it by own very detailed study carried out on the Transdanubian crustal conductivity anomaly (TCA). The material of the conductor and their possible relation to the seismicity of the area will also be demonstrated by the TCA anomaly. In the closing chapters the origin of the graphite and its accumulation in the shear zones is discussed including the role of the (geothermal) water.

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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.

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Two effects have been studied concerning the former Wiese arrows and the newly determined complex induction vectors in the Pannonian backarc basin (Hungary): the remote effect of the curved Carpathian Conductivity Anomaly (CA) on the direction of the long period vectors, the local effect of the thickness (or conductance) of the conductive sediments on the induction vectors. The curvature of the Carpathian CA is clearly seen in the direction of the induction vectors as a remote effect dividing the Pannonian Basin into two great parts from this point of view. Following Zhang et al. (1993) who stated that the length (absolute value) of the induction vector becomes also constant in the “S-interval” as the magnetotelluric (MT) impedance which is related to the conductance of the sedimentary cover, it has been studied whether there is also any relation between the length of the induction vectors and the conductance of the same sedimentary cover (or thickness of sediment if its resistivity is constant). Due to the structural inhomogeneities to which the induction vectors are very sensitive, and to their great remote (side) effect, only a weak statistical relation has been found, nevertheless, its trend could be approximated by Ritter and Banks' (1998) theory. Exceptional cases are demonstrated.

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Magnetotelluric method (MT) offers opportunity to detect crustal fluids along faults due to their high conductivity anomaly. Supposing that fluids deposited minerals in the conductive fractures (faults, dykes) decreasing the resistivity, the high seismicity in the area can be explained by the presence of these fluids. MT measurements were carried out in the period range 0.001–420 s crossing the Kalabsha fault (Aswan, Egypt) and Remiremont fault (Southern Vosges, France). In these work we detect geoelectrical resistivity anomalies of the Earth’s crust and link them to local seismic activity. Seismic events having magnitude (M<5) are found along fault zones in Kalabsha and Remiremont. The goals of our measurements are various. We would like to determine the precise location of the active faults, to study the connection of the Remiremont and Kalabsha seismicity to the MT resistivity structures, and to support the idea of the influence of the fluid-bearing conducting faults in the Remiremont and Kalabsha areas to the earthquake. These applications afford the unusual opportunity to study the percolation of water into the faults system and its effect on the seismicity, to reveal geological structures and the stress field covered by thin Quaternary formations. Data are analysed by 2D simultaneous inversion of both polarizations. The resulting models are compared with the local seismicity map. Our MT model reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in active regions.

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: New data about the Carpathian conductivity anomaly. In: To the 75th Birthday of Professor Antal Tárczy-Hornoch, Geodetic and Geophysical Research Institute, Sopron, 220--240. New data about the Carpathian

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