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1 BRGM - Bureau de Recherches Géologiques et Minières

Abstract : A geophysical survey has been performed in the area of Litoměřice Czech Republic where a geothermal exploration drilling has been performed in the Bohemian massif in 2000. Measurements have been performed from 6 to 11th September by BRGM and Geomedia staff, with the logistic help of Litoměřice city. The survey aims at measuring electrical resistivity at depth to image the main geological features underground until few kilometers in urban areas with the challenging problem of cultural noise.Because the first method MT can be severely affected by electromagnetic EM noise induced by anthropogenic activities such as cities and industries and DC trains such as in Czech Republic and many regions in Europe, CSEM active controlled-source electromagnetics and MT passive method, natural sources data were both collected to be able to provide a resistivity imaging of the area. 27 CSEM stations and 7 collocated MT stations have been recorded.The CSEM survey has been conducted using surface and borehole high power emissions and two pseudo-orthogonal polarizations. A set of frequencies ranging between 0.125 Hz to 128 Hz has been selected to investigate the medium. Results provide apparent resistivity maps at various frequencies and for both polarizations reflecting the geological variations from north to south.MT robust signal processing has been performed using synchronous recordings performed locally 30 km and at far > 500 km remote stations. Data were processed using advanced robust techniques in each possible configurations 3 remote reference stations and single site processing. As the ambient noise and MT sources are not stationary, a time-lapse approach based on an error threshold was used to select the best MT estimates. MT soundings were obtained in the 7mHz-1kHz frequency band such as station 06 shown on Figure 1. Their quality will be discussed in the presentation.

Keywords : CSEM Magnetotellurics

Autor: Pierre Wawrzyniak - Nicolas Coppo - Jean-François Girard -



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