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Cerv, V.
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Cerv, V.
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- PublicationOpen AccessStochastic interpretation of magnetotelluric data, comparison of methods(2007-02)
; ; ; ;Cerv, V.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague 4, Czech Republic ;Menvielle, M.; Centre d’Études des Environnements Terrestre et Planétaire, Saint Maur des Fosses Cedex, France ;Pek, J.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague 4, Czech Republic; ; Global optimization and stochastic approaches to the interpretation of measured data have recently gained particular attraction as tools for directed search for and/or verification of characteristic structural details and quantitative parameters of the deep structure, which is a task often arising when interpreting geoelectrical induction data in seismoactive and volcanic areas. We present a comparison of three common global optimization and stochastic approaches to the solution of a magnetotelluric inverse problem for thick layer structures, specifically the controlled random search algorithm, the stochastic sampling by the Monte Carlo method with Markov chains and its newly suggested approximate, but largely accelerated, version, the neighbourhood algorithm. We test the algorithms on a notoriously difficult synthetic 5-layer structure with two conductors situated at different depths, as well as on the experimental COPROD1 data set standardly used to benchmark 1D magnetotelluric inversion codes. The controlled random search algorithm is a fast and reliable global minimization procedure if a relatively small number of parameters is involved and a search for a single target minimum is the main objective of the inversion. By repeated runs with different starting test model pools, a sufficiently exhaustive mapping of the parameter space can be accomplished. The Markov chain Monte Carlo gives the most complete information for the parameter estimation and their uncertainty assessment by providing samples from the posterior probability distribution of the model parameters conditioned on the experimental data. Though computationally intensive, this method shows good performance provided the model parameters are sufficiently decorrelated. For layered models with mixed resistivities and layer thicknesses, where strong correlations occur and even different model classes may conform to the target function, the method often converges poorly and even very long chains do not guarantee fair distributions of the model parameters according to their probability densities. The neighbourhood resampling procedure attempts to accelerate the Monte Carlo simulation by approximating the computationally expensive true target function by a simpler, piecewise constant interpolant on a Voronoi mesh constructed over a set of pre-generated models. The method performs relatively fast but seems to suggest systematically larger uncertainties for the model parameters. The results of the stochastic simulations are compared with the standard linearized solutions both for thick layer models and for smooth Occam solutions.196 357 - PublicationOpen AccessMagnetotelluric and deep geomagnetic induction data in the Bohemian Massif(1997-03)
; ; ; ; ; ;Cerv, V.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ;Pek, J.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ;Praus, O.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ;Zaja, A.; Dipartimento di Geologia, Paleontologia e Geofisica, Università di Padova, Italy ;Manzella, A.; Istituto Internazionale di Ricerche Geotermiche, C.N.R., Pisa, Italy; ; ; ; Manifestations of the increased tectonic activity (seismic activity, remnants of Tertiary volcanism and riftogenesis) in the Bohemian Massif are bound to geologically extremely complicated regions with a system of discordant structures and a mosaic of fault zones. The presented results concern the deep geoelectrical features of the tectonic transition between the Saxothuringian and Moldanubian tectonoblocks on the territory of West Bohemia, in close vicinity to the German deep drilling experiment KTB (Kontinentale Tiefbohrung der BRD). Three first-order tectonic lines demarcate the region under study í the Litomerice deep fault to the north, the West Bohemian deep fault zone to the east, and the Central Bohemian fault to the south. As a whole, the region involved is characterized by a regionally increased seismic activity with the most active zone just beyond the northern end-point of the profile investigated. The contribution can be considered an example of possible interpretation of MT/MV/AMT data in geologically extremely complicated conditions with evidently discordant structures affecting the geoelectrical data.146 168 - PublicationOpen AccessMagnetotelluric investigations of the seismically active region of Northwest Bohemia: preliminary results(1999-02)
; ; ; ; ; ; ; ; ;Di Mauro, D.; Istituto Nazionale di Geofisica, Roma, Italy ;Volpi, G.; Istituto Internazionale per le Ricerche Geotermiche, Pisa, Italy ;Manzella, A.; Istituto Internazionale per le Ricerche Geotermiche, Pisa, Italy ;Zaja, A.; Università di Padova, Italy ;Praticelli, N.; Università di Padova, Italy ;Cerv, V.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ;Pek, J.; Geophysical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ;De Santis, A.; Istituto Nazionale di Geofisica, Roma, Italy; ; ; ; ; ; ; During 1997, within the framework of an Italian-funded scientific cooperation between Italy and the Czech Republic, a series of magnetotelluric (MT) soundings was carried out in the region of Northwest Bohemia (Czech Republic). This is one of the most seismically active areas in Central Europe, where micro-earthquake swarms frequently occur during the apparently quiescent intervals between large macro-seismic swarms. Fifteen MT stations were installed in an area of about 15 ´ 20 km2 where 80% of the seismicity of the entire region has been recorded since 1986. The area showed a high electromagnetic noise, possibly of high cultural origin from the nearby industrial zone of the Sokolov basin, which affected both the electric and the magnetic signals. The final data, carefully selected, were modeled by 2D and 3D techniques. The results show an extensive conductive structure in the depth range from 0.5 to 3 km. This structure could be connected with the locally buried granitic massif in the inhomogeneous metamorphic basement, probably accompanied by fracturation, thermo-metamorphism or paleofluids. Moreover, the presence of a conductive anomaly in the northern part of the investigated region could be linked to a lithological change in the metamorphic rocks (prevalence of phyllites over mica schists), which would even increase the effect of the granite.197 235 - PublicationRestrictedGeomagnetic depth sounding in the Northern Apennines (Italy)(2001)
; ; ; ; ; ; ; ; ; ;Armadillo, E.; Dipartimento per lo studio del terrirorio e delle sue risorse, Viale Benedetto XV, 5-16132 Genova, Italy ;Bozzo, E.; Dipartimento per lo studio del terrirorio e delle sue risorse, Viale Benedetto XV, 5-16132 Genova, Italy ;Cerv, V.; GFU CSAV, 141 31 Praha 4, Czech Republic ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Di Mauro, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Gambetta, M.; Dipartimento per lo studio del terrirorio e delle sue risorse, Viale Benedetto XV, 5-16132 Genova, Italy ;Meloni, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Pek, J.; GFU CSAV, 141 31 Praha 4, Czech Republic ;Speranza, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; ; ; ; ; ; ; ; A Geomagnetic Depth Sounding (GDS) survey covering the Northern Apennines of Italy has been carried out in the period 1992–94. Induction arrows maps and hypothetical event Fourier maps were constructed to obtain an electromagnetic imaging of this area. Since the two dimensional (2-D) character comes out from those maps for periods greater than 32 minutes, a 2-D inverse modeling was carried out. The model responses show that a deep conductive layer (>5000 S) underlies the Apennine chain at about 20 km depth. The transition between the Adriatic and the Tyrrhenian domains is marked by a rather sharp vertical offset in this conducting layer. In the northwest sector of the studied area an anomalous high conductivity behavior is superimposed on the regional trend, which corresponds to the geothermal field of Larderello-Travale.336 32 - PublicationOpen AccessGDS (Geomagnetic Depth Sounding) in Italy: applications and perspectives(1998-08)
; ; ; ; ; ; ; ;Di Mauro, D.; Istituto Nazionale di Geofisica, Roma, Italy ;Armadillo, E.; Dipartimento di Scienze della Terra, Università di Genova, Italy ;Bozzo, E.; Dipartimento di Scienze della Terra, Università di Genova, Italy ;Cerv, V.; GFU CSAV, Praha, Czech Republic ;De Santis, A.; Istituto Nazionale di Geofisica, Roma, Italy ;Gambetta, M.; Dipartimento di Scienze della Terra, Università di Genova, Italy ;Meloni, A.; Istituto Nazionale di Geofisica, Roma, Italy; ; ; ; ; ; The analysis of geomagnetic field variations is a useful tool to detect electrical conductivity contrasts within the Earth. Lateral resolution of outlined patterns depends on the array dimensions and density of measurement sites over the investigated area. The inspection depth is constrained by the period of geomagnetic variations considered in data processing. Regions with significant geological features such as boundaries of continental plates, marginal areas of contact between tectonic units or other geodynamical processes, are of primary interest for the application of the MagnetoVariational (MV) method. In the last ten years, in the frame of the ElectroMagnetic (EM) sounding techniques in applied geophysics, this method has been applied in Italy by researchers of the Istituto Nazionale di Geofisica, Rome, the Dipartimento di Scienze della Terra, Universitá di Genova and the Czech Science Academy of Prague. The Ivrea body in the Northwestern Alps and their junction with the Apennine chain, the micro-plate of the Sardinian-Corsican system and, recently, the central part of the peninsula along Tyrrhenian-Adriatic lithospheric transects were investigated. Studies in time and frequency-domain used in the first investigations, have been followed by more refined analysis involving tests on the induced EM field dimension, computations of single site Transfer Functions (TFs) through Parkinson arrows' and Fourier maps in the Hypothetical Event technique (HE). It was possible to describe the electrical conductivity distribution in the inner part of the SW Alpine arc and to confirm the presence of lithospheric and asthenospheric anomalies obtained by other geophysical methods. For the Sardinia-Corsica system, 2D and 3D inversion models highlighted the existence of two major conducting bodies, one north of Corsica, and the other south of Sardinia. In Central Italy, the regional electrical conductivity distribution pointed out a deep conductive structure beneath the Apennines and a very resistive root for this part of the mountain chain.255 420