Session: 2 Paper type: Oral Title: 2.2 Combined LOTEM and MT measurements in the Odenwald area in Germany Authors: S. Dautel, T. Hanstein, A.Hoerdt, J. Michel, M. Mueller, B. Tezkan, H. Stefan Dautel University of Cologne Institute for Geophysics and Meteorology Albertus Magnus Platz 50933 Koeln Germany Tel.: 49-221-470-5476 Fax: 49-221-470-5481 email: dautel@geo.uni-koeln.de Thern The Rhenohercynian/Saxothuringian suture zone in central Germany is characterized by a conductive crustal anomaly at 7 - 10 km depth. In the Spessart area, the interpretation of MT data revealed a characteristic shape of the conductor. To see whether and at which depth the anomaly extends further South-West, a combined long-offset transient electromagnetics (LOTEM) and magnetotellurics (MT) survey was carried out in 1994 in the Odenwald area. A 30 km North-South profile was covered with 10 electric dipole transmitters, 96 LOTEM receivers, built on 6 separate profiles, and 10 MT stations. Because of the high cultural noise level, new digital filtering techniques had to be developed for both the LOTEM and the MT data. The data were interpreted using decomposition techniques, joint 1-D inversion, 2-D inversion and 3-D modelling. The LOTEM electric fields have a particular value for the interpretation, as they contain the DC signal in their late-time asymptote. This is sensitive to vertical current flow and thus provides complementary information. The results show a southward dipping crustal conductor in both the MT and the LOTEM data, which could be the extension of the feature in the Spessart, but the anomaly seems to be less distinct, and closer to the surface. Session: 2 Paper type: Oral Title: 2.3 Relationships between hydraulic and electric parameters in porous and fissured media Authors: S. Hautot (1), O. Bour (2), P. Tarits (1), and P. Davy (2) (1) IUEM, UMR 'Domaines Oceaniques', Place Nicolas Copernic F-29280 Plouzane, France (2) Geosciences Rennes, Campus Beaulieu, Universite de Rennes I F-35000 Rennes, France email: hautot@univ-brest.fr The EM (frequency domain) and DC soundings responses over a porous or fissured, water saturated, medium are different because the two techniques do not probe the same bulk electrical conductivity with depth. We use this property to evaluate the capability of the combination of the DC and EM response functions to characterize the hydraulic properties of a fissured medium. We present a numerical study of the relationship between the hydraulic and the electric properties of a porous fissured medium. We computed the EM and DC response functions for random fault networks with known hydraulic parameters. Results are analysed using percolation theory which predicts that the effective hydraulic permeability of a fissured medium is a function of the length of connected fissures as well as their density. At the macroscopic scale, and without any surface conduction effect, the preliminary results show that the EM response function depends mainly on the porosity and on the water conductivity whereas the DC response function depends also on the matrix conductivity and on the geometry of the porous medium. Different combinations of the DC and the EM bulk conductivities are tested to compare the effect of the connected fissured network on the DC bulk conducivity to the hydraulic properties. Title: 2.4 ELECTRICAL INTERACTION BETWEEN CARBON AND PHYLLOSILICATES IN BLACK SHALES. Authors: G. Losito (1), G. Pratesi (2), A. Trova (1) (1)Department of Civil Engineering, Via di S. Marta, 3, I-50139, Firenze, (2) Mineralogical and Lithological Museum, Via La Pira,4, I-50100 Firenze, Italy. LOSITO@DICEA.UNIFI.IT The electrical conductivity of black shales is an actual and discussed problem, given the large interval of values observed both in situ and in laboratory. We studied the electrical behaviour of black shale samples coming from Italian and Swiss boreholes at simulated physical conditions (Hydrostatic pressure up to 39 MPa; Internal fluid pressure up to 36 MPa; temperature up to 230 'C) and analysed the petrographic structure, the mineralogical and chemical composition of the samples by using optical and electronic microscopes (TEM, SEM) and chemical analysis. Thanks to new chemical techniques we measured the whole carbon content, that resulted more less than expected (only few percent). The main research result is the evidenced role of no-graphitic crystallites of phyllosilicates in activating the electronic conduction mechanism of the carbon minerals in the whole rock sample. In fact the requested 'continuity' of the carbon net can be created also in presence of few percent of carbon, if the rock crystallites have dimensions under a critical size and the phyllosilicates can joint to the graphite creating a global layered structure, thanks to the common 2D crystalline structure of these minerals. A physical interpretative model of the black shale electrical behaviour is described. ----------------------- Oral presentation G. Losito Department of Civil Engineering, Via di S. Marta, 3, I-50139, Italy tel. +39-55-4796323 ; fax +39-55-495333. LOSITO@DICEA.UNIFI.IT -------------------------------------------------------------- Gabriella LOSITO mailto:Losito@dicea.UniFi.It Applied Geophysical Laboratory http://www.dicea.unifi.it/gfis/ Civil Engineering Department Florence University http://www.unifi.it/ Via di S. Marta, 3 I-50141 FIRENZE ITALY Voice +39-(0)55-47.96.323 Fax +39-(0)55-49.53.33 -------------------------------------------------------------- Title: 2.5 MAGNETOTELLURIC AND DC RESISTIVITY SURVEY ALONG SOME SELECTED TRAVERSES OF THE SINGHBHUM ARCHAEAN CRATON Authors: By K.K. Roy, A.K. Singh, K.K. Mukherjee, S. Srivastava and P.S. Routh Abstract Magnetotelluric and DC resistivity soundings were done across the Archaean Singhbhum granite batholith. Azimuthal dipole sounding with a maximum dipole separation of 30km and Schlumberger sounding with a maximum electrode separation of 5 km were carried out. DC resistivity deep sounding model are obtained using very fast Simulated Annealing (VFSA) and Genetic algorithm (GA). Magnetotelluric model is obtained using the Rapid Relaxation Inversion (RRI) of Smith and Booker (1991). Both MT and DC resistivity have shown that the granites near the Keonjhargarh are more resistive than that of Bisoi. Gravity, Geochronology, trace element geochemistry have supported our observation. It is now accepted by the geologists that the two granite bodies evolved from the two separate parent magma and originated within a time gap of 1.4 million year. MT model revealed that the granites near Keonjhargarh are deep rooted. Thickness of the weathered zone over the unaltered granites varies from 10 m to 1 km. Location of the major contacts coul;d be established. Magnetotelluric model has also confirmed that the granitic body near Keonjhargarh is much more resistive than those near Bisoi. Signature of mantle upwarping and thinning of the lithosphere is observed in the MT model near the Simlipal volcanics. High Bouguer gravity anomaly supports this view point. Thickness of the lithosphere is found to vary from 70 to 130 Km. DC collinear dipole-dipole traversing (with the dipole length of 500 m and for dipole separation varying from n=1 to 6) across the Singhbhum shear zone near Rajkharsawan reveals the contact of the Proterozoic and Archaeans . Both DC resistivity and MT has reflected the presence of the conducting shear zone and resistive north Singhbhum fold belts. Title: 2.6 MID CRUSTAL STUDIES USING A COMBINATION OF LONGOFFSET DC-MEASUREMNTS AND MAGNETOTELLURIC Authors: Johannes B. Stoll, Alexander Gatzemeier Johannes B. Stoll Institute of Geophysics, University of Goettingen Herzberger Landstr. 180 D-37075 Goettingen fon : +49 (0)551 3974 66 fax : +49 (0)551 3974 59 e-mail: josto@willi.uni-geophys.gwdg.de To investigate the resistivity of the middle crust using DC-methods long distances between the source and the receiver up to 100 km are required. However the resolution of the signal of the controlled soucre can be improved through removing the inductive electrical field from the observed field. This requires the registration of the time varying electrical and magnetic field before and during the injection of electrical current in the ground. Field experiments in the northern part of Bavaria (Gemany) using long offset dipole-dipole measuremnts up to 60 km indicate an anisotropic behaviour of the resistivity curve at distances of 30 km. Title: 2.7 Induced Polarization by the Transient EM Sounding Method in the Multilayer Sections Authors: V.Yu.Zadorozhnaya, V.P.Lepoyshkin Nizhnevolzhsky Geology and Geopysics Research Institute 70, Moskovskaya Str., 410600, Saratov, Russia e-mail ccvg@mail.saratov.ru session 2, oral Numerous investigations showed that during registration of TEM processes in ungrounded loops in multilayered media there can arise decay phenomena and the change of a sign of the curves E(t), that can be explained by the processes of induced polarization,The induced polarization makes it impossible to interpret the result TEM curves with typical techniques. For solving of such fine geological problems as definition of lithological, hydrogeological and criogenic characteristics of sectoin layers the mathematical modelling would be the most efficient but it reguires high-perfor- mance and accurate programs.The authors present a technique for a definition of electromagnetic field in S-plains during the work of the receiver-transmitter loops.The problem was solved by sequential approximation that allowed to obtain the analitic solution as well as to divide and analyze induction and polarization components of electromagnetic field. The set of programs of polarized S-plaines was developed and tested by interpretation of TEM data from the Tumen region (per- mafrost). Induced polarization effects were recorded during the conto- uring of oil field at boarding zone of Surkhandaryinskaya depres- sion in Uzbekistan. Induced polarization effects can be the indicators of leaka- ge of technogenic gas along permeable layers through poorly cased well. Such anomalies are recorded by TEM in the Orenburg region. Created set of programs allowed to contour zones of technogenic gas penetrations into sandy water saturated sediments. Title: 2.8p High resolution of pulsed DC-signals using MT-impedance tensor Author: Gatzemeier Alexander Institute for Geophysics, University of Goettingen Address Herzberger Landstr. 180 City Goettingen Postal code 37075 Country: Germany Phone +49 (0)551 397491 Fax: E-mail: gatze@willi.uni-geophys.gwdg.de The estimation of DC-resistivity within the middle crust requires transmitter-receiver distances up to 100 km. For those distances, the electrical field due to the electrical dipole source amounts amplitudes of a few nanovolts per meter only. In order to resolve such signals two different ways are used to determine the amplitude: The inductive part of the time series of the E-field is removed from field data through the estimation of the MT-impedance tensor. Secondly, the pulsed DC-signal must be stacked many times. The results of a megascale dipole-dipole-experiment with distances up to 60 km are presented. Title: 2.9p SIMULTANEOUS INVERSION OF ELECTRIC AND MT DATA IN 1-D SITUATIONS USING MCMC METHODS Authors: M. Menvielle(1,2), M. Roussignol(3), and J.J. Schott(4) (1) Centre d'Etudes des Environnements Terrestre et PlanThetataires, CNRS/UVSQ, 4 Avenue de Neptune, F-94100 SAINT MAUR, FRANCE (2) UniversitTheta Paris Sud, ORSAY, FRANCE (3) Equipe d'Analyse et de MathThetamatiques AppliquThetaes, UniversitTheta de Marne la VallThetae, 5 Boulevard Descartes, F-77454 MARNE LA VALLEE CEDEX 2, FRANCE. (4) Ecole et Observatoire des Sciences de la Terre, 5 rue RenTheta Descartes, F-67084 STRASBOURG CEDEX, FRANCE In the Bayesian approach, the inverse problem solution can be obtained from a posteriori probability distribution of the model given the data and a priori information on model parameters. Stochastic algorithms based upon MCMC (Monte carlo Markov Chain) methods have already been developed to solve the Bayesian inverse problems in both electric and MT 1-D situations. The domain under study is divided into a large number of thin horizontal homogeneous layers whose thickness is fixed, and the model parameters are the conductivity of each layer. We use an a priori law of the parameters which favour smooth models. For each layer, the a priori and a posteriori laws are digitised over a limited set of conductivity values. The Markov chain relies on updating the model parameters during successive scanning of the domain under study. For each step of the scanning, the conductivity is updated in one layer given the actual value of the conductivity in the other layers. Thus we designed an ergodic Markov chain, the invariant law of which is the a posteriori law of the parameters, provided the forward problem is completely solved at each step. The a posteriori marginal probability distributions are estimated from the simulated successive values of the Markov Chain. We present an application of this MCMC algorithm to the simultaneous inversion of electric and magnetotelluric data. Title: 2.10p ENHANCED RESOLUTION OF SUB-BASALTIC SEDIMENTS BY THE JOINT-INVERSION OF MT, DC RESISTIVITY AND SESMIC DATA Authors: Saurabh K. Verma and Ajay Manglik National Geophysical Research Institute Uppal Road, Hyderabad 500 007 (AP), INDIA. In many flood basalt provinces in the world, sediments underlying thick lava flows have potential for hydrocarbons. Conventionally used seismic methods in such terrains fail to clearly resolve the sediments due to masking by the high velocity basalts. On the other hand, the electrical contrast between the basalts and the sediments can be exploited by MT and DC resistivity methods to provide an effective solution to the problem. Resolutions of sub-basaltic sediments yielded by seismic, MT and DC methods are analysed for representative geological situations. Seismic methods expectedly map the high velocity basalts well, while the MT method efficiently resolves the sediments and is insensitive to small variations in the basalts. The DC method is very sensitive to changes in the basalts and less sensitive to those in the sediments. A joint-inversion algorithm is developed to numerically combine the diverse sets of information deciphered by the three methods. This scheme can provide results for individual or any combination of select data sets. Enhanced resolution of the sediments is studied for various combinations of data sets. For thin sediments under a thick basaltic cover, the resolution is found to be critically dependent on the noise in data. Title: 2.11p The Vertical Gradient Method in Combination with Magnetotelluric, DC Resistivity, and Borehole Geophysical Methods Authors: Klaus Spitzer^1, Wolfgang Storz^2, Erich Steveling^3 & Thomas Wonik^1 1) Geowissenschaftliche Gemeinschaftsaufgaben, Hannover, Germany 2) Hydrogeologie GmbH, Torgau, Germany 3) Institut fuer Geophysik der Universitaet Goettingen, Germany Klaus Spitzer Geowissenschaftliche Gemeinschaftsaufgaben Stilleweg 2 30655 Hannover Germany Tel.: (+49) 511-643-3490 Fax: (+49) 511-643-3665 klaus.spitzer@bgr.de In summer 1994, we could carry out an experiment to determine the skin effect of horizontal variations and pulsations of the earthmagnetic field employing a borehole magnetometer that has been developed by the Institute of Geophysics of the University of Goettingen in the frame of the Continental Deep Drilling Project KTB several years before. Unfortunately, the skin effect had been weak due to the low conductivity in the KTB target area. This time, however, we encountered a strong skin effect of approximately 20 - 25 % at a depth of only 832 m in a borehole located in the spa of Bad Koenigshofen / Unterfranken in south-eastern central Germany. The vertical gradient method is similar to magnetotellurics, but substitutes the gradient of the horizontal magnetic componets for the telluric field thereby by-passing the distortions that are usually associated with the telluric components. This has far-reaching consequences. Firstly, the magnetotelluric response, that has been determined simultaneously in our experiment, can be freed from local distortions by decomposition into its regular part and its locally distorted one and, secondly, existing decomposition techniques can be crosschecked. Another aim is to derive a consistent conductivity model of the earth's crust in the target area using additional techniques as long offset DC surveys and borehole geophysical standard methods covering several magnitudes of scale lenghts. 2.12p Natural Electric Field of Electrokinetic Nature from Deep Source Authors: V.Yu.Zadorozhnaya, V.P.Lepyoshkin Nizhnevolzhsky Geology and Geophysics Research Institute 70, Moskovskaya Str., 410600, Saratov, Russia e-mail ccvg@mail.saratov.ru session 2, oral, poster Self-potential (SP) anomalies registered in a very wide va- riety of geological phenomena, including geodynamic processes, have been explained on the basis of capillary model of porous me- dia. We presuppose that regions of tense deformed state of the medium can also cause SP anomalies includind those on ancient consolidated platforms. To verify our hypothesis we need the fol- lowing: - to perform mathematical modelling of SP conditioned by strea- ming potentials in heterogeneous medium; - to determine the range of electrokinetic parameters of tense medium at large depths and estimate an amplitude of the potential and an intensity of the electric field registered on the Earth's surface. The problem of determination of the field, caused by an hete- rogeneity contained in one of the layers, on the surface of a multilayered medium is solved with the use of the electrodynamic duality principle. The range of the parameters of the medium appearing in the Gelmholtz-Smolukhovsky formula (viscosity, dielectric permeabili- ty, electroconductivity and surface conductivity of electrolytes, dzeta-potential) is considered. Calculation performed for medium models, heterogeneous inclu- sion, having excellent electrokinetic features, is placed at the depth of 2600-5000 m and has horizontal dimensions calculated by the first dozens of kilometers. Such inclusions can create anoma- lies on the Earth's surface calculated by dozens of mV.