4.2Electrical anisotropy and skew in the mid crust Author: Karsten Bahr Address: Institut fuer Geophysik, Herzberger Landstr. 180, 37075 Goettingen, Germany Tel 49 551 397452 Fax 49 551 397459 kbahr@willi.uni-geophys.gwdg.de The superposition of surface and midcrustal conductivity anomalies is considered for the case of an anisotropic middle crust. If this mid crust is inhomogeneous, then it will act on long period electric fields like a local scatterer and impose static shifts on these. Most static shift removal techniques then have serious limitations: techniques by which the surface is scanned yield factors which can't be used to correct long periods. The 'long period reference impedance' yields factors which must not be applied to shorter periods, e.g. they can#t be used to find the depth of the mid crustal structure. AMT or the regional skew can sometimes be used to find this depth. Here we present a simple technique for estimating the two conductances of the anisotropic middle crust from the two regional MT phases, if the depth is known. Field data examples for two frequently occuring type are shown: 1) regular anisotropy - neighboured sites have similar MT phases, conductances and regional strike. 2) irregular anisotropy - those parameters vary smoothly from site to site, and regional skew occurs. Title: 4.3 The ANCORP Project: a seismic and magnetotelluric study of the central Andean subduction zone Authors: H. Brasse (1), S. Friedel (1), P. Lezaeta (1), K. Schwalenberg (2) (1) Fachrichtung Geophysik, FU Berlin, Malteserstr. 74-100, 12249 Berlin, Germany (2) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany e-mail: h.brasse@geophysik.fu-berlin.de The seismic and magnetotelluric project ANCORP (Andean Continental Research Program) has been conducted in North Chile and South Bolivia to study the entral Andean subduction zone. A seismic reflection survey(a line of 400 km length from the Pacific coast to the Eastern Cordillera) revealed prominent deep reflectors in the Chilean forearc, but less pronounced, diffuse patterns in the backarc of the Bolivian Altiplano. Seismological investigations show a deep-crustal zone of high absorption (low Q) below the magmatic arc in the south of the study area (24=B0S), which gradually vanishes towards the North (20=B0S). This anomalous zone and its disappearance to the north corresponds to a formerly detected good electrical conductor. The deep crust below the magmatic arc at 21=B0S was again confirmed as bad conducting by recent MT= measurements on the Bolivian part of the transect. After dimensionality analysis and decomposition a 2-D inversion was performed and revealed a highly conductive zone at mid crustal depths (30 km) below the Altiplano, in certain accordance with the reflective zone mentioned above. The pattern of induction arrows, however, opposes the two-dimensionality deduced from the impedances in parts of the profile. Possible explanations and implications will be addressed in detail. 4.4 Single station C-response function estimates for Indian region using 27-day variation and its harmonics. Author: E. Chandrasekhar Indian Institute of Geomagnetism Dr. Nanabhoy Moos Road Colaba, Mumbai - 400 005, India. esekhar@iig.iigm.res.in; Fax: 0091-22-2189568 Complex demodulation technique has been applied to the data of 27-day variation and its harmonics to determine the single station C-response function estimates by robust technique for the stations confined to the 75 E longitude band extending from equator to the polar regions of Russia. This unique distribution of chain of stations has facilitated to test statistically the validity of P_1^0 assumption of the inducing source field. Statistically reliable C-response estimates are obtained only for the mid-latitude stations. The average depth of the perfect substitute conductor at stations free from lateral inhomogeneities is found to be about 1200 (+/-200)km with an average conductivity of 0.7 (+/-0.3)S/m. An integrated conductivity depth profile is obtained by using all the stations, and by combining with the Sq analysis results obtained for the same chain of stations. This has revealed the presence of mid-mantle conductor at a depth of about 800 km with a conductivity of 0.15 S/m beneath the Indian region, compatible with the other regional and global models, reported elsewhere. The obtained results are compared with those of the global ones and discussed. 4.5 An Electromagnetic Array Survey for the Detection of a Hypothetical Mantle Plume: Simulation of the Induction Process by Optimized Threedimensional Modelling Authors: Kuras, O., Leibecker, J., Bahr, K. (Goettingen) Institut fuer Geophysik der Georg-August-Universitaet Herzberger Landstr. 180 37075 Goettingen Germany Phone: +49-551-39-7469/7478 Fax: +49-551-39-7459 E-Mail: oliver@geo.physik.uni-goettingen.de The hypothesis of a mantle plume has directed the interest of geosciences towards the structural reconnaissance of the Western Rhenish Massif (Germany). In our research project "Electromagnetic Scanning of the Upper Mantle beneath the Eifel Mountains" an array of 23 long period MT stations has been used for the synchronous recording of magnetic and telluric variations. In contrast to successive measurements, a synchronous configuration allows for the area-wide evaluation of the geomagnetic perturbation tensor with arbitrary reference stations on the array. This leads to an improved quality of the Geomagnetic Depth Sounding analysis. A prominent medium-period anomaly in the horizontal magnetic transfer functions has been discovered in the Northwestern Eifel Mountains. Extensive 3D modelling suggests a highly conductive anomalous structure at depths corresponding to the lower crust or uppermost mantle. A mechanism of local channelling is proposed to explain the high amplitude and small halfwidth of the anomaly. Optimization of the modelling process using a minimization algorithm based on polynomial fitting methods is the subject of current research. 4.6 Crustal Structure Beneath South Pole Region, East Antarctica, from MT Measurements Authors: Philip E. Wannamaker(1), John A. Stodt (1), Louise Pellerin (2) and Darrell Hall (3) (1) University of Utah, Energy and Geoscience Institute 423 Wakara Way, Suite 300 Salt Lake City, UT U.S.A. 84108 pewanna@mines.utah.edu (2) Lawrence Berkeley Laboratory, University of California Berkeley, CA U.S.A. 94720 (3) University of Utah, Department of Geology and Geophysics Salt Lake City, UT U.S.A. 84112 Across South Pole area, East Antarctica, in the 1997-1998 austral summer season, we collected 10 tensor magnetotelluric (MT) soundings using instrumentation designed and constructed in-house. These soundings were obtained to extend knowledge of the structure and physico-chemical state (temperatures, fluids, melts) of the crust and upper mantle of East Antarctica. The effort was offset from South Pole station about 5 km and oriented 210 grid north, where north is the Greenwich meridian, and about normal to the Trans-Antarctic Mountains (TAM). The main purposes are four- fold. First, we illuminate first-order structure and test the cratonic character of the lithosphere of this part of East Antarctica. Second, climatic conditions around South Pole are relatively extreme, and this project should help define MT surveying feasibility over the entire continent. Third, the results will provide the crustal response baseline for possible long- term MT monitoring to lower mantle depths at South Pole. Fourth, the profile takes advantage of existing logistical facilities at South Pole station. Good quality data were obtained, but rate of collection was hampered by low geomagnetic activity and wind-generated, electrostatic noise induced in the ice. The most striking inference of the data is a thick, low-resistivity sedimentary sections immediately below the polar ice cap. Initial interpretations also suggest a lower crust of higher conductivity than typically is attributed to cratonic regions. 4.7 ELECTRICAL CONDUCTIVITY STRUCTURE OF THE HIKURANGI SUBDUCTION ZONE, NEW ZEALAND Authors: Kathy Whaler(1), Malcolm Ingham(2) and Don McKnight(3) (1)Department of Geology & Geophysics, University of Edinburgh, email: whaler@mail.glg.ed.ac.uk (2)School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand, email: ingham@matai.vuw.ac.nc (3)Institute of Geological and Nuclear Sciences, PO Box 1320, Wellington, New Zealand, email: d.mcknight@gns.cri.nz Whilst some subducted slabs have been imaged electrically (eg Juan de Fuca), others do not appear to have a discernible resistivity contrast with their surroundings (eg beneath Chile), perhaps related to trench roll-back rate. This study describes the data collected and their preliminary interpretation in the first stages of a collaborative NZ/UK study of the Hikurangi Margin (HM), where the Pacific plate is being subducted to the north-west beneath the Australian-Indian plate. MT soundings have thus far been made at a total of 36 locations on the east coast of the North Island of New Zealand. Soundings cover the frequency range from 1000-0.01Hz with longer period data from 12 of these sites, including 5 using new Narod ring core fluxgate LMT systems. Shallow dc resistivity soundings have been used for static-shift control. Preliminary results show that to the east of the Main Ranges low resistivities (1-20 Ohm.m) associated with Tertiary and Cretaceous sediments of the on-shore part of the accretionary prism, exist to several kilometres depth. At the southern end of the HM, where Jurassic and Triassic rocks outcrop, resistivities are higher. Pseudosections of TM apparent resistivities and phases for traverses across the subduction zone suggest higher resistivities at depth beneath and to the north-west of the Main Ranges. 4.8p The Electromagnetic Soundings Across T-T Zone in East-South Poland Authors: Ernst T., Jankowski J., Semenov V., Jozwiak W. Institute of Geophysics, Polish Academy of Sciences ul. Ksiecia Janusza 64, 01-452 Warszawa There are very few EM soundings across the Teisseyre-Tornquiste Zone, one of the most prominent tectonic feature of Central Europe. We are presenting some of the results from a profile crossing T-T Zone in East-South Poland. The 2D interpretation was carried out on the base of the complex induction vectors obtained from six points, supplemented by information derived from deep magnetotelluric soundings. The results have shown that to the north of the well known Carpatian Geomagnetic Anomaly, in the area of T-T Zone, there is a deep fault which probably has prolongation westward, to the Holy Cross Mountains. The joint interpretation of the deep magnetotelluric sounding comprising the point Pawlowka of the profile and magnetovariation data from Lvov observatory have shown that astenosphere can exist at the depths of about 100-200 km. 4.9p KOLA PROBE Authors: Fainberg Ed., Abramova L, Barsukov O., Khabensky E. Institute of Geoelectromagnetic Investigations of United Inst. of Physics of the Earth, 142092, TROITSK, Moscow region, box 30, RUSSIA. E-mail: abramova@geo.igemi.troitsk.ru During two decades strong discussion on deep structure of Kola Peninsula takes place. There is discrepancy between MT soundings data and the results based on controlled source soundings. To solve this problem new experiment using transient electromagnetic method is proposed. The target of the experiment is crustal layer laying at 8-12 km depth at Kola Peninsula. The instrumentation and mathematical methods of interpretation of the field data taking into account large sounding antennae and intensive magnetotelluric noise generated by external variations in high latitudes were developed. The prototype of instrumental complex for the super-deep pulse sounding is under development. This work is supported by the Russian Foundation of Basic Researches, grant 97-05-64585 4.10p ELECTRICAL CONDUCTIVITY OF THE EARTH CRUST IN KIROVOGRAD BLOCK OF THE UKRAINIAN SHIELD Authors: Ingerov A.I.(1), Rokityansky I.I.(2), Shirkunov V.V.(3) (1) Geophys.res.lab. "Slavuta", (2) Inst.of Geophys. POB-338/7, Kiev-146, Ukraina, (3) Inst.of Mineral Resourses The Kirovograd block of proterozoic age is the youngest and most conductive one in the Ukrainian shield. Here exist two largest crustal conducting zones: Ryasnopol and Kirovograd anomalies. The first one is situated in SW part of the block, it is manifested by graphite gneiss of Ryasnopol structure on the surface and by long descending branch of MTS curves. MTS array yealds the width of the anomaly of 10-20 km and the azimuth of electrical axis of 315 degrees, the conductor is gently sloping in NE direction, its total longitudinal conductivity is as high as 3000-10000 Sm according to formal interpretation of MTS data. The Kirovograd conductivity anomaly is primarily manifested by regional anomaly of geomagnetic variations at rather long periods of 50-10000 s. MTS data yeald the conductivity of 1000-2000 Sm and conductor's depth of 10 km. In the Kirovograd block EM data manifest a system of faults with inherent azimuths of 0, 17, 90 and 315 degrees forming regularnetwork with characteristic distance between parallel faults of approximately 18, 35, 53...km. MTS made on few fault's crosses show their high conductivity near the Earth surface. The crosses are also the most penetrable for fluids,sulfide and graphite. Geological survey of the conducting zones reveals deposits of ore minerals. 4.11p DEEP GEOLOGICAL STRUCTURE OF THE NEOGENE VOLCANIC AREA OAS-GUTAI (EAST-CARPATHIANS) Authors: Ivanov Ana, Furnica Cornelia, Nistor H., Asimopolos L. Geological Institute of Romania, 78344 1, Caransebes Str, Bucharest 32 e-mail : anaiv@igr.sfos.ro The investigated area corresponds to a part of the Inner Carpathians realm where the major structural group of units belongs to the Inner Dacides, Pienides, Median and Outer Dacides. The North - Transylvanian Fault, the two gaps of the Moho surface, below the Pienidic area and in the front of the Inner Dacides, respectively - corresponding to two "consumption" paleoplanes - are the most interesting geostructural features the work is concerned. Contributions on the deep structure, particularly of the Neogen volcanic structure, have been brought by electromagnetic methods. The available material consists of seven magnetotelluric profiles and the telluric network carried out in this area. Finally a geoelectric model has been obtained. 4.12p AMT-measurings with the new Goettinger "midband"- system Authors: Christiane Jantos, Karsten Bahr Institut fuer Geophysik,Herzberger Landstrasse 180, 37075 Goettingen, Germany Tel.: +49 551 397454 Fax: +49 551 397459 jane@willi.uni-geophysik.gwdg.de In September 1997 and March 1998 electric and magnetic measurements were carried out in Bad Koenigshofen consisting of audiomagnetotelluric measurements (16 Hz until 0,0625 Hz) with a clock rate of 64 Hz and 8 Hz and magnetotelluric measurements (0,1 Hz until 0,00024 Hz) with a clock rate of 2 s. For both recordings the Goettinger RAP-Data-Logger were used, which are able to registrate from 120 s to 64 Hz. The low frequency measurements were made with the Magson-Fluxgate-Magnetometer, which unfits for periods lower than 10 s because of their own noise (20 pT- 30 pT). In this frequency range coils (Metronix MFS05) were used. They are more sensitive for measuring higher frequencies. The intention of this campain was to prove how good the both data-curves fits together. This leads to astonishing good results. With this frequency range it was possible to examine the middle crust (10 km to 40 km). A good conductor have been found in about 14 km. 4.13p GEOELECTRIC AND SEISMIC IMAGE OF THE RHEOLOGICALLY WEAK LAYER AT THE BASE OF THE UPPER CRUST. Authors: V.Kouznetsov, N.Palshin and L.Vanyan, Shirshov Institute of Oceanology, Moscow E-mail:vanyan@geo.sio.rssi.ru and B.Tezkan, University of Cologne. Conductivity models of the Baltic Shield constructed by means of magnetotelluric soundings ( Golod and Klabukov, 1989; Koria and Hjelt, 1993; Kovtun et al., 1992) for the domains with Archaean and Proterozoic age revealed a similar feature for both domains. There is a conductive layer at the depth of about 10 km, i.e. at the base of the resistive upper crust, with the conductance about 2-10 S. In frames of the International "EUROPROBE" project deep electromagnetic investigations have been carried out along 550 km long profile at the Precambrian Byelourussian uplift ( Fainberg et al, 1997). The methodology of field measurements included synchronous measurements in mobile and base sites. According to interpretation results there is a layer with average conductance of 50 S beneath high resistive upper crust at the depth of 11 km. Thus, in two regions of the East-European Platform MT soundings reveal a conductive (i.e.porous) layer at the base of the upper crust. There is a seismic low-velocity wave-guide at the same depth in both regions with the compressional wave velosity decrease about 0.2 km/s. Probably metheoric waters have a significant contribution to that rheologically weak layer. Conductivity model for the Rhenish Massif (Volbers et al,1990) shows similar conductive layer at the depth of 12-15km with greater conductance of a few hundreds S. At the same depth seismic boundaries change their dip from almost vertical at small depth to horizontal that indicates a rheological weakness. Greater conductance value can be a result of higher geothermal regime. This work was supported by the Russian Basic Research Foundation (project 96-05-64051) 4.14p Deep electromagnetic investigation of recent activization zones in tectonosphere Authors: S.N.Kulik, T.K.Burakhovich Institute of geophysics of the Ukrainian National Academy of Sciences, Ukraine. e-mail:earth@igph.kiev.ua The local conductivity anomalies (CA) were discovered on the West of the Eastern-European platform (EEP) by means of Earth's electromagnetic fields analysis. The region of high conductivity characterised by the longitudinal conductivity near 1000 S was found on the West part of Lithuania (Klaipeda CA). Suppose the rocks of upper mantle have minimum degree of melting then the thickness of this CA will be 70 km. Conductive objects were observed in the Earth crust and upper mantle in the northern part of Pripyat' depression with the depth from 30 to 100 km, with the longitudinal conductivity near 3500 S. Yavorov CA is situated in frames of Lvov Palaeozoic deep. Geoelectric model includes the conductor in the Earth crust with the thickness of 10 km. The depth of the top of CA dips from 20 km to 26 km (West-East direction). The conductor is characterised by the integral conductivity 2000 S and the width of 60 km. The Volhyn-Podolian plate we also found CA in the depth interval from 50 to 100 km (Tchernovtsy CA), the longitudinal conductivity near 2000 S. These two latter anomalies are situated in the transition zone from "hot" geoelectric section under Pannonian basin and Folding Carpathians to "cold" one under EEP. In the northern part of Moldovian plate CA with the depth from 50 to 120 km and the longitudinal conductivity near 3500 S is discovered. In the Earth's crust and upper mantle of Near Dobrudgian foredeep are observed CA (thickness 10-20 km and 40 km, the longitudinal conductivity is equal to 2000 S (Tarkhankut CA). These CA in the Earth crust and upper mantle constituted original prolonged zone from Klaipeda anomaly (inside of EEP) though Yavorov and Tchernovtsy anomalies (on the West and the South-West boundaries of the EEP) to Tarkhankut anomaly (outside of the EEP). In the different regions characterised high heat flow (Lithuanian syneclise, Pripyat depression, Volhyno-Podolian and Moldovian plates, Near Dobrudgian foredeep, Tarkhankut raise) high electrical conductivity domains connected with zones of partial melting - asthenosphere. Complex interpretation of the geophysical data manifest the current process of the tectonic activization in the tectonosphere. 4.15p Percolation theory with embedded networks -a model for the conduction mechanism in the middle crust Author: Daniel Labendz Institut fuer Geophysik, Herzberger Landstrasse 180, 37075 Goettingen, Germany Tel.: +49 551 397469 Fax: +49 551 397459 dan@willi.uni-geophys.gwdg.de Electromagnetic deep sounding measurements often indicate a high conductive layer(HCL) in the middle crust (15-20 km) which very often exhibits an electrical anisotropy with respect to the two horizontal directions. A cause of the high conductivities has not been identified inambigiously but graphite or saline fluids are the major players. With the aid of mixing laws, percolation theory and an embedded network approach we model the heterogeneous rock matrix in order to determine the "electrical connectivity" of the highly conductive phase. An essential result is the electrical anisotropy, which indicates a deviation from the perfectly interconnected case. We will present results of the dependence between connectivity and percolation probability as well as the embedding factor. It is not necessary to impose a fractal geometry on the model by incoporating embedded networks. However, in percolation experiments with large networks the resulting cluster at the percolation threshold has fractal geometry. Using magnetotelluric data containing anisotropy it becomes possible to calculate an indicator which can help to determine the cause of the high conductive layers. 4.16p Electromagnetic Crustal structure of southern and central Canadian Cordillera Authors: Juanjo Ledo, University of Barcelona, Spain, and Geological Survey of Canada Ottawa, Canada Alan G. Jones, Geological Survey of Canada, Ottawa, Canada The western margin of the Canadian Cordillera has grown by lithospheric accretionary processes since the Jurassic. The crust of the Canadian Cordillera is formed by terranes originated due to the consumption of oceanic plates beneath Ancestral North America craton and docking of their exotic terranes on top of the craton. To first approximation, the upper crust of the Canadian Cordillera is characterized by high electrical resistivity compared with the more conductive lower crust. However, a more intense study of the data reveal that the electrical structures are also characterized by important lateral changes at all scales, being the electromagnetic response for short periods variable from nearby sites. Previous studies had shown a strike variation with depth, being around N15E degrees in the lower crust, and in the upper crust being more erratic. The regional electric structure can be considered as a 3D upper crust over a 2D lower crust. Moreover, in some areas the relationship between the apparent resistivities and the phases doesn't follow a Hilbert transform relationship, which suggest important 3D effects. The main topic of this paper is to present a regional electrical structure of the Canadian Cordillera crust to explain the main features of the MT data and its interpretation in terms of the known geological features of the region. 4.17p The structure of the lower crust and upper mantle below an area with young volcanism explored with geomagnetic studies Authors: Joerg Leibecker, Karsten Bahr Institute of Geophysics University of Goettingen Herzberger Landstrasse 180 37075 Goettingen jleibec@carl-f.uni-geophys.gwdg.dephone: +49 551 397454 fax: +49 551 397456 The Rhenish Shield is characterized by its quarternary volcanism and the continuing plateau uplift. It was suggested due to various geochemical and seismological explorations that a small mantle plume could be the cause of this volcanism and uplift. Supposed that a plume contains partial melts with increased electric conductivity, it should be possible to detect it with electromagnetic methods. From July 1997 to September 1997 simultaneous measurements of the magnetic and electric field at 23 stations were made in order to explore the structure of this geological interesting area. The stations covered an area of 100 x 120 km in the western part of the Rhenish Massif. Here the magnetic data will be presented. The transferfunctions of the horizontal magnetic field show a distinct anomaly of local increased conductivity in the lower crust/upper mantle. The problem of magnetic distortion caused by a possibly anisotropic crust below the Rhenish Shield and the structure mentioned above will be discussed 4.18p The Solution of the Inverse Problem on the Example of CSAMT Sounding in Central Finland. Author: Alexander N. Shevtsov (Geological Institute of the Kola Science Centre of RAS, Apatity, RUSSIA) In the summer 1997 an experiment of the deep CSAMT sounding has been made in the Central Finland. At this presentation it is introduced an example of the data interpretation with the use of inversion problem realized on the basis of effective linearization technique. The Frechet kernels of the components EM fields were calculated as A.D. Chave article 1984. Algorithms of D.E. Boerner and G.F. West (1989) for layered model and A.D. Chave and Ch. S. Cox (1982) for continuous model used for the forward problem. The nonlinear part of the forward problem was estimated by M.M. Kharlamov technique. The start models for iterations was constructed by Molotchnov - Viet transformation of apparent resistivity curve in bar-zone of the source and from other side it was conductive half - space with resistivity, that was equal mean-geometrical value of the measured apparent resistive values. The technique used for estimations of the penetration depth for the EM soundings in Central Finland with the use of controlled sources and for resolution ability of data, and for estimation of the parameters of electrical cross-section in this region. The upper part of cross-section was estimated by inverse of the DC data. 4.19p Geoelectrical Cross-section of the Ukrainian Part of Eurobridge Profile Authors: Soldatenko V.P.(1), Ingerov A.I.(2), Vlasov Y.T.(2), Rokytyansky I.I.(3) (1)National Mining Academy of Ukraine, 19, K.Marx Ave, Dnipropetrovsk,320027, Ukraine,Fax (380562) 44-08-35 (2)GRL Slavuta Co., Lid, 33, K.Marx Ave,Dnipropetrovsk, 320044,Ukraine, Fax(380562) 476777 (3)Institute of Geophysics,POB-338/7, Kiev-146, Ukraine (380044) 4743251 e-mail:root@slavuta.dnepropetrovsk.ua prte@nmuu.dp.ua (2)GRL Slavuta Co., Lid, 33, K.Marx Ave,Dnipropetrovsk, 320044,Ukraine, Fax(380562) 476777 (3)Institute of Geophysics,POB-338/7, Kiev-146, Ukraine (380044) 4743251 e-mail:root@slavuta.dnepropetrovsk.ua prte@nmuu.dp.ua In central and southern part of Ukraine EUROBRIDGE profile passes on the line geotraverse 6 on which MTS with step 5-10 km and deep sesmic sounding are carried out. Geotraverse 6 passes almost parallel and in 30 km to north-east of the Ryasnopol anomaly conductivity