Title: 9.1p Tectonic structure of the Transdanubian graphitic conductor Author: A. Adam Geodetic and Geophysical Research Institute, H-9401 Sopron, POB 5, adam@ggki.hu Two Hungarian geophysical institutes (GGRI, ELGI) carried out about 300 magnetotelluric (MT) deep soundings in the NW part of Transdanubia (W-Hungary) partly along basic profiles, partly in areal distribution. In case of two-third of the measurements the period range has been beween 1/20 and (1000 s (ELGI) in the other cases between 10 and 3600 s (GGRI). These measurements aimed first of all at the determination of the tectonic structures represented by a strong conductivity anomaly detected in the early sixties in the upper crust of NW Transdanubia between two great tectonic lines i.e Raba and Balaton-line. The analysis of the electromagnetic distortion of the MT sounding curves proved that the parameters of this conductor (TCA) (its depth, conductance etc.) could be best approximated by the Rhomin curves on the basis of a conducting dike model. Using the 1D/2D inversion techniques, the tectonic structure of this Paleozoic graphitic conductor - as supposed from its extreme resistivity and its position below the Mesozoic formations - has been determined, including the synclinal axis of the Paleozoic basin, the strike slip and the thrust sheets, etc. The former ones follow the typical NE-SW longitudinal fractures/faults of the Pannonian Basin. According to the MT measurements, the low viscosity graphite smeared into the tectonic zones as it appears in the highly conducting dikes, which emerge above the average depth of the conductor. The graphite can influence the stress accumulation of tectonic earthquakes and reduce the area affected by them as shown by high attenuation factors characterizing the area of the TCA. 9.2p Establishing the Structural Relations of Geological Units in the North Dobrogea through Magnetotelluric Researches Authors: L.Asimopolos, C.Furnica, A.Ivanov, H.Nistor Geological Institute of Romania, 78344 1, Caransebes Str, Bucharest 32 e-mail :anaiv@igr.sfos.ro Dobrogea is an area within the Carpathian Foreland, where parts of two major units crop out: the North Dobrogea Orogen and the Moesian Platform. North Dobrogea Orogen is the westernmost part of an Early Alpine or Cimmerian Folded Belt. Several nappes (Macin, Consul, Niculitel, Tulcea) of Jurasic or Neocomian ages have been there recognized. They have a North or North-East vergency, toward the Scythian Platform. In accordance with magnetotellurical researches was elucidated the order of nappes, structural elements and Mohorovicic discontinuity. We have analyzed geological structures dimensionality using more parameters: skew, elipticity, those six parameters defined by Kao and Orr (1982), and other. We presented in this poster the sections function of these parameters, the sections of phase and resistivity in both cases B-polarization and E-polarization as well as the behaviour of the telluric parameters. ABSTRACT OF A POSTER FOR PRESENTATION AT THE E.M.WORKSHOP Title: 9.3p Analysis of magnetotelluric and magnetic variation data from networks of sites Author: Roger Banks Department of Geology & Geophysics, University of Edinburgh, Edinburgh EH9 3JW, U.K. Roger.Banks@ed.ac.uk While the analysis of magnetic variation data relies very heavily on knowledge of the spatial structure of the electromagnetic field, magnetotelluric measurements are usually based entirely on measurements at a single site (except where remote reference techniques are used to combat noise). One exception is the E-Map approach to analysing data from dense profiles of observations. The availability of the Kayabe database, with its dense network of impedance values, prompts the question whether more information can be extracted from such an array before modelling or inversion begins. Spatial analysis has to be based on the predictions of simple models of the interaction between the e.m. field and the conductive structure. For instance, if the "large- scale" structure can be treated as no more than two-dimensional, and the "small-scale" structure distorts the electric field without modifying its phase, Argand plots of the array impedance elements will share a common phase, provided the impedance tensor is expressed in regional strike coordinates. The viability of these and other approaches will be tested on data sets with different spatial densities, hopefully including a revised Kayabe database. POSTER TITLE :9.4p MT measurements in Variscan SW Ireland as part of the VARNET Project AUTHORS : Patrick Denny, GFZ-Potsdam, Telegrafenberg, D-14473 Potsdam, Germany Colin Brown, Applied Geophysics Unit, NUI Galway, Co. Galway, Ireland MT measurements were performed at thirty sites in southwest Ireland in 1995 and 1996 as part of an integrated study of the European Variscan Front.The data were processed using robust and remote reference methods to remove as much cultural noise as possible.The large scale geology was shown to be a complex mixture of both Caledonian and Variscan processes. Many different inversion approaches were used using the Occam method (DeGroot-Hedlin, 1995) due to the low quality of the electrical data on the short profile. A previously under-utilized method involving a subset of good quality electrical data and all magnetic data was used to create models with a reasonable magnetic fit. Due to the larger scale 3D nature of the region, attempts were made to find the large scale inductive effects with 3D modeling code (Mackie, 1983) and magnetic functions. Modeling suggests a conductor which may represent a continuation of the Iapetus Suture into SW Ireland in addition to the presence of Variscan structures. Previous MT data was remodeled to create a clearer regional geoelectric model. Certain aspects of confidence limit calculation for impedance parameters were also investigated. The findings are described and explained in more detail below. Title: 9.5p PRELIMINARY RESULTS OF THE SCALE MODELING OF THE SEA-FLOOR CSEM Authors: Z.Djatieva, L.Vanyan, Shirshov Institute of Oceanology, Moscow, Russia, A.Kouznetsov and Yu.Popov, Vniigeofizika, Narofominsk, Russia vanyan@geo.sio.rssi.ru djatieva@geo.sio.rssi.ru Nakhimovsky Prospect 36, Shirshov Institute of Oceanology, Moscow 117851 Russia tel.: +7 (095) 124 7956 fax: +7 (095) 124 5983 Kievskoe Avn. 5, Vniigeofizika, Narofominsk 143300, Russia tel.: +7 (095) 592 1882 There was a significant development of the sea-floor frequency sounding theory during the last decade. The most up-to-date knowledge concerns 1D models. In order to obtain information about the features of CSEM for 3D case the scale modeling was carried out in an electolythic tank. The model consists of three layers. The brine layer (resistivity 0.05 Ohmm, thickness 24 cm) simulates the sea water, the ceramic layer (resistivity 0.25 Ohmm, thickness 2.5 cm) simulates pillow lavas and resistive tank bottom simulates the gabbro complex (resistivity is more then 1000 Ohmm). The absence of the pillow lavas at the square 30x30 cm simulates the gabbro complex uplift. Electric field of the electric dipole was measured with the spacing 26 cm at frequency band 0.63 MHz-8.0 MHz. According to the EM scaling these frequencies correspond to the low frequencies used in practice. A strong increase of the electric field was revealed when the dipole-dipole array is moving across the uplift. The increase was about 2 times for the azimutal component and 4 times for the radial one. These values are much greater then the difference of the electric components above 1D models with pillow lava and without it. Thus the 3D edge effect plays a significant role in the sea-floor CSEM. This work was supported by Russian Basic Research Foundation (project 98-05-64025). Title: 9.6p Two-dimensional anisotropic and three-dimensional modeling of crustal anisotropy: An Example from the German Deep Drilling Project (KTB) Authors: Markus Eisel, Oregon State University, College of Oceanic and Atmospheric Sciences, Ocean. Admin. Bldg. 104, Corvallis, Oregon 97331, e-mail: eisel@oce.orst.edu Josef Pek, Geophysical Institute, Academy of Sciences of the Czech Republic, Bocni II, Prague 4 -Sporilov, 14131 CR, e-mail: jpk@ig.cas.cr Vaclav Cerv, Geophysical Institute, Academy of Sciences of the Czech Republic, Bocni II, Prague 4 -Sporilov, 14131 CR, e-mail: vcv@ig.cas.cr Volker Haak, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, e-mail: vhaak@gfz-potsdam.de Gary D. Egbert, Oregon State University, College of Oceanic and Atmospheric Sciences, Ocean. Admin. Bldg. 104, Corvallis, Oregon 97331, e-mail: egbert@oce.orst.edu Previous magneto-telluric measurements in the KTB surrounding revealed an extreme electrical anisotropy of the upper to middle crust. Almost neglect-able intrinsic anisotropy of drill cores and steeply dipping cataclastic zones enriched with graphite support the models of macroscopic anisotropy. Nevertheless, these models fail to constrain the depth extend of the anisotropic layer and therefore are in conflict with models of a regional mid-crustal conductor. Poor data quality in the 'dead band' prevents resolving details at depths where the two structures interact. In an attempt to answer these critical questions wide-band MT measurements along a 40 km SW-NE profile crossing the cataclastic block of the KTB vicinity and reaching into Bohemia were carried out. Time series recorded simultaneously at up to 8 sites were processed with the multiple-station approach (Egbert, 1997) revealing the existence of coherent noise in the whole survey area. Detailed studies of the time variations of coherent noise could eliminate the strong but not obvious bias of the sounding curves. The cleaned-up transfer functions allow detailed studies of the superposition of the anisotropic and regional structures. The coupling of the two structures is discussed by means of two-dimensional anisotropic and three-dimensional modeling. Title: 9.7p Induction Studies in Greenland: GDS at High Latitudes Author: Martin Engels, University of Uppsala, Sweden Address: Uppsala University, Department of Geophysics, Villavagen 16, S-75236 Uppsala, Sweden Tel: (+46) 18 4713327 Fax: (+46) 18 501110 E-mail: me@geofys.uu.se On Greenland a magnetometer array has been in operation for several years. The array consists of 17 coastal sites (Danish Meteorological Institute, Eigil Friis-Christensen) and 4 sites on the ice sheet (University of Michigan, Robert F. Clauer). The sampling rate is 20 seconds. >From these data the normal conductivity structure (1D) is estimated by using i) the anomalous magnetic field due to the coast effect ii) the normal magnetic field applying a modified gradient method i) The data indicate strong coast effects, represented by induction arrows. Using Peter Weidelt's code, thin-sheet modeling is performed, representing the known land-ocean conductivity contrast within the thin-theet as top-layer. Results from the modeling are adjusted to fit the results from robust data analysis, by varying the layered conductivity structure below the thin-sheet. Periods from several minutes to hours are considered. ii) Greenland is situated at high latitudes within the polar oval, a very complex, dynamic source field region. The gradient method is applied to the first daily harmonics, which are mainly due to the Sq-polar and DP2 current systems, in the main consisting of two current vortices. A correction term is taking strong source field inhomogeneities into account (both the gradient method and the usual magnetotelluric relation can be considered as the first term of a space domain series expansion). The anomalous magnetic field due to the coast effect is subtracted and an ideal phase is assumed for the west-east gradient. The often problematic source field geometry requires a strong pre-selection of usable 3-day data intervals and optionally an iterative weightening of residuals. One year of data has been analysed, reasonable C-responses are obtained and inverted by a three-layer model. Title: 9.8p DEEP GEOELECTRIC STRUCTURE OF THE BAIKAL MOUNTAIN AREA Authors: Feldman, I.S., Okulessky, B.A. ( Associated Institute for High Temperature, RAS, Izhorskaya str, 13/19, Moscow, 124412, Russia) and Shpak, I.P. (Ministry of natural resources of the Russian Federation, B.Gruzinskaya str, 4, Moscow, 123812, Russia) MT-data obtained at about 200 field stations in the Baikal mountain area were analyzed. The interpretation was carried out by means of selection of 2-D models for individual sites of the area. The models were selected together with the simultaneous coordination of MTS curves for longitudinal and transversal directions in relation to the structures. The models were constructed on the basis of results of geological studies of the Proterozoic formations and both TEM and DC data. The distinguishing feature of these formations is a wide occurrence of interbeds of carbon-bearing material causing the resistivity of formations in a massif to decrease up to the tenth fractions of Ohm-meter. All basic peculiarities of MT-curves are determined by sharp contrasts of resistivity of the upper part of the Earth crust. Narrow conductive troughs up to depths of 6 - 9 km are corresponding to root parts of ancient greenstone belts. The majority of earthquakes in this region are associated just with these zones. A conductive layer with conductivity of about 1000 1/Ohm-meter is traced in the upper mantle within the 45- to 70-km depth interval over the entire region considered, including a narrow zone contiguous to the Baikal on the side of the Siberian platform. Title: 9.9p Studies of the Georesistivity Structure in the Central Part of the Northeastern Japan Arc Authors: Yukio FUJINAWA1(NIED), Noriaki KAWAKAMI2(GERD), and Yoshimori HONKURA3(TIT) 1 Nat. Res. Inst. Earth Science and Disaster Prevention(NIED), Tennodai 3-1, Tsukuba-shi, Ibaraki-ken, 305-0006, Japan, E-mail: fujinawa@bosai.go.jp 2 Geothermal Energy Research and Development Co., Ltd.(GERD), 11-7,Kabuto -cho, Nihonbashi, Chuo-ku, Tokyo, 103, Japan, E-mail: kawakami@gerd.co.jp 3 Tokyo Institute of Technology(TIT), Ookayama 2-12-1, Meguro-ku, Tokyo, 152, Japan, E-mail: yhonkura@cc.titech.ac.jp Wideband (0.002~20,000Hz) magnetotelluric measurements(MT) observations have been conducted along three traverses in the central Tohoku district of the northeastern Japan arc at 90 observation sites. The data are expected to provide independent constraints for understanding of seismo-tectonics and geology in the region where the Pacific plate subducts beneath the Eurasian plate. In order to have refined model to the previous one the MT impedance tensors were corrected for the effects of the three-dimensional galvanic distortions induced by the subsurface 3D inhomogeneity using the Groom and Bailey(G/B) tensor decomposition technique and for the static shift effect using results of the TEM measurements at each site on the three transects. Smooth two-dimensional georesistivity models were obtained along the three transects assuming NS direction as the regional 2D structural strike as inferred from the G/B decomposition procedure. The refined resistivity profiles indicate two clear conductive anomalies in the Central Basin Range and in the Kitakami, Abukuma river regions as well as smaller resistivity anomalies with considerable variability among the three 2-D profiles. The resistivity distributions are combined with the seismic velocity, temperature, gravity, geomagnetic anomalies, seismic wave dissipation parameter, and seismic activity to understand the deep crustal regime associated with the subduction processes. Title: 9.10p Electrical Properties Correlation in the Kola Super Deep Hole and Surrounding Area. Authors: Larissa D. Galitchanina & Abdoulkhay A. Zhamaletdinov (Geological Institute of the Kola Science Centre of RAS, Apatity, RUSSIA) Electrical properties of rocks in the Kola super deep hole (SDH) and surrounding area are characterized by sharp changes from 0.1 Ohm.m in graphite and sulfide bearing phyllites till 10\6 Ohm.m in gneisses and granite-gneisses. For Proterozoic part of section (0-6.8 km) electrical properties of rocks depend an geological composition and distribution of graphite and sulfide bearing layers. Most of them can be traced from the depth to the surface where they spread along the strike of geological structures on many dozens of kilometers. The most thick of them - nickel prospective carbonaceous layer - has thickness about 2 km and longitudinal conductivity about 5- 10 thousand siemences. The layer spread on more then 100 km and heavily effects on results of the deep EM soundings with natural and control sources. Archaean part of electrical section depends on both geological composition of rocks and physical phenomena : temperature, fluid distribution, porosity, pressure etc. Fluids distribution along interlyering causes the apparent vertical anisotropy with coefficient 1.5-2 at the depth of 7- 10 km. With the use of auto- and cross power functions the relationship of laterolog data with inclinometry data has been studied. These results showed absence of correlation between apparent resistivity curve and variations of the hole diameter. Two possible models of the Kola SDH electrical properties extrapolation to the depth are supposed. The first, based on statistical extrapolation, suppose the fast decrease of resistivity till 100 Ohm.m at the depth of 20 km; the second one, based on taking into account geological composition of rocks, suppose the slow decrease of resistivity till 10\4 Ohm.m at the depth of 20 km. The obtained gradient of resistivity with depth is equal to 0.1 order per 1 km of depth. The first version is in more agreement with the AMT soundings. The second version of extrapolation is more consistent with the data of the deep soundings with powerful controlled sources and with observation for the electrical properties of Archaean rocks on the surface. Further investigations are necessary to find consent between the deep soundings data, results of the super deep drillings and electrical properties of rocks in the surrounding area. Title: 9.11p Interpretation of MT data across the exposed internides of the Trans-Hudson Orogen. Authors: Xavier Garcia, University of Barcelona, Spain Alan G Jones, Geological Survey of Canada, Ottawa, Canada The THO94 MT data set was recorded in northern Saskatchewan during the Fall of 1994. The main goal of this transect was to trace the enigmatic NACP anomaly onto the exposed rocks of the internides of the Trans-Hudson orogen. After processing the time series data from the 23 MT stations to remove strong auroral effects, the MT responses were tensor decomposed, modelled, and geologically interpreted. The GB-based tensor decomposition gave a N60W strike as most appropriate, which confirms the rotation of the deep conductivity structures to the east as suggested by Gupta et al. The regional responses were inverted, using RRI, in this coordinate system. The conductivity model from the profile further south correlates with the seismic interpretation of subduction of La Ronge metasediments to the Hearne craton edge. The NACP anomaly is imaged in the La Ronge Domain, in the contact with the Rottenstone Domain, just over the Guncoat thrust, suggesting the same origin for the anomaly in the north of the Province of Saskatchewan as the one suggested to the south. To the south of the profile there is a body deeping to the North interpreted as the Sask craton. Title: 9.12p GEOELECTRIC MODEL IN THE CENTRAL SECTION OF THE BULGARIAN BALKANS. Authors: N.G. Golubev, E.R. Martanus, E.Yu. Sokolova, Iv.M. Varentsov (Geoelectromagnetic Research Institute RAS, Troitsk, Russia; e-mail: igemi1@pop.transit.ru) The deep MT sounding project was held in the 80s along seven profiles crossing the whole country. MT and induction transfer functions were estimated in the interval from 600 to 10000 s in about 70 sites. It was the first attempt to study the deep geoelectric structure in Bulgaria and preliminary publications reported interesting objects like crustal and asthenospheric conducting structures in the Western Rhodopeans. The subject of this paper is the joint analysis of deep MT soundings together with a few prospecting MT sites along the most detailed meridional Ruse-Madan transect running from the Romanian border across the Balkans to the Eastern Rhodopians. The conductivity model derived for this section within bi-modal 2D inversion procedure is presented in comparison with previous results. Title: 9.13p Magnetic distortion of the GDS transfer functions. An example from the penninic Alps of eastern Switzerland (Graubuenden) Authors: Marcus Gurk and P.-A. Schnegg MARKUS GURK Geomagnetism Group of the University Rue de l'Observatoire 58 CH-2000 NEUCHATEL Switzerland Phone: +4132 889 68 70 Direct: 889 88 17 Fax: +4132 889 62 81 email: Pierre-Andre.Schnegg@on.unine.ch markus gurk@geol.unine.ch URL: www-geol.unine.ch/GEOMAGNETISME/HomePage.html Geomagnetism Group, Geological Institute, University of Neuchatel, Switzerland marcus.gurk@geol.unine.ch In the last two years 64 MT and GDS soundings have been carried out in the Alps of eastern Switzerland (Graubuenden) to get information about the electrical conductivity distribution of this region. One of the main findings was an anomalous directional behaviour of the real induction arrows over the entire period range (T= 1-300 s) on the mesozoic Buendnerschiefer together with a strong vertical magnetic field. The Buendnerschiefer in the investigation area are situated between the crystalline Aar and Gotthard massifs to the north and the crystalline penninic nappes (Adula, Tambo and Suretta) to the south. The sediments form an elongated eastwards plunging ramp with a possible conductive link to the Permo- carboniferous trough (Molasse basin) of northern Switzerland (and the Rhinegraben?). We consider electric currents, presumably induced in the Molasse basin and leaking into the Buendnerschiefer as possible cause for the observed effects upon the electromagnetic fields. Title: 9.14p High Frequency Audio-MT in complex geological and topographic environment Authors: S. Hautot (1), M. Trique (2), F. Perrier (2), and P. Tarits (1) (1) IUEM, UMR 'Domaines Oceaniques', Place Nicolas Copernic F-29280 Plouzane, France (2) Laboratoire de Geophysique, Commisariat a l'Energie Atomique BP12, F-91680 Bruyeres-Le-Chatel, France VLF and audio-MT soundings were carried out on the Sur-fretes ridge in the French Alps to help the characterization of the hydrological properties of the geological structures. The zone is characterized by a large elongated (2D) topographic ridge, 400 m high and 1 km wide. The geology is sub-vertical and normal to the ridge axis. As a consequence, the TE mode for the geology corresponds to the TM mode for the topography and conversely. We investigate this extreme situation to improve the understanding of the electrical coupling between the large topography effect and the geology and to characterize the electrical structure of the site. 2D and 3D models are presented. Conductive bodies that cross some of the geological vertical layers are evidenced within the ridge. These conductors are interpreted as perched aquifers. The results are in good agreement with the data from the hydrogeological and hydrogeochemical studies. Title: 9.15p A MAGNETOTELLURIC STUDY OF THE ALPINE FAULT, NEW ZEALAND Auhors: Malcolm Ingham(1) and Colin Brown(2) (1) School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand, email: ingham@matai.vuw.ac.nz (2) Applied Geophysics Unit, University College Galway, Galway, Ireland, email: Colin.Brown@ucg.ie Magnetotelluric soundings have been made at seven locations on a 4km profile crossing the Alpine Fault in the South Island of New Zealand. The "distortion" techniques of Groom & Bailey (1989) and Lilley (1998a, 1998b) have been used to derive regional apparent resistivity and phase curves which correspond to electromagnetic induction in orientations parallel and perpendicular to the fault. Two-dimensional inversion of the regional responses reveals a narrow (<1km wide) conductive zone to be associated with the Alpine Fault. This conductor is most probably related to the heating of deep circulating meteoric water in a region in which enhanced temperatures occur at shallow depth due to the tectonic uplift of the Southern Alps. Title: 9.16p ON THE USE OF MOHR CIRCLES TO INTERPRET MT DATA. AN EXAMPLE FROM SOUTHERN PORTUGAL Author: F. W. Jones, Univ. of Alberta. E-mail: wjones@terra.phys.ualberta.ca A. Correia, Univ. of Evora Mohr circles have been plotted from magnetotelluric impedance tensors for thirty-four sites in southern Portugal for five periods. The array of sites lies over the intersection of the Messejana fault and the Ferreira-Ficalho overthrust that separates the Ossa-Morena and South-Portuguese zones. Contour plots of parameters calculated from the Mohr circles show distinct differences between these two zones. Both anisotropy and skew are more variable and generally greater in the more geologically complex Ossa-Morena zone. These parameters also show considerable variation with period in their spatial character, indicating that both the anisotropy and dimensionality of the structures vary with depth. However, comparisons of the contour plots raises questions such as: (1) Can we infer the scales of anisotropy and dimensionality from the contour plots? (2) Do the scales of the anisotropies and dimensional characteristics increase with depth or is this an artifact of the method? (3) Does the geological complexity change with depth as indicated by the data or is this observation a reault of the method? (4) What is the relationship between anisotropy and dimensionality anyway? (5) What is the meaning of the parameters we are calculating in terms of the real geological structures? In the poster we present the contour plots with some of the observed conclusions and invite comments and discussion on the results and the above questions. Title: 9.17 Magnetotelluric measurements in southern Mexico' Authors: A.Jording, H.Joedicke (WWU Muenster, Germany), J.Arzate (UNAM, Mexico) email: A. Jording : jording@uni-muenster.de H. Joedicke: jodickeQuni-muenster.de J. Arzate : arzatej@igeofcu.unam.mx The Mexican crust is characterized by terrain-tectonics and the subduction process of the Cocos plate at the Pacific coast. To study the distribution of the electrical conductivity in the Mexican crust down to the subducting plate, magnetotelluric measurements have been carried out along the profile GEOLIMEX (Geoseccion litosfere de Mexico). This SSW - NNE profile in the south of Mexico is running from Puerto Escondido at the Pacific coast via Oaxaca up to Tlacotalpan at the Gulf of Mexico. On this global transect refraction seismics and gravimetric measurements have been carried out, as well. Preliminary results of 2D modeling give clear indications of the existence of a conducting zone paralleling the top of the subducting slab. Title: 9.18p The rezults of physical modelling of magnetotelluric field of Carpathian region. Authors: Kobzova V., Bilinsky A. Carpathian Branch of Subbotin Institute of Geophysics NASU Ukraine 290601 Lviv Naukova str.3b E-mail va@carp.lviv.ua Poster for session 9 Scale 3D modelling of magnetotelluric field of Carpathian region a total area about 1000 x 750 km has been carried out using electrolytic tank 5.0 x 5.0 x 0.7 m. Geoelectromagnetic variations in the range periods 0.2 - 10 hours were modelling by field of plane harmonic electromagnetic wave. MT field for assumed three types of a deep geoelectric section in the Carpathian region has been studied using physical modelling techniques. The first model is an inhomogeneous surface layer which is in full agreement with the map S of a sedimentary cover. The second model is a sedimentary layer and a crust conducting zone under the whol Carpathian arc. The third model is a sedimentary layer, a crust conducting zone and 3D asthenosphere. Comparison of spatial and frequency characteristics of these models with those similar in nature make it possible to conclude that geoelectromagnetic variation anomaly in the Carpathian region is mainly (60 - 70%) due to geoelectric inhomogeneity of a sedimentary cover. Model data are in accord with natural data within (80 - 90)% due to crust and asthenospheric conducting zones included into a model. Title: 9.19p Genetic relationship between Palaeoproterozoic collisional processes and conductivity anomalies: Implications from the North Karelia region in the Fennoscandian Shield Authors: T. Korja1), D.E. Boerner2) J. Kohonen3) and J. Lerssi3) 1) University of Oulu, Institute of Geosciences, Oulu, Finland Currently: Geological Survey of Finland POB 96, FI-02151 Espoo, Finland, tel: +358-205 502 530; fax: +358-205 5012; email: toivo.korja@gsf.fi 2) Geological Survey of Canada, Ottawa, Canada 3) Geological Survey of Finland, Espoo, Finland Electromagnetic studies of Precambrian regions have shown that elongated conductivity structures are parallel to, and approximately colocated with, collisional orogenic belts. Recently a hypothesis associating regional conductivity anomalies with the sedimentary response from the initial phases of passive margin destruction was advanced by Boerner et al. (1996). As the orogens consist of highly deformed basement and cover rocks, it is possible that other facies or tectonic elements of the destroyed margin contribute to the observed conductivity within the orogenic belt. Our purpose is to test further the association between orogenic conductors and tectonic sedimentary facies. The comparison is based on recent detailed geological mapping from the North Karelia Schist Belt in Finland at a scale commensurate with available high-resolution airborne EM data, allowing rigorous spatial comparisons between rock units and surficial conductivity anomalies. Major observations are: (1) Graphite provides a major electrical conduction. (2) An euxinic depositional environment can be implicated in all cases of a conductive assemblage. (3) It is possible to identify two major depositional settings that are common to all conductors. Extensional basins are initial rift or pull apart basins that developed higher energy sediment input sometime after initial formation while compressional basins represent foreland basin system that developed on the Karelian foreland in response to the advancing Svecofennian domain. (4) The pre-collisional stage of basin and margin formation (extensional basins) does not seem to produce elongated conductors and large amounts of conducting material whereas the post-collisional stage (compressional basins) contains the dominant proportion of conducting material. Title: 9.20p Magnetotelluric Sounding in Choukotka Authors: V.V.Kotok, A.S.Lisin and V.V.Yevstigneyev (Troitsk Institute for Innovation and Fusion Research (TRINITI), 142092, Troitsk, Moscow Reg., Russia). MTS was carried out in Choukotka in Summer, 1990. The sounding was aimed at searching deep geological-tectonic situation near the Bilibino Atomic Power Plant (BAPP). Total number of the MTS points are 20, with mean step between them - 15km. The analysis and interpretation of the curve of the apparent resistively (Reff) profile distributions allows to make the following conclusions: 1. Profile distributions of (Reff) are noticeably changeable in horizontal and vertical directions. 2. A decrease of (Reff) is observed on the depth below 30km that can be connected with the crust-mantle conductive zone. 3. In central part of the profiles there is an emergence of the high resistance paleozoic rocks. The average (Reff) is 5600 Ohm at 10km depth here. 4. The vertical zone of the maximum gradient (Reff) corresponds to the crust fault that is observed due to cosmogeological data at a distance of 10km from BAPP. 5. In MTS points situated near tectonic disturbance obtained from airocosmic data, local falls of Reff occur. Title: 9.21p Electromagnetic signature of an ancient collision zone: the Trans-Hudson Orogen and the NACP Authors: Juanjo Ledo, University of Barcelona, Spain, and Geological Survey of Canada Ottawa, Canada Alan G. Jones, Geological Survey of Canada, Ottawa, Canada The Paleoproterozoic TransiHudson Orogen (THO) extends from South Dakota north through North Dakota and Saskatchewan, across Hudson Bay and is believed to extend into Greenland, with possible counterparts in Scandinavia. Magnetotelluric studies along eight profiles from North Dakota to northern Saskatchewan map the NACP as a predominantly 2D structure. The NACP, as a multiibody structure, appears in all profiles, but it presents significant variation in its depth, horizontal position and extent, as well as its strike, along its length. Due to the blanketing effect of the Paleozoic sediments, the multiibody structure cannot be distinguished from an anisotropic body. The Behaviour of both polarizations is very similar for the different profiles, except for the northernmost one due to the lack of the sedimentary Paleozoic layer at the surface. The profiles located in southern Saskatchewan and the northernmost one in North Dakota display bodies of enhanced conductivity deep in the crust, beneath the midicrustal NACP. The tectonic implications of the location, geometrical shape and nature of the NACP will be the main topics of this paper Title: 9.22p SHALLOW AND DEEP RESISTIVITY STRUCTURES IN THE AMIATA VOLCANIC AREA DEFINED BY MAGNETOTELLURICS Authors: A. Manzella (1), R. Mackie (2), J. Larsen (3), A. Fiordelisi (3) (1) CNR - International Institute for Geothermal Research, Pisa, Italy (2) Geosystem s.r.l., San Francisco U.S.A. (3) PMEL, NOAA Seattle, U.S.A. (4) ENEL VDTG, Pisa, Italy. e-mail address: Adele.Manzella@iirg.pi.cnr.it Poster presentation to Session 9: "Local and regional electromagnetic studies" 28 magnetotelluric (MT) measurements were collected in the Mt. Amiata geothermal region (Italy) during September 1994. They were alligned along a profile with an average spacing of 1 km. Due to the strong electromagnetic noise in the area a simultaneous recording at a quiet reference site was performed. To improve the resolution of the shallow part a continuous telluric profiling was also adopted. Direct current data at all the sites and the continuous profile methodology allowed the removal of static shift along the profile. Two-dimensional inversion of TM and TM-TE data was performed, taking into account topographic and coast effects. The use of telluric continuous profiling produced a significant increase in resolution of the first 2 km depth. A positive correlation with known geology and other geophysical data was achieved. The resistivity values we modeled defined new exploration areas besides the already known geothermal system. Title: 9.23p Geoelectric structure of the North Armorican cadomian belt Authors: Guy Marquis (EOST Strasbourg), Benoit Tournerie, Dominique Gibert (Geosciences Rennes), Sebastien Theetten (UBO Brest) As part of the GeoFrance 3D Armor project, a magnetotelluric profile has been acquired through northern Brittany to determine the geometry of Cadomian and Hercynian structures from the Channel to the North Armorican Shear Zone. Tensor analysis shows a preferred orientation of conductivity NE in the Cadomian and EW in the Hercynian units, in good agreement with the regional geological strikes. These directions are consistent throughout the whole period range for the northernmost sites. Inversion of the data reveals: i) a thin, NW-dipping, high-conductivity horizon that corresponds to the Major Cadomian Contact (MCC) underlain by a 5 to 8 km-thick resistive body, interpreted as being migmatites, ii) a complex high-conductivity structure around the North Armorican Shear Zone documenting the importance of the deformation in this area, iii) a high contrast in conductivity between the conductive lower crust in the north and south portions of the profile to a resistive lower crust in the centre, probably caused by a large fluid-releasing event related to Hercynian tectonics, as documented by Hercynian veins observed east of the profile. Our MT data bring an important complement to the coincident seismic, magnetic and gravity data also acquired in this project. Guy MARQUIS Laboratoire Imagerie Tectonique CNRS UMR 7516 Ecole et Observatoire des Sciences de la Terre 5 rue Rene Descartes 67084 STRASBOURG FRANCE marquis@eost.u-strasbg.fr Tel: +33 (0)3 88 41 64 86 Fax: +33 (0)3 88 61 67 47 Title: Regional conductivity structure of the island of Newfoundland Authors: 9.24p Gary McNeice, Memorial University of Newfoundland, St. John's and Geological Survey of Canada, Ottawa (now at Phoenix Geophysics, Scarborough) Alan G. Jones, Geological Survey of Canada, Ottawa Toivo Korja, Oulu University and Geological Survey of Canada, Ottawa Jim Craven, Geological Survey of Canada, Ottawa James A. Wright, Memorial University of Newfoundland, St. John's, and Robert G. Ellis, University of British Columbia, Vancouver (now at BHP Minerals, Golden, Colorado) Data from 77 magnetotelluric (MT) soundings across the Appalachian orogen in Newfoundland, Canada, are analysed to determine the conductivity structure of major tectonic boundaries of the orogen and the regional conductivity structure. An extended GroomiBailey tensor decomposition analyses is developed and employed to remove galvanic distortion effects produced by the complex near surface geology of Newfoundland. In the proposed decomposition analysis a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. The recovered 2D responses are representative of regionali scale structures and 2D inversion algorithms can be employed to recover the regional conductivity structure. The regional conductivity structure of the orogen suggests that the Baie Verte Line, Red Indian Line and Dover fault i Hermitage flexure represent significant crustal tectonic boundaries. The conductivity structure changes both across and along strike in Newfoundland. The magnetotelluric data do not support a near twoidimensional model of three lower crustal blocks but indicate a more complex threeidimensional geometry in southern Newfoundland. The Baie Verte Line is imaged as an upperi middle crustal boundary separating the deformed, overthrust passive margin of Laurentia (Humber zone) and resistive arc terrain of the Notre Dame subzone. The Red Indian Line marks the boundary between the Notre Dame and Exploits subzones of the Dunnage zone and is roughly coincident with a change in lower crustal conductivity. The Dover is imaged as a near vertical boundary between upperimiddle crusts of contrasting electrical properties, extending to depth of at least 20 km. The sharp change in upperi middle crustal conductivity observed in the magnetotelluric data suggest that orogen parallel motions were important in the development of many of the major boundaries of the orogen. The data support a three part division of the lower crust. Title: 9.25p Study of an EM background by means of the Japan Sea Cable (JASC). Authors: R. Medzhitov(1), H.Utada(2), L.Vanyan(1), N. Palshin(1). 1-Shirshov Institute of Oceanology, 23 Nahimovsky Prospekt, Moscow 117218, Russia 2-Earthquake Research Institute, The University of Tokyo, Bukyo-ku, Tokyo 113, Japan. Since 1996 cable voltage in the Japan Sea from Nakhodka to Naoetsu has been measured by Earthquake Research Institute (Tokyo), Shirshov Institute of Oceanology (Moscow) and Pacific Oceanological Institute (Vladivostok). The data obtained is used for study the lithosphere resistivity and for water transport monitoring in the Japan Sea. The same data has been also analyzed with the purpose of getting information about statistical parameters of the EM background. The daily, monthly and seasonal variability of power spectra of cable voltage induced by different sources has been analyzed. Comparison of obtained data from GN,GP, GM, HAW-1 cables and data of experiments, was carried out specially for study the electromagnetic background in the Japan Sea was made. Title: 9.26p KACMHATKA: DEEP MODEL FOR THE ELECTRICAL CONDUCTIVITY OF THE SEISMOACTIVE ZONE Authors: Yu.F.Moroz, A.G.Nurmukhamedov. Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences. Petropavlovsk-Kamchatsky 683006. Russia. E-Mail: ivgg@svyaz.Kamchatka.su The study area is located at the Pacific coast of Kamchatka which seismofocal zone is confined to. It is a 7 region of high seismicity. The Avacha-Koryakskiy group of active volcanoes is located here. Almost 1000 magnetotelluric soundings at period range of 0.3 to 3000 s have been carried out at the area of 8000 sq.km. The MTS curves are subdivided according to the along and across strike directions. The longitudinal and transverse curves undergo local galvanic-type effects. An attempt has been made to smooth out these effects through statistical averaging. The obtained average curves are not free from the influence of the large sedimentary- volcanogenic cover structures oriented across the strike of major tectonic zones of Kamchatka. Using numerical and physical modelling the distortion of the MTS curves caused by the influence of the horizontal geoelectrical inhomo- geneities as well as volcanoes and aquatorium of the ocean has been studied. On the basis of these data methods of MTS interpretation have been determined. Longitudinal MTS curves generally free from the coastal effect are assumed to be the main ones. However these curves in the low- frequency branch are distored by the S effect due to the influence of the transverse structure of the sedimentary cover. The reverse task was solved using numerical 2D modelling. As a result the geoelectrical model has been obtained in which two zones were distinguished: western zone with developed crustal layer of higher electrical conductivity and eastern zone with extremely low earth electrical conductivity where this layer was not manifested. Eastern zone is adjacent to the seismofocal layer. Active volcanoes are located at the boundary of these zones. In upper areas of the earth's crust they are confined to the bort of the graben, filled with a conducting (8 ohm) up to 6 km-thick rock mass. Crustal conducting layer rises from the 25-km depth at the west up to 10 km beneath recent volcanoes. The nature of crustal layer is associated with presence of the hydrothermal liquids and melts. Lower earth's crust electrical conduc- tivity of the eastern zone seems to be defined by the existence of ultrabasic rocks of enhanced thickness and relatively high temperatures. Title: 9.27p Processing and modelling of noisy magnetotelluric data from the Saxonian Granulite Massif, SE Germany Author: G. Oettinger In 1995, time series recorded along a profile on the Saxonian Granulite Massif were contaminated by strong correlated noise signals and robust single-site estimates of the transfer functions give erroneous results. By using a smoothly varying transfer function, which facilitates identification and removal of electric and magnetic outliers, and by simultaneously estimating MT and correlated noise transfer functions (Two-Source-Method), we succeeded in separating the time series into MT signal and correlated noise signal. Stations from a simultaneous MT campaign in the Odenwald (350 km away) were available as magnetic remote sites. One station has an extremely low noise level and it proved to be an ideal remote station. The 2-D conductivity model of the Granulite Massif does not show a good conductor in the upper crust marking the contours of the granulite, but a high conductive zone in the lower crust which ends south of the Granulite Massif. This zone seems to be directly above a prominent heightening of seismic velocity at a depth of 23 km. G. Oettinger, GeoForschungsZentrum Potsdam, Telegrafenberg, Bereich 2/3, 14473 Potsdam, Gernamy e-mail: oetti@gfz-potsdam.de Title: 9.28p A MT PROFILE ACROSS THE WESTERN PART OF THE CANTABRIAN MOUNTAINS (North of Iberian Peninsula) Authors: J. Pous, P. Queralt and A. Marcuello Dept. Geodinamica i Geofisica. Universitat de Barcelona Marti i Franques s/n. 08028 Barcelona. Spain jaume@natura.geo.ub.es A MT profile across the western part of the Cantabrian Mountains has been carried out. The Cantabrian Mountains constitute a range that extends more than 250 km E-W, along the northern border of the Iberian Peninsula, in the western extension of the Pyrenees. They were developed during Late Cretaceous to Early Miocene by the oblique convergence and collision of the Iberian plate with European plate. The MT profile is 100 km long from the Duero basin to near the shoreline. It is oriented N-S and consists of thirteen magnetotelluric soundings where the five MT components were recorded from 0.004 s to 2000 s. After the static shift correction and the study of the dimensionality, the data of the profile were interpreted as a 2D structure striking WNW-ESE. The main results from the magnetotelluric data are: (1) the conductive sediments of the foreland Duero Basin dip to the North beneath the Cantabrian range which appears as a resistive zone down to 6 km depth; (2) a high conductive zone correlates with the Iberian lower crust beneath the Cantabrian range which dips progressively to the North, reaching a depth of more than 50 km, and (3) these features suggest a northward subduction of the Iberian plate. Title: 9.29p ELECTRICAL CONDUCTIVITY OF THE CRUST ACROSS THE OSSA MORENA AND SOUTH PORTUGUESE ZONE SUTURE Authors: Pous,J.(1), Santos, F.(2), Almeida, E.(2-3), Marcuello, A.(1), Queralt, P.(1), Matias, H.(2) and Mendes-Victor, L.(2) (1) University of Barcelona, Zona Universitaria de Pedralbes, 08028-Barcelona, Spain. (2) University of Lisbon-CGUL, R. Escola Politecnica 58, 1250 Lisboa, Portugal. (3) Instituto Politecnico de Tomar, Tomar, Portugal. The transition between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) is made up of a major geosuture, which is indicated by the Beja-Acebuches ophiolite. Several geological data suggest that this suture is the result of an oblique collision, with a northward propagation, between the SPZ and OMZ. From the structural point of view the OMZ is much more complex than the adjacent Central Iberian Zone and the SPZ. The frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events. With the aim of providing new constraints to this complex deep structure the Universities of Lisbon and Barcelona designed a magnetotelluric (MT) survey in the scope of the bilateral cooperation and the Europrobe programs. A first NNE-SSW profile across the SPZ and OMZ was carried out in September 1997. The profile is 40 km long and consists of nine deep MT soundings with periods ranging from 256 Hz to 4000 s. The time series were processed using a robust algorithm, after visual inspection. To study the dimensionality of the regional structure and to select a preferential direction for which 2-D modeling should be carried out, the induction arrows were analysed. A strike of N55W was determined in accordance with regional geological features.2D inversion was undertaken on both TE and TM modes using the RRI program. The main feature of the obtained model is a large conductive body (1-4 Ohm m) which depth and thickness increases from south to north. Beneath the OMZ this structure lies at a depth between 20 and 27 km. This reduced resistivity can be tentatively associated with the lower crustal detachment surface connected with the old collision/subducted zone. Title: 9.30p MAGNETOVARIATIONAL SOUNDINGS ACROSS THE SOUTH ISLAND OF NEW ZEALAND: DIFFERENCE INDUCTION ARROWS AND THE SOUTHERN ALPS CONDUCTOR Authors: Daniel Pringle(1), Malcolm Ingham(2) and Don McKnight(3) (1)School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand (2)School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand, email: ingham@matai.vuw.ac.nz (3)Institute of Geological and Nuclear Sciences, PO Box 1320, Wellington, New Zealand, email: d.mcknight@gns.cri.nz Magnetovariational soundings have been made at 14 sites along the MT transect of the South Island, New Zealand reported by Ingham (1997). The observed induction arrows are strongly influenced by the effect of the surrounding oceans. This effect has been removed, to first order, by subtraction of the analogue model induction arrows of Chen et al. (1993). The resulting difference induction arrows show a clear reversal across the previously identified Southern Alps conductor (SAC). This occurs for the real arrows between 5 and 15min period and at longer periods for the quadrature arrows. The real difference arrows at longer periods show evidence of three-dimensinal structure not accounted for in the model of Chen et al. Two-dimensional modelling of the projections of the difference arrows onto the transect, shows that the real and quadrature arrows between 5 and 20min can be well fit by a model containing a conductive body in the central part of the transect between 10 and 60km depth. Additional near-surface conductors representing the conductive sediments beneath the Canterbury Plains and on the west coast are also modelled. Investigation of the 2- and 3-dimensional responses of the anomalous structure and the oceans shows that, for the derived model, the subtraction of induction arrows to yield difference arrows is a valid approximation. Title: 9.31p Furrowed conducting zones in the Frontal Himalaya and their Tectonic Interpretation Author: C.D. Reddy Indian Institute of Geomagnetism, Colaba, Bombay 400 005, India (email: cdreddy@iig.iigm.res.in) A 3-D conductance map has been developed for Frontal Himalaya region using magnetovariational (MV) data collected during 1979-1991. The perspective map of the conductance clearly indicates three prominent furrowed conducting zones, approximately parallel to Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT). In the 2-D modelling, the northern most conducting zone is seen as obduction zone at MCT,while the other two are seen as humps near northern and southern limits of Indo- Gangetic Foredeep. Further, the presence of structural heterogeneity associated with these conducting zones is supplemented by gravity and seismic wave velocity information. The presence of furrowed conducting zones parallel to MFT/MBT/MCT raises an interesting question whether the mapped crustal upwarps are independent manifestations of the flexing of the lithosphere during successive subduction /underthrusting at the Indo Tsangpo Suture Zone, MCT, MBT and MFT. Title: 9.32p New magnetotelluric data from Namibia. Author: Oliver Ritter, GeoForschungsZentrum Potsdam, Germany Oliver Ritter (email: oritter@gfz-potsdam.de) GeoForschungsZentrum Potsdam Telegrafenberg D-14473 Potsdam Germany Continental rifting and the early Cretaceous opening of the South Atlantic formed the passive, volcanic margin off Namibia. Just prior to the onset of sea floor spreading a large igneous province was emplaced. This included voluminous extrusives, continental flood basalts and subvolcanic ring complexes. GFZ, in cooperation with other research institutions, intends to explore the deep structure of the entire ocean-continent transition with several geophysical and geological investigations. Early magnetic variation and Schlumberger soundings from de Beer and Gough from the mid-seventies indicated a conductive zone in the Damara Orogenic Belt. In Spring 1998 we recorded at 33 sites in the Brandberg area in Namibia. This was probably the first magnetotelluric experiment ever caried out in the region. The data were collected synchronously with 4 instruments in the period range 0.001s - 1000s. The sites were arranged in 3 profiles, the site spacing varying between 4 and 12 kilometers. In this paper we present first modelling results. The most prominent feature in the data is an extensive lower crustal conductor which is visible from all sites. We observe minimum apparent resistivities of below 1 Ohmm and maximum induction arrow amplitudes larger than 0.75. The upper crust is generally resistive (> 1000 Ohmm), but several small scale anomalies are resolvable. Title: 9.33p RELIABLE MAP OF CRUSTAL ELECTRICAL CONDUCTIVITY Author: Rokityansky I.I. POB-338/7, Kiev-146, Ukraina, earth@igph.kiev.ua A map is a commonly accepted form of representation of any quantities on the Earth surface. Mapping of integral surface electrical conductivity of water and sediments overlying the poorly conducting basement (S-map) is a rather well developed approach. Deep geoelectric data interpretation is rather sofisticated and ambiguous and can be made only by highly skilled experts. What are the most adequate methods to transfer the geoelectric response functions into reliable physically and geologically meaningful quantities? Can a conductive layer be an adequate model for deep crustal studies? Can multifaulting be an alternative to a layered model? Can the fractal geometry be an adequate description of multifault structure? What is the relationship of deterministic and stochastic approaches in representation of geoelectric data on a map? What survey density and period interval are optimal to mantain a reliable map of crustal conductivity? The goal of the Symposium is to collect different approaches on representation of deep geoelectric data for geologists, (geo)physisists, engineers - all nonexperts who need information about Earth crust. Development of world-widelly recognized mapping procedure is the programme maximum for the Symposium. We welcome theoretical, methodological, computer simulating and practical contributions leading to the goal. Title: 9.34p CONDUCTIVE STRUCTURE OF UKRAINIAN CARPATHIANS FROM EM OBSERVATIONS Authors: Rokityansky I.I., Ingerov A.I. Geomagnetic variation observations in Carpathian region gave the data for tracing the axis of 1200 km long Carpathian electrical conductivity anomaly (CA) and estimation of its integral longitudinal conductivity. We made also 35 MTS in the south-east part of the Ukrainian Carpathians. The form of the MT curves regularly changes from NE to SW forming 6 zones of similar behaviour. Most interesting MTS curves are above the CA. The longitudinal curves define CA on the 10 km depth; the transverse ones are not sensitive to crustal CA but they define a mantle conductor on the 100-200 km depth with conductivity more than 5000 Sm which is probably the asthenosphere. CA transparency for transverse polarisation yields estimation of the anomaly width. Thus, the two principal crustal conductors manifested by MTS data in Carpathians are CA subducting in SW direction from moderately conductive sediments to main conductor at 10-20 km depth and a conductive zone of Transcarpathian deep fault. Correlation of CA structure with seismicity and thunderstorm activity is discussed. Title: 9.35p MAGNETOTELLURIC STUDY OF A PLIO-QUATERNARY TECTONIC DEPRESSION: THE VILARIGA BASIN (NE PORTUGAL) Authors: Fernando A. M. Santos(1), Liliana Matos(1), Hugo C. Matias(1), Eugenio P. Almeida(1,2) and Luis A. Mendes-Victor(1) (1) Departamento de F=A1sica da Universidade de Lisboa and Centro de Geof=A1sica da Universidade de Lisboa R. Escola Politecnica, 58, 1250 Lisboa, Portugal E-mail: dfams@fc.ul.pt (2) Instituto Politecnico de Tomar The Vilariga basin, located northeast Portugal astride a major late-Variscan NNE-SSW reactivated strike-slip fault, is an excellent example of interplate neotectonic activity whose development has been interpreted mainly as a result of left-lateral displacement. Twenty-five magnetotelluric soundings were carried out in the Sta Comba da Vilariga/Sampaio region (northern of the tectonic basin) in order to investigate the internal structure of the basin and its relationship with the main tectonic accidents. Distortions of the impedance tensors were removed using Groom-Bailey decomposition technique. An analysis of the parameters of the tensor decomposition allowed to distinguish the main electrical regional structures. Two main regional strikes (N20E and N50-58E) which can be explained for the reactivation of previous structures, were found. Using two-dimensional inversion and three-dimensional modeling, the resistivity distribution in the studied area was obtained. The graben has low resistivity of less than 40 Ohm m and is asymmetric. This asymmetry suggests that a transform-normal extension process generated the basin, with the transform fault located on the eastern side of the basin. The electrical resistivity increasing at depths greater than 2 km is related to the Hesperian basement rocks. Using the obtained MT model a first picture of the tectonic stress pattern along the graben is presented. Title: 9.36p Study on deep crustal resistivity structure using wideband magnetotellurics -Southwesternmost part of the Kurile arc- Authors: Hideyuki Satoh(1), Yasunori Nishida(1), Yasuo Ogawa(2) and Masamitsu Takada(3) (1)Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Japan (2)Geological Survey of Japan, Japan (3)Research Center for Earthquake Prediction, Faculty of Science, Hokkaido University, Japan (1)N-10 W-8 kita-ku Sapporo 060-0810 Japan (2)1-1-3 Higashi Tsukuba Ibaraki 305-0046 Japan (3)N-10 W-8 kita-ku Sapporo 060-0810 Japan (1)satoh@ares.sci.hokudai.ac.jp, nishida@ares.sci.hokudai.ac.jp (2)oga@gsj.go.jp (3)takada@eos.hokudai.ac.jp We collected wideband(0.01 to 1000 sec) magnetotelluric data in the eastern part of Hokkaido, Japan. The main purpose of this study is to clarify the tectonic features of the southwesternmost part of the Kurile arc. The MT stations were aligned in the NNW-SSE direction, which were almost perpendicular to the strike direction of the Kuriles island arc. Two-dimensional resistivity models were constructed by inversion method (Ogawa and Uchida, 1996). (1)A conductive(a few to 10 Ohm-m) layer distributes widely and thickly(maximum thickness is about 5 km) in the shallow place. The layer well correlates with the distribution of the Quaternary, Neogene and Pleogene sediments containing a large amount of alteration minerals. (2)A resistive layer(more than 1000 Ohm-m) is located at upper to middle crust of the forearc side. The depth to this resistor(about 5 km) coincides with the seismic reflector(K-T boundary) revealed by a seismic reflection study. As the distribution of the resistor is consistent with that of the high positive gravity anomaly amounting to 227mgal, the causative material is thought to be common:Cretaceous volcanic body. (3)A conductive layer(10 to 50 Ohm-m) is situated at lower crust, extending from the volcanic front toward the backarc side. Two possiblities can be considered as the origin of this conductive layer:one is trapped free water which is supplied from the dehydration of the present subducting slab(Pacific plate), and the other is trapped oceanic crust by the former complex tectonic movements, collision of the Okhotsk Paleo-land and the Eurasia plate at late Cretaceous. Title: 9.37p MT mapping of an outcropping, carbonated dipping slab in the Alps Authors: P.-A. Schnegg and Markus Gurk Geomagnetism Group, Geological Institute, University of Neuchatel, Switzerland pierre-andre.schnegg@geol.unine.ch Pierre-Andre Schnegg Geomagnetism Group of the University Rue de l'Observatoire 58 CH-2000 NEUCHATEL Switzerland Phone: +4132 889 68 70 Direct: 889 88 17 Fax: +4132 889 62 81 Pierre-Andre.Schnegg@on.unine.ch URL: www-geol.unine.ch/GEOMAGNETISME/HomePage.html Some 34 MT sites were sounded on an area of 80x40 km limited to the north by the Rhone river and to the south by the Switzerland-Italy border. Two-D modelling was carried out, using simultaneously TE, TM and Hz along a profile parallel to an existing seismic line, revealing the existence of a very low resistive (0.2 Ohm-m), 1 km-thick slab embedded in a resistive matrix. The slab plunges below the Alps with a dip angle of 35 degrees. With a total length of 20 km, its lower tip reaches a depth of 8 km. At its NNW edge, it crops out at the surface in the Zone Houillere. Measurements on rock cores from an available borehole revealed resistivities of as low as 0.8 Ohm-m, which further decreased to 0.6 under pressure of 40 MPa, indicating graphite film reconnection. Comparison with seismic profile shows good agreement between model slope and seismic reflectors. 3D modelling allowed to map the conductor shape and depth beneath the Alps. The final result confirms the large lateral extension of the conducting slab over the measuring area, and a smaller width (<20 km) in the NNW-SSE direction. The 3D model assumes the shape of the alpine arc. Title: 9.38p Scalar impedance identity for unhomogeneous medium Authors: V.N.Shuman, L.K.Lankis, S.N.Kulik, T.K.Burakhovich, E.S.Lysenko Institute of Geophysics National Academy Science of the Ukraine. E-mail:earth@igph.kiev.ua A new approach to the analysis of magnetotelluric observation in proposed. It is based on the introduction of two scalar parameters of impedances. According Aboul-Atta and Boerner (Can. J. Phys. 1975, 53, p. 1404) two scalar parameters may be determined on the closed interface of media with different electromagnetic parameters as the coefficients of expansion of double vectorial product n x E x n based on the orthogonal basis of the vectors n x H and n x H* x n, where n is the electrical external normal to the interface, a star designates a complex conjugation. The specific feature of the coefficients of expansion is they are determined by the relationships between the tangential components of the electromagnetic field on closed interface, i.e. analogously to Leontovich's surface condition. Properties of scalar impedances is studied and compared with ordinary tensorial representation by numerical modelling mt-field of S-film which approximated main heterogeneous geological structures of Ukraine. Title: 9.39p DEEP GEOELECTRICAL STRUCTURE ALONG THE PANNONO-CARPATHIAN GEOTRANSECT Authors: D.Stanica, M. Stanica (all at the Geological Institute of Romania 78344 1, Caransebes Str, Bucharest 32) e-mail: stanica@igr.sfos.ro In the last four years an extensive program of magnetotelluric soundings (MTS) was developed in the the Pannono-Carpathian area, in the framework of the EUROPROBE Project (PANCARDI). The goal of this paper is to show how the MT data can provide valuable information about present physical state of the geotectonic structure along Varsand-Alba Iulia-Persani Mts. geotransect. A 2D forward model was carried out using finite element code of Wannamaker et al. (1987). In this respect, we parametrized the deep structure by means of a grid of rectangular cells, devided in four triangles, each of them having an uniform conductivity distribution. Both the E and B-polarization responses were computed at each MT site for nine frequencies ranging from 0.001Hz to 1Hz. The final model reveals the following aspects: the presence of the conductivity belts associated with both the remnants of the crust (ophiolitic formation) bearing to the Tethys Ocean and the "consumption" palaeo-plane of the primary socle of the Eastern Carpathians' flysch nappes system; a large alohtony to the East of the units belonging to the overthrusted Transylvanides, composed by ophiolitic and crystalline formation; the electric properties distribution in the crust and upper mantle along the geotransect. Title: 9.40p A MAGNETOTELLURIC SURVEY ACROSS THE NAMAQUA METAMORPHIC BELT, KHEIS PROVINCE AND WESTERN MARGIN OF THE KAAPVAAL CRATON, SOUTH AFRICA Authors: E.H. Stettler, J. Prinsloo, M.E. Hauger and P.J. Kulper Geophysics Division, Council for Geoscience, Private Bag X112, Pretoria, 0001, South Africa South Africa has a very rich geological history. It starts with the 3.7 Ga Kaapvaal Craton that underlies the central and northern parts of the country. A number of tectonic provinces and mobile belts accumulated around the western and southern margins of the craton during Euburnian (2Ga), Kibaran (1.1Ga) and later orogenic events. During the late eighties a deep seismic survey was conducted along a traverse across the Namaqua Metamorphic Province, Kheis Tectonic Province and western margin of the Kaapvaal Craton in the Northern Cape Province of South Africa. Subsequently, eleven magnetotelluric sounding stations were completed at a 20 km station interval along the same traverse, using in-house developed equipment and software. The seismic section reveals a package of westward dipping reflectors at the Craton margin. Two dimensional MT and gravity models show dense, conductive material that correlates well with the western edge of the reflector package. It is interpreted to be ophiolitic material caught up between the Craton and Kheis Province during the Euburnian collisional event. Title: 9.41p Magnetotelluric investigation of the Aso caldera, central Kyushu, Japan Authors : Shinichi Takakura, Geological Survey of Japan Takeshi Hashimoto, Kyoto University Yasuo Ogawa, Geological Survey of Japan 1-1-3 Higashi, Tsukuba, Ibaraki 305-8567, Japan shin@gsj.go.jp A magnetotelluric (MT) survey was conducted at the Aso caldera with a number of central cones. The purpose of this survey was to investigate the deep structure of the caldera. Measurements were performed at 16 sites, which were located along a NNE-SSW profile traversing the caldera. A two-dimensional inversion was applied to the MT data. The analyzed resistivity structure is consistent with the resistivity logging data of the nearby wells. The caldera is basically interpreted as a two-layer structure; an upper conductive layer and a lower resistive layer. The conductive layer corresponds to the caldera deposits. A very conductive zone appears at the surface of the Yunotani hot spring area. The low resistivity may be due to hot-water or clay minerals which were produced by the geothermal alteration. The resistive layer correlates well with the basement interpreted from gravity, which reflects pre-Tertiary basement rocks. However the upper part of the basement is conductive around the central cones, indicating the fractures with geothermal fluid are present in the basement. There are no conductors which correspond to magma at least to a depth of 10 km. Title: 9.42p The conductivity structure of the Southern Uplands - Are crustal conductors thick or thin? Authors: S. Tauber (1), Dr R. J. Banks (1), Dr O. Ritter (2), G. Dawes (1) (1) The University of Edinburgh, Department of Geology and Geophysics, Grant Institute, West Mains Road, Edinburgh EH9 3JW, Scotland (2) GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany The resolution of MT is inherently limited by the attenuation of the electromagnetic fields. Nonetheless relatively thin vertical conductors associated with fault and shear zones have been detected in recent MT studies indicating the ability of the MT method to map zones of past or present zones of weakness in the earth crust. Previous MT surveys within the Iapetus Suture Zone show extended, flat lying mid-crustal conductors beneath the Scottish Southern Uplands and Irish Longford Down Massif and shallow, steep dipping conductors within the Southern Uplands. The question is if there is a transition between the shallow and deep crustal conductors which might help to unravel the nature of the deeper conductive structures. To address this problem a detailed AMT cross profile - running parallel and perpendicular toregional strike - with increased site density (site separation 1.5 km) and broadened frequency range (0.001 Hz to 2 kHz), employing S.P.A.M. MkIII AMT-system, was carried out last autumn. 2D inverse modelling reveals two steep dipping conductive zones which coincide with outcrops of black shales. A flattening of these conductive features with depth has yet not been observed; additional LMT measurements are planned. Title: 9.43p Geoelectric structure beneath limestones of a pericratonic basin Authors: Jandyr de Menezes Travassos - CNPq-On - jandyr@on.br Paulo de Tarso L. Menezes - UERJ - Departamento de Geologia Aplicada - ptarso@uerj.br Jorge Luis Porsani - UNESP/S|o Carlos- jporsani@caviar.igce.unesp.br The metasedimentary rocks overlying the S|o Francisco craton are of interest for hydrocarbon exploration. The high-velocity limestones of the neoproterozoic Bambu_ Group make the seismic techniques difficult to interpret the subsurface. The knowledge of the regional structure as well as the mapping of the underlying crustal blocks is very limited. The only geophysical available information comprises of gravimetric data that shows spatial wavelengths greater than 100 km. A profile made up with 7 RR EMAP segments was done in order to reveal the regional structure of the region. The profile was 200 km long, with a E-W trend. Such an hybrid technique was employed in order to deal with local 3D distortions. Each EMAP segment is equivalent to 5 RR sites, so the whole profile is equivalent to 35 RR MT sites. The frequency rage was 0.001 to 400 Hz. The final interpretation is based on a 2D regional model made up correlating laterally 7 2D inversions, one for each EMAP segment. Title: 9.44p Along Strike Variations in the Structure of the San Andreas Fault Imaged by Continuous MT Profiling Authors: Martyn Unsworth and Paul Bedrosian Geophysics Program, Univ. of Washington, Seattle, WA 98195 Markus Eisel and Gary Egbert COAS, Oregon State University, Corvallis, Oregon, 97331-5503 The Parkfield, California segment of the San Andreas Fault (SAF) represents a transition in mechanical behavior, between fault creep to the north, and a locked segment to the south. Recent MT surveys by a number of groups have suggested significant variations in fault zone resistivity between these segments. In Fall 1997 data were collected on 3 profiles in the Parkfield area. Each profile consisted of continuous coverage close to the fault trace, with more widely spaced stations at greater distances from the SAF. Combined with continuous MT profile data from 1994, the inversion of these data indicates the following: (a) A low resistivity (probably high fluid content) fault zone appears to be a continuous feature along the Parkfield segment. The structure appears to be quite 2-D to a depth of 5 km. (b) The Cholame segment (south of Parkfield, at the northern edge of the locked segment) lacks this low resistivity core, suggesting a lower fault zone fluid content. (c) The pull-apart basin associated with the en echelon offset of the SAF in Cholame valley has produced a 1 km deep basin. At depth the two fault strands comprise a flower structure some 5 km wide. Title: 9.45p PRELIMINARY RESULTS OF THIN-SHEET MODELING OF THE MAGNETOTELLURIC FIELD STRUCTURE ON THE IBERIAN PENINSULA. Authors: L. Vanyan, V. Kouznetsov (Shirshov Institute of Oceanology, Moscow) and J. Pous (University of Barcelona). e-mail:vanyan@geo.sio.rssi.ru Resistive landmass of the Iberian Peninsula is almost surrounded by the well-conductive sea water. Therefore the electric charges accumulation on the coast-line gives a significant contribution to the primary magnetotelluric (MT) field. Thin-sheet modeling was used for study the coast-line effect on the regional effect MT field. The mesh consists of 38*38 square cells 38 km on side. Period band is 100-10000 s. A map of the uppermost layer conductance was constructed using geological, geophysical and bathymetric data. The maximal values are about 6000 S and minimal ones are 10 S. The calculated regional MT impedance is affected for periods longer than 1000 s at places moderately away from the coast line (e g in the Ebro Basin). Other more complex models including the resistivity distribution found in 2D previous models are going to be tested. Title: 9.46p FIRST EM INVESTIGATIONS OF THE SEISMO-ACTIVE REGION OF NORTHWEST BOHEMIA Authors: G. Volpi (1), A. Manzella (1), D. Di Mauro (2), A. Zaja (3), N. Praticelli (3), V. Cerv (4), J. Pek (4), A. De Santis (2) (1) International Institute for Geothermal Research, Pisa, Italy (2) Istituto Nazionale di Geofisica, Roma, Italy (3) Padua University, Padua, Italy (4) Academy of Science, Prague, Czech Republic During 1997 a series of AMT soundings was carried out in the region of Northwest Bohemia. This area is known by relatively frequent occurence of micro-earthquake swarms. 15 AMT stations were installed in an area of about 20 x 25 km2, where almost 80% of seismicity of the whole region was recorded since 1986. The area showed a high electromagnetic noise which affected both the electric and magnetic records. Nevertheless, a preliminary analysis of the data shows some electromagnetic evidence with regard to the seismic characteristics of the region. 2-D and 3-D modelling results indicate a contrast between a conductive and a non-conductive zone approximately along the focal area. The obtained results are compared with earlier MT and MV data from a broader area, as well as with results of other geophysical methods. Title: 9.47 AMT soundings in Spessart mountains Author: Ute Weckmann Institute for Meteorology and Geophysics Feldbergstr.47 D-60323 Frankfurt/Main, Germany email: weckmann@geophysik.uni-frankfurt.de phone [++49] 69 798 24908 fax: [++49] 69 798 23280 Within the framework of audiomagnetotelluric measurements in the Spessart Mountains (Germany) magnetic and telluric time variations in the frequency range from 0.001s up to 1000s were observed. The time series were disturbed, however, by noise from cultural and industrial sources. Although a robust processing was used to compute magnetotelluric transfer functions, the results are severely biased if the number of bad data or outliers is too large. Therefore a pre-selection has to be done. In most cases it is nearly impossible to distinguish between disturbed and acceptable time windows. But in frequency domain there are several parameters like power or coherencies marking bad data quality. A program was developed which allows to visualize all relevant MT parameters and to select data relating to many criteria. AMT results will be presented which lead to a conductivity model. Title: 9.48p A magnetotelluric study of the southwestern Northwest Territories, Canada Authors: X. Wu, I.J. Ferguson, Department of Geological Sciences, U. of Manitoba, Winnipeg, MB, Canada, R3T 2N2, E-mail: umwux@cc.umanitoba.ca A.G. Jones, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, ON, K1A OY3, Canada As part of the LITHOPROBE SNORCLE (Slave-Northern Cordillera Lithospheric Evolution) Transect, magnetotelluric soundings were made at 58 sites in southwestern Northwest Territories, Canada. The sites cross the exposed Archean Slave Province and the buried Proterozoic Hottah, Fort Simpson, and Nahanni terranes. The objectives of the present study are to resolve the conductivity structure of the crust and mantle lithosphere of the Proterozoic terranes and overlying Phanerozoic sediments. Acquisition at each site included both wide-band, using Phoenix V5 systems, and long periods, using GSC LiMS instruments. At the geomagnetic latitudes of survey the responses are potentially contaminated by non-uniform sources requiring application of robust processing methods. Groom-Bailey decompositions and static-shift removal were applied in data analysis. The MT responses indicate that at shallow depth the conductivity structure is approximately 1D. Within the bulk of the crust it is 2D with a strike ~N30E. At greater depth it is 2D with again a strike ~N30E except on the Great Bear magnetic arc where it is weakly 2D with a direction ~N30W. Apparent resistivity and phase sections reveal both lateral and vertical conductivity variations. A conductive surface layer, corresponding to Phanerozoic cover, thickens from east (0 m) to west (1000 m), with the most conductive rocks occurring at several hundred metres depth. A relatively resistive crust overlies a more conductive region at depth. Longer-period (>1 s) spatial variations in several groups of sites correspond with tectonic boundaries. Title: 9.49p Resistivity mapping using VLF-MT and a resistivity survey, using hole-to-surface configuration, around the surface fault ruptures of the 1995 Hyogo-ken Nanbu earthquake, Japan. AUTHORS: Satoru Yamaguchi1), Takahisa Murakami1), Hiroo Inokuchi2) N.Oshiman3), Y.Makino1), H.Murakami4), N.Sumitomo3) 1) Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe, 657, Japan. 2) Research Center for Inland Seas, Kobe University, Iwaya, Kobe, 656-24, Japan. 3) Disaster Prevention Research Institute, Kyoto University, Uji, 611, Kyoto, Japan. 4) Department of Geology, Faculty of Science, Kochi University, Kochi University, Kochi, 780, Japan. Associated with the 1995 Hyogo-ken Nanbu earthquake (M=7.2), distinctive ruptures in surface faulting appeared along the Nojima Fault, on the northwest coast of Awaji Island, central Japan. The surface ruptures consist of a NE-SW trending right-lateral fault system, with a high angle reverse component. They have two fault strands, namely the Nojima Earthquake Fault and the Ogura Earthquake Fault. Surface displacement is prominent along the Ogura fault. in the northern part of the Nojima fault. VLF-MT surveys were made along nineteen transects across both faults, and pseudo two-dimensional conductivity structures were computed for each transect. Also resistivity measurements, using hole-to-surface configuration, were conducted around the Ogura fault and at the southern part of the Nojima fault. A three-dimensional conductivity model was then computed for the area by using a finite-difference method. Highly conductive zones were found along the faults where surface displacements were prominent. The resistive zones coincided with areas of granitic rock, inland from the Nojima fault. Conductive zones map areas of Tertiary and Quaternary sediments, on the coastal side of it. The coastal side of the Ogura fault is more conductive than the inland side. Title: 9.50p On the Graphite nature of Carpathian Anomaly of electrical Conductivity. Authors: Abdoulkhay A. Zhamaletdinov (Geological Institute of the Kola Science Centre of RAS, Apatity, RUSSIA). Vladimir Chermak ( Geophysical Institute of Czech Academy of Sciences, Prague. CZECH. Alexander I. Ivliev (Geological Institute of Russian Academy of Sciences, Moscow, RUSSIA). At the middle of 60-th an extensive anomaly of electrical conductivity was recognized by magnetometer array methods in the Carpathians [Jankovsky, 1967; Adam, 1969].The anomaly follows approximately up to 1000 km in length. With the use of numerical modeling the depth to the conductor is estimated equal to 10-20 km. The nature of anomaly was explained by different phenomena - fluids distribution, temperature, subduction etc. We have made the research for to explain the nature of Carpathian anomaly by presence of graphite bearing rocks. For the purpose we have made the following: (I) analysis of appearance of graphite bearing rocks and their electrical properties in Carpathians; (ii) analysis of the heat flow field distribution in Carpathian region and its relationship with conductivity anomaly location; (iii) digital modeling of MT-effect from sedimentary cover and analysis of MT-modeling from the Earth crust electrical conductivity [Zhdanov, 1985]. Results of this study makes possible to explain the nature of anomaly by electron conductive graphite bearing rocks appearing on the surface along marmaroshsky and peninian zones. Geotectonical origin of the high grade metamorphed graphite could be explained by subduction-upduction mechanism, same as it has been used for to explain the nature of graphite bearing rocks in Tien-Shan electrical conductivity anomaly [Bakirov, 1984; Zhamaletdinov, 1996].