Local and Regional Electromagnetic Studies
Chairpersons: M.C. Pomposiello, A.L. Padilha, and A.G. Jones


8.1   EFFECTIVE MODELLING OF LOCAL AND REGIONAL EM DATA
      (INVITED REVIEW PAPER)

Yasuo Ogawa

Geological Survey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 305-8567, 
Japan
oga@gsj.go.jp




8.2   THE ELECTRIC MOHO		

Alan G. Jones(1) and Ian J. Ferguson(2)

(1) Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 
    0E9, Canada
(2) Department of Geological Sciences, University of Manitoba, Winnipeg 
    Manitoba, R3T 2N2, Canada
ajones@cg.nrcan.gc.ca

Since Mohovicic discovered a dramatic increase in compressional 
seismic velocity from 5.68 km/s to 7.75 km/s at a depth of 54 km 
beneath the Kulpa Valley in Croatia, the "Moho" has become 
arguably the most important seismic horizon in the Earth in its role 
as defining the crust-mantle boundary.  It is now known to be a 
ubiquitous feature of the Earth, and is usually assumed to separate 
lower crustal mafic rocks from upper mantle ultramafic rocks. 
Electromagnetic experiments conducted to date have failed to 
demonstrate a convincing change in electrical conductivity at the 
base of the crust. Here we report on the interpretation of 
magnetotelluric data from the southwestern edge of the Slave 
craton which show an unequivocal change at the Moho, the 
seismically-defined base of the crust. This change is a conductivity 
increase with depth, contrary to expectations, and requires a 
conducting phase in the upper mantle beneath the Slave craton.




8.3   CARPATHIAN CONDUCTIVITY ANOMALY AND ITS RELATION WITH
      TRANS-EUROPEAN SUTURE ZONE (TESZ)
                    
Dumitru Stanica

Geological Institute of Romania, 78344-Caransebes  Str., 1 ,Bucharest 
32, Romania
stanica@ns.igr.ro

Some models derived from 2D inversion and  modelling  of the 
magnetotelluric (MT) data are presented, in order to identify the 
role of the Trans-European Suture Zone (TESZ) on the 
geoelectrical conductivity anomaly induced by it in lithosphere.  In 
connection with this suture, other relevant geophysical and 
geological information have been used to emphasize its crustal and 
subcrustal features, without neglecting certain aspects concerning 
the nature of the deposits which are comprised in the subduction 
process, mainly metamorphosed sediments and graphite, and are 
keenly involved in the conductivity anomaly.  All the images 
provided by the electromagnetic parameters supply conclusive 
information related to the suture emplacement, its depth extent 
(from 16 km to 100 km or more), width (20-25 km) and anomalous 
values of geoelectrical conductivity (about 0.1 S/m), as well as the 
relationship between two types of lithosphere that belong to 
Precambrian East-European Platform, towards East and North-
East, and Phanerozoic terranes, towards West and South-West. 
Also, quantitative DATA regarding the sedimentary cover, the 
thickness of the crust and upper mantle, as well as tectonical 
elements - some of them playing an essential role in explaining the 
geodynamic evolution of the TESZ, are revealed.




8.4   DEEP FAULT MAPPING BY MAGNETOTELLURICS IN THE UKRAINE

O.I. Ingerov(1), I.I. Rokityansky(2), and A.L. Lozovoy(3)

(1) Phoenix Geophysics Ltd., Toronto, Canada
(2) Institute of Geophysics of Ukrainian Academy of Science, Kyiv, 
    Ukraine
(3) National Mining University of Ukraine, Dnipropetrovsk, Ukraine
alingers@arvotek.net

More then 8000 magnetotelluric soundings (MTS) covering all 
Ukrainian territory give the possibility to interpret the systems of 
conductive faults which appear in the maps of MT parameters as 
linear zones of low resistivity, the border between blocks with 
different level of crust conductivity or different character of 
conductivity distribution. All six systems of faults (known in the 
Ukrainian shield territory from geological and potential field 
geophysical methods) are clearly seen in MT data. These data 
permit one to extrapolate the fault systems throughout Ukraine and 
to estimate the order (i.e. magnitude) of faults.  1.Three systems of 
faults are most clearly seen in MT data: 0-270, 15-285, 45-315 
degrees. 2. Different forms of MTS curves permitting conductive 
fault identification characterize conductive faults at each of these 
azimuths by MTS data. 3. The faults have different orders, which 
can be estimated from MT data. 4. The faults form a spatially 
periodic structure (net) where the distance between the parallel 
faults tends to be approx. 17.5; 35; 53.5, 70; 107; 214 km. 5.Some 
mineral deposits are situated at the intersection of the faults of 
definite azimuths; such intersections appear favorable for mineral 
prospecting. 6. In Ukraine, MT methods have became standard 
methods in mineral and oil prospecting. The growth of these 
methods in other countries may be predicted, considering the 
recent availability of 5th generation MT equipment with increased 
productivity and reduced cost of MT surveys. 7. Spatial regularities 
of the fault Systems permits an economical, 2-step approach to MT 
fault mapping: 1- use a dense sounding net on limited territories to 
develop a good understanding of the fault systems; 2 
subsequently, check the predicted locations with limited profiles.




8.5   VARNET-MT: INTERPRETATION OF MAGNETOTELLURIC AND
      MAGNETOVARIATIONAL DATA OVER VARISCAN AND CALEDONIAN 
      STRUCTURES IN SW IRELAND

C. Brown(1), P. Denny(2), and V. Haak(2)

(1) Applied Geophysics Unit, National University of Ireland, Galway, 
    Ireland
(2) GFZ-Potsdam, Telegrafenberg, D-14473, Potsdam, Germany
colin.brown@nuigalway.ie

VARNET is a network of institutes with an interest in the European 
Variscides, a major orogenic front that extends 1500km from 
Ireland to Germany. It is well-exposed at the surface in southwest 
Ireland, making this region an obvious choice for a study of 
Variscan tectonic processes. The principal aims of VARNET are to 
quantify the effect of older, Caledonian structures on the structural 
style of Variscan geology and to provide an integrated model for 
the evolution of the Variscan Front. This will be based upon new 
geophysical data (P- and S-wave wide-angle seismic reflection, 
gravity and MT/MV) in southwest Ireland and pre-existing data in 
the Rhenish Massif in Germany. We will present an interpretation 
of the MT/MV data and integrate this with the seismic and gravity 
studies. The MV data have located 3 crustal conductors. One of 
these is related to the Iapetus Suture Zone extending from 
Scotland through Ireland. The second appears to resolve the upper 
surface of a ramp structure associated with the major Variscan 
frontal thrust fault. The origin of the third conductor is puzzling: its 
conductance is modest (50S) and its 60km of strike is orthogonal to 
al major geological structures. It may be due to current channeling. 
The geometry of these conductors has been approximated using a 
3D forward modelling code that has also included the effect of the 
conductive seawater and sediments that surround Ireland.  The MT 
data are of lower quality because of troublesome electric fence 
noise. Nevertheless, we have located several Caledonian fault 
zones that constrain the evolution of Variscan structures. The 2D 
MT inversions corroborate seismic results concerning the geometry 
and style of post-Variscan basin development.




8.6   REGIONAL ELECTRICAL CONDUCTIVITY STRUCTURE OF THE SOUTHERN 
      CANADIAN CORDILLERA AND ITS PHYSICAL INTERPRETATION

Juanjo Ledo and Alan G. Jones

Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 0E9, 
Canada
ledo@cg.nrcan.gc.ca

The regional crustal structure of the southern Canadian Cordillera 
of western Canada is interpreted from the inversion of 
magnetotelluric data along five transects crossing the 
morphogeological belts. Decomposition of the data demonstrates 
that regional structures can be validly interpreted along the 
transects as two-dimensional, but with varying strike from profile to 
profile. The strike directions suggest a local clockwise rotation of 
crustal structures in the southern Intermontane and Omineca Belts, 
but not along the northern profiles in central British Columbia. 
Comparing the models from each profile allows us to obtain the 
regional-scale three-dimensional electrical crustal structure. The 
models show generally an upper resistive crust that contrasts with 
a more conductive lower crust. The conductivity in the Intermonate 
Belt is virtually the same along all profiles. In contrast, a strong 
along-strike variation in lower crustal conductivity is observed in the 
Omineca Belt, with much higher conductivities in the region of 
Eocene extension, and lower conductivities to the north in the 
unextended part of the Omineca. The joint interpretation of these 
magnetotelluric models with other geophysical data suggests that 
the presence of fluids in the lower crust can explain the observed 
data.




8.7   TWO-DIMENSIONAL MODELING OF MAGNETOTELLURIC DATA FROM THE 
      CENTRAL ANDES

Katrin Schwalenberg(1), Heinrich Brasse(2), Pamela Lezaeta(2), 
Volker Rath(2), Wolfgang Soyer(2), and Volker Haak(1)

(1) GFZ Potsdam, Telegrafenberg, 14473 Potsdam, Germany
(2) FU Berlin, Fachrichtung Geophysik, Malteserstr. 74-100, Haus D, 
    12249 Berlin, Germany
Katrin@gfz-potsdam.de

A 400 km long magnetotelluric profile was carried out across the 
Central Andes to investigate the conductivity structure from crustal 
levels of the upper continental plate down to the subducted Pacific 
Nazca plate.  Dimensionality studies of the data show that they are 
clearly influenced by three-dimensional effects, but in a first step a 
2-D interpretation seems applicable. We used Mackie's 2D 
inversion code to generate smooth and stable models. The most 
prominent result in the model is a huge conductor beneath the 
Altiplano, a 3700 m high plateau in the backarc with an anomalous 
thickened crust of ca. 70 km. Sensitivity and resolution studies 
have been applied to constrain a minimal vertical extension to 
lower crustal levels, leading to an integrated conductivity of more 
than 30.000 S/m. Since other geophysical investigations also show 
an anomalous behavior in that region the most probable 
explanation are partial melts.




8.8   IMAGES ABOUT A DEEP EXTENSIONAL BASIN BY USING ROTATIONAL 
      INVARIANTS OF THE MAGNETOTELLURIC IMPEDANCE TENSOR

A. Adam and L. Szarka

Geodetic and Geophysical Research Institute of the Hungarian Academy 
of Sciences, H-9401 Sopron, POB 5, Hungary
adam@ggki.hu
szarka@ggki.hu

The so-called Bekes graben in Hungary is considered as a 
characteristic extensional sub-basin of the Pannonian basin. Its 
bottom under thick Pannonian sediments - having a maximum 
depth of 7 km - is more or less known by deep drill holes and by 
geophysical studies. Among others about 40 MT soundings were 
carried out in an are of some 1200 km2 along PGT Geoscience 
Transects - Adam,A. et al., 1996, Geophys. Trans., 40, 45-79, 
Adam,A., Bielik,M., 1998, Geophys, J. Int., 134, 157-171.  In this 
paper various magnetotelluric interpretational parameters are 
presented for this area. A special attention is paid to the 
magnetotelluric rotational invariants, since according to some 
three-dimensional numerical modelling experiments -Szarka,L., 
Menvielle,M., 1997, Geophys. J. Int., 129, 133-142, Szarka,L. et 
al., 2000, Acta geod. Geoph. Hung., 35, 1-27 - any rotational 
invariant computed from the real tensor provides in general better 
images than any other invariants. All magnetotelluric maps give 
useful information about the subsurface structure of the sub-basin, 
but the most realistic picture about the bottom is given by rotational 
invariants, computed from the real elements of the magnetotelluric 
impedance tensor, as it was expected from theoretical studies.




8.9   3-D MODELING OF CONDUCTIVITY STRUCTURE: INTEGRATING 	
      MAGNETOTELLURICS, INDUCTION VECTORS AND THE EFFECT OF STEEP 
      TOPOGRAPHY - A CASE STUDY FROM MERAPI VOLCANO (CENTRAL JAVA)

A. Muller

GeoForschungsZentrum Potsdam, 14471 Potsdam, Germany
grassus@gfz-potsdam.de

Very few examples of successful 3-D interpretation of 
magnetotelluric (MT) sounding curves are known. In the case of 
Merapi volcano 3-D structures are hidden by the effect of steep 
topography. Here, we present 3-D models derived mainly from 
induction vectors. However, in addition, topography is considered 
explicitly, and the overall magnitude of the conductivities is 
constrained by MT measurements. Two 3-D structures have been 
identified uniquely; one is situated in the center of the volcano, and 
the other 4km southwest of the summit.  Knowledge of the two 3-D 
features provides important information about the internal structure 
of the volcano, and for the interpretation of several monitoring 
experiments. While the first conductor might reflect the level of a  
hydrothermal water system, the origin of the second conductor 
remains unclear. Its position and extent correspond to the eruptiva 
of New Merapi. However, the conductivity of 1 Sm-1 is surprisingly 
high, comparable to values found for a layer at about 1500 m depth 
everywhere in the vicinity of the volcano. 1-D modeling suggests 
that this high conductivity is reached at less than 300 m depth 
below the center of the 3-D anomaly. I consider ways of 
differentiating between these two possible causes for the second 
conductor: new eruptiva or ascending material. 




8.10   DISTORTION OF MT IMPEDANCES CAUSED BY A SHALLOW 3D HIGH 
       CONDUCTIVITY STRUCTURE

Edgar Schneider

University of Göttingen, Herzberger Landstr. 180, 37075 Göttingen, 
Germany
edgar@geo.physik.uni-goettingen.de

The Münchberg Gneiss Massif in southern Germany was target of 
geoelectric and magnetotelluric measurements in 1998 and 1999. 
Both methods resolve a high conductive structure above 10 km 
depth. The magnetotelluric impedance is calculated for periods 
from 10 to 10000 s. A big resistivity contrast can be found between 
the polarisations of the tensor, so 3-dimensional forward modelling 
is done to explain the data. The strike of conductivity can be 
explained by high conductive shear zones at the boundary of the 
Gneiss Massif. Special atention is paid to the frequency distribution 
of strike angels. Due to the distortion the penetration depth of the 
electromagnetic field depends on the coordinate system in which 
the tensors are rotated.  




8.11   A MT PROFILE ACROSS THE ELBE LINEAMENT IN NORTHERN GERMANY

Bernhard Friedrichs

Metronix, Kocherstrasse 3, 38120 Braunschweig, Germany
friedrib@cooperpowertools.com
  
Metronix has carried out magnetotelluric measurements for the 
BGR, Federal Institute for Geosciences and Natural Resources, in 
the last years. The soundings took place in the North German 
Basin, new models of the temporal and spatial development of the 
deep sub-surface and, in particular, of the regional distribution of 
potential source rock in the pre-Westphalian have been derived by 
the BGR. Good electrical conductors in depths of 7-11 km have 
been found in the northern and southern areas of the North 
German Basin. In the central part of the North German Basin any 
indications of good conductors within the pre-Westphalian 
sediments are missing.  The data quality was decreasing in the 
western parts of Germany together with the increasing density of 
population. First measurements with the new 24-bit data logger 
ADU-06 were carried out on a new profile North of Braunschweig. 
One of the questions was whether the better dynamic range can 
help to reduce the influence of man made noise.




8.12p  INVESTIGATION OF THE DEEP CONDUCTIVITY STRUCTURE OF THE 
       SW IBERIA USING LOW FREQUENCY MAGNETOTELLURICS

F. Monteiro Santos, Rafael Luzio, Antonio Soares, and L. Mendes 
Victor

Centro de Geofisica da Universidade de Lisboa, R. Escola Politecnica, 
58, 1269-102 Lisboa, Portugal
dfams@fc.ul.pt

As part of the EUROPROBE/IBERIA program a set of low 
frequency magnetotelluric data was collected in the SW Iberia 
(Portugal) to study the conductivity structure of the lower crust and 
upper mantle. It was the first attempt to study the deep geoelectric 
structure in Iberia. MT and induction transfer functions were 
estimated and a preliminary interpretation of the data is presented. 
The conductivity model derived using three-dimensional forward 
modelling is compared with published seismic results.




8.13p  NEW MT MEASUREMENTS IN THE TRANSITION BETWEEN SOUTH PORTUGUESE, 
       OSSA MORENA AND CENTRO IBERIA ZONES

Eugenio Pina de Almeida(1), Jaume Pous(2), Alex Marcuello(2), 
Pilar Queralt(2), F. Monteiro Santos(3), Rita Nolasco(3), Liliana 
Matos(3), and L.A. Mendes Victor(3)

(1) Instituto Politecnico de Tomar, Escola Superior de Tecnologia de 
    Abrantes, 2200-Abrantes, Portugal
(2) Universidade de Barcelona, Faculdade de Geologia, Zona 
    Universitaria de Pedralbes, 08071, Barcelona, Espanha
(3) Universidade de Lisboa and Centro de Geofisica da Universidade de 
    Lisboa, R. da Escola Politécnica, 58, 1250-Lisboa, Portugal
epalmeida@ipt.pt

The Variscan Belt in the Iberian Pen-insula is the result of a 
continent-continent collision between an Ibero-Aquitanian indentor 
and a northern continent. This Belt offers the opportunity to 
observe and study a transpressional orogen.  Forty magnetotelluric 
soundings were carried out along a 200 km-long profile, 
approximately NNE-SSW, across the three major geotectonic units 
in SW Iberia.  A model obtained from two-dimensional inversion of 
the soundings shows four conductive regions at depths from 10 to 
25 km, corresponding to the middle-lower crust. Two of these high-
conductivity zones correspond to transition zones between main 
geotectonic units, SPZ-OMZ and OMZ-CIZ.  The third conductive 
zone is tentatively associated with black shales with major graphite 
impregnation. The model shows several resistive features, which 
are preferentially located in the upper crust,1-10 km, related with 
gabbroic and granitic complexes.  




8.14p  MAGNETOTELLURIC SIGNATURE OF NORTHERN IBERIAN PENINSULA: 
       CRUSTAL STRUCTURE OF THE CANTABRIAN MOUNTAINS AND
       ASSOCIATED FORELAND BASINS

Jaume Pous, Pilar Queralt, Alex Marcuello, and Wiebke Heise

Departament de Geodinamica i Geofisica Universitat de Barcelona, Martí 
Franqués s/n, 08028 Barcelona, Spain
jaume@geo.ub.es

The electrical conductivity structure of the crust of the northern 
Iberian Peninsula, along the Cantabrian Mountains and their 
southern foreland, the Duero basin, is investigated from 
magnetotelluric studies. The Cantabrian  Mountains extend more 
than 250 km along the northern border of the Iberian Peninsula and 
constitute the western prolongation of the Pyrenees. They 
developed from Late Cretaceous to Early Miocene as a result of 
the collision between Iberian and European plates. The data set 
consist of 70 magnetotelluric soundings distributed along 4 profiles. 
Three profiles, oriented N-S, are located in the western and central 
part of the range and the fourth profile, oriented NE-SW, is located 
more to the east, across  the western Pyrenees.  After the analysis 
of the electrical dimensionality a consistent 2D strike was found for 
each profile and the recovered 2D regional responses were 
inverted by using the REBOCC code. The correlation between 
these 2D resistivity models gives a complete electromagnetic 
image of the crust in the northern Iberian Peninsula. Comparison 
with available deep seismic reflection data  shows a good 
correlation between major reflectors and high conductivity zones in 
the upper and middle crust related to Alpine faults cutting deep 
across the basement. At lower crustal depths a number of  high 
conductivity  zones were detected, in general being thickened 
towards the north. All these features reveal a conductivity 
behaviour similar to the one found in the central Pyrenees related 
to the north dipping subduction of the Iberian lower crust.




8.15p  INVESTIGATION OF COAST EFFECT AND ITS CORRECTION ON 
       MAGNETOTELLURIC DATA

Rita Nolasco(1), F. Monteiro Santos(1), Eugenio Almeida(2), 
Jaume Pous(3), and Luis A. Mendes-Victor(1)

(1) Centro de Geofisica da Universidade de Lisboa, R. Escola Politecnica, 
    58, 1269-102 Lisboa, Portugal
(2) Instituto Politecnico de Tomar, Tomar, Portugal
(3) University of Barcelona, Spain
dubert@teleweb.pt

The oceanic effects on MT data acquired near the SW coast in the 
Iberian Peninsula were investigated using three-dimensional 
modelling and a methodology for the correction of those effects is 
proposed. The ocean was modelled considering a constant 
conductivity, 0.3 ohm m, and its mean depth, 3.5 km. The 
continental resistivity model was constructed taking into account its 
electrical structure obtained from previous magnetotelluric studies. 
The coastlines were represented by straight lines to avoid 
numerical instabilities. The model shows that ocean-coast effects 
are important for periods greater than 100 s affecting mainly the 
induction vectors. For sites located close to the coast, less than 
100 km, a distortion of the magnetotelluric impedance tensors is 
also observed. This effect must be corrected before any strike 
analysis based on the impedance tensor was performed. 




8.16p  ELECTRIC FIELD OBSERVATIONS IN THE CAM-1 CABLE (BETWEEN 
       MADEIRA AND LISBON)

Antonio Soares, F. Monteiro Santos, and ISO-3D Group

Centro de Geofisica da Universidade de Lisboa, R. Esc Politrcnica, 58, 
1250 Lisboa, Portugal
dfams@fc.ul.pt

Electric potential measurements between Sesimbra and Madeira 
(Portugal) at the CAM-1 cable, a retired telephone cable, has been 
conducted since beginnings of 1999. The cable is 1100 km long 
and its end near Lisbon is grounded to the ocean bottom. An 
analysis of the cable showed that the cable electrical conditions 
have not significantly changed during the last year.  Monitoring of 
the cable is being done at a sampling rate of 1 s using a Campbell 
data logger with 2 Mb memory, but only the average values, taken 
over 30 s, are recorded.  We present a preliminary analysis of the 
results (data series statistics and spectral analysis). Finally we 
discuss the possibilities of using this data to measure the water 
transport and carry out a large scale magnetotelluric sounding to 
study the deep electrical structure.




8.17p  LONG PERIOD MAGNETOTELLURICS ON THE AZORES ISLANDS

Andreas Junge(1), Anja Kreutzmann(1), and Fernando Santos(2)

(1) University of Frankfurt, Germany
(2) Univ. of  Lisboa, Portugal
junge@geophysik.uni-frankfurt.de

Time variations of the three magnetic and two horizontal telluric 
field components were observed last summer on the islands 
Terceira, Faial and Flores of the Azores Archipelago in the period 
range between 50 and 20.000 seconds.  The aim of the experiment 
was the investigation of the oceanic crust and mantle for the 
American and European/African plate west and east of the Mid-
Atlantic Ridge.  The data do hardly show any cultural noise, thus 
the results are of excellent quality. The magnetotelluric transfer 
functions of the three islands do not reveal significant differences. 
The data can be explained by a simple 3D island model with 100 
Ohm*m resistivity for the crust and mantle.




8.18p  MAGNETOTELLURICS IN THE EASTERN EUROPEAN ALPS
 
Andreas Junge(1), Graham Dawes(2), and Kathy Whaler(2)

(1) University of Frankfurt, Germany
(2) University of Edinburgh, Scotland
junge@geophysik.uni-frankfurt.de
 
Last summer a pilot study was performed to investigate the 
topographic effect and the influence of cultural noise on 
observations of the electromagnetic field in the frequency range of 
1 mHz to 1 kHz along a seismic traverse across the Eastern 
European Alps.  5 simultaneously recording sites were set up: a) 1 
far remote site in Scotland in a distance of 1500 km, 1 near remote 
site in Italy in a distance of 60 km and 3 sites in Austria on the 
slope of a large valley at different levels (1100 m, 1400 m, 1900 m); 
b) 1 far remote site in Scotland, 1 near remote site in Austria, 3 
sites in Italy on the same level at 1800 m around the entrance of a 
valley.  While the influence of topography could be neglected there 
was highly correlated local noise. The preselection of time 
segments using either remote site enabled the interpretation of 
otherwise strongly misleading data.




8.19p  INVESTIGATION OF THE GOOD CONDUCTOR BENEATH ICELAND WITH MT

Anja Kreutzmann(1), Andreas Junge(1), and Hjalmar Eysteinsson(2)

(1) Institute for Meteorology and Geophysics, University of Frankfurt, 
    Feldbergstr. 47, D-60323 Frankfurt, Germany
(2) Orkustofnun, Reykjavik, Iceland
akreutz@geophysik.uni-frankfurt.de

Nearly 250 MT sites were measured on Iceland in the last 20 
years. 1-D and 2-D interpretations indicated a good conductor with 
increasing depth from the island's centre by 8 km down to 25 km. 
Nearly all available data were reprocessed and reinterpreted. Data 
with a similar shape in the transfer functions were organized into 
groups to develop a comprehensive model of the conductive zone 
beneath Iceland.  3D modelling allowed an improved determination 
of the shape and size of the conductor. The conductive zone in the 
crust region is obviously bounded by the coastal line of Iceland. 
Therefore transfer functions in the vicinity of the coastal range will 
be influenced by this lateral change of conductivity as well as by 
the transition of crust/seawater conductivity. Special care is taken 
to distinguish between these two effects. Finally the origin, nature 
and composition of the crust conductor is related to dynamic 
processes like crust generation and it is discussed whether the 
upwelling plume beneath Iceland might influence the conductor's 
geometry.




8.20p  CRUSTAL CONDUCTIVITY MAP OF THE FENNOSCANDIAN SHIELD

Toivo Korja(1), V. Asming(2), A.A. Kovtun(3), N.A. Palshin(4), 
M. Smirnov(3), A. Tokarev(2), A.A. Zhamaletdinov(2), and BEAR 
Working Group

(1) Geological Survey of Finland, Espoo, Finland
(2) Kola Science Centre of RAS, Apatity, Russia
(3) St. Petersburg University, St. Petersburg, Russia
(4) Shirsov Oceanological Centre, Moscow, Russia
toivo.korja@gsf.fi

Knowledge on subsurface conductivity structure is required in 
many applications that are analysing and modelling the BEAR 
electromagnetic data. Such information is also necessary to 
understand the nature and tectono-geological significance of the 
major crustal conductors identified in the Shield. We shall present a 
preliminary version of a crustal conductivity map, i.e. crustal 
conductance, for the Baltic Shield and the surrounding oceans, sea 
basins and continental areas. The map - and the underlying digital 
database - consists of several separate layers including layers for 
sea waters, sediments and basement. The last is divided into 
several sub-layers to enable a feasible use of the map in various 
applications as well as to allow a rough 3D description of the 
conductivity structure. The database for the shield area, proper, 
consists of observations, e.g. 2D models from magnetotelluric data, 
and their extrapolations based on existing 2D data such as 
magnetometer array and airborne electromagnetic survey data. In 
practice, the database are compiled using a multipurpose BEAR 
Map software that allows to input data on electrical conductivity in 
different forms, to compute electrical conductivity in nodes of 
rectangular grids by set of isolines or data points.  In the current 
version, the final database is extrapolated into a 5 min x 5 min grid.




8.21p  INTERPRETING A THREE-DIMENSIONAL RESISTIVITY STRUCTURE 
       USING DIFFERENT ALGORITHMS

Edgar Schneider

University of Göttingen, Herzberger Landstr. 180, 37075 Göttingen, 
Germany
edgar@geo.physik.uni-goettingen.de

A comparison between several forward modelling algorithms is 
always useful to manifest the model of a resistivity structure. 
Parameters of the model such as grid sizes can be varied to 
estimate the stability of the model. On this poster the thin-sheet-
algorithm of Vasseur and Weidelt and Mackie's 3D-forward-
algorihtm are compared. The model contains a shallow conductivity 
anomaly in a layered halfspace. With this two modelling algorithms 
it can be found out, how the thickness of the structure influences 
the results. This is applied to the data of the Münchberg Gneiss 
Massif in southern Germany.




8.22p  ARRAY TENSOR-GEOELECTRICS ON A HIGH CONDUCTIVITY NAPPE PILE

J.B. Stoll

Institute of Geophysics, University of Goettingen, Herzberger Landstr. 
180, 37075 Goettingen, Germany
josto@willi.uni-geophys.gwdg.de
 
The scalar apparent resistivity was originally defined for simple 
linear electrode resistivity arrays such as Wenner and 
Schlumberger arrays. For bipole mapping, it has become common 
practice to measure the total electric field vector at each survey 
point. There are many ways that an apparent resistivity can be 
defined for an electric bipole but all make use of the current density 
vector for the uniform halfspace. It will be shown that when a pair of 
current sources (or quadripole source) is used, and the 
corresponding electric field vectors are measured at each field 
station, the most comprehensive expression of the reduced data is 
as an apparent resistivity tensor. Long range direct current-
soundings were carried out in the Muenchberg Gneiss Massif using 
multiple-source-quadripole geoelectrics. The soundings revealed a 
very high conductivity layer at shallow depth with a conductance of 
about 10000 Siemens.




8.23p  THE HIGH CONDUCTIVITY ZONE OF MUENCHBERG: A MARKER HORIZON 
       OF THE CARBON CYCLE

J.B. Stoll

Institute of Geophysics, University of Goettingen, Herzberger Landstr. 
180, 37075 Goettingen, Germany
josto@willi.uni-geophys.gwdg.de
 
DC- and MT-array soundings on the metamorphic nappe pile of 
Muenchberg (Germany) detected a very high conductivity zone 
(10000 Siemens) in the upper crust, which correlates with the 
overthrusting shearplanes of this nappe. The estimated 
conductance of this zone suggests an electronically conducting 
phase rather than high salinity fluids. Graphite is an important 
source of conductivity in the form of metasediments trapped in 
continent-continent paleosuture zones. Fluid inclusion studies in 
the Muenchberg area observed CO2-, CO4-rich phases in certain 
lithostratigraphic units, which were trapped from a paleofluid 
system. Under certain P-T-conditions the organic matter of Corg-
rich sediments is reduces to graphite in weakly-metamorphosed 
black shales in underthrust sedimentary basins. Graphite can also 
precipitated from CO2-rich fluids on shearplanes from active 
enrichment by fluid invasion and causes the presence of continous 
graphite films within fault zones. Inasmuch as, resistivity soundings 
are essential for the understanding of tectonism involving recycling 
of large volumes of fluids and underthrusting of metasedimentary 
rocks.




8.24p  THE DIFFERENCE IN DEEP CONDUCTIVITY STRUCTURE ACROSS THE 
       NORTH ANATOLIAN FAULT ZONE (NAFZ): ON THE GOLCUK 1999 
       EARTHQUAKE SEGMENT

Aysan Gurer

Geophysical Engineering Department, University of Istanbul, 34850, 
Avcilar, Istanbul, Turkey
agurer@akasya.istanbul.edu.tr

Magnetotelluric measurements at six locations along  a 90-km 
profile across the area between Golpazari-Izmit and Akcaova have 
been modeled in two dimensions to increase the understanding of 
deep structure of western part of  the North Anatolian Fault Zone 
(NAFZ) where the last Golcuk (Ms=7.4, 1999) and Duzce (Ms=7.2, 
1999) erthquakes took place. It is well known from surface geology 
that the branch of NAFZ that pass through the Pamukova Valley 
with an E-W strike separates the region into two sub-areas, 
containing contrasting sets of geological features. These two areas 
also exhibit significant differences in terms of their deep 
conductivity structure Electrical resistivity, which is quite low (10 
ohm-m) south of the fault at an approximate depth of 25 km in 
contrast to the area north of the fault zone.  In the northern part of 
the profile beneath  Ucgaziler (DAK site) and Akcaova (DUC site), 
a five layered conductivity sequence obtained. In the light of the 
geological knowledge, these layers in the conductivity model are 
interpreted as the fragments of the Istanbul zone and the Sakarya 
continent as well as the ophiolitic slab in between.




8.25p  GEODYNAMIC EVOLUTION OF THE VRANCEA SEISMOGENIC AREA 
       REVEALED BY MAGNETOTELLURIC TOMOGRAPHY
                    
Dumitru Stanica and Maria Stanica

Geological Institute of Romania, 78344 - Caransebes Str., 1, Bucharest 
32,  Romania
stanica@ns.igr.ro
                         
As a sequence of its placement at the joint point of the three major 
tectonic plates, of different thicknesses: East-European, Moesian 
and Intra-Alpine, Vrancea zone is characterized by numberless 
motions triggered especially in the intermediate depth of 70-200km.  
In the context created by the existence of more geotectonic models 
which may not explain the whole mechanism regarding the genesis 
and the occurrence of Vrancea earthquakes, new techniques of 
investigation of the lithosphere are imposed, in order to define  
more reliable information. Therefore, this paper aims at revealing 
the deep structure in Vrancea zone by means of a magnetotelluric 
tomography, able to bring into the light new aspects related to the 
electromagnetic parameters, owing to the impedance tensor 
decomposition technique. Derived from this, the separation of the 
regional effects and the strike direction at various depths are 
emphasized.  The tomographic images (the conductivity and phase 
distribution) reveal that the intermediate depth extent of 
seismogenic block is 70-200km and it is charecterized by low 
conductivity surrounded by high conductivity areas, what may 
constitute an evidence of a lithosphere slab (linked with the East-
Europen Platform) embeded into the mantle during the subduction 
process. The seismogenic volume displays a peculiar structure, its 
main feature being a change of strike from top to bottom.  At the 
same time, its strike orientation changes in the depth, displaying a 
NE-SW direction (in conformity with the subduction trend towards 
NW), in the upper part, and N-S (corresponding to the strike 
direction of the subduction before the last segment of the Vrancea 
lithosphere was incorporated into the mantle), in the lower part. 




8.26p  CONDUCTIVITY ANOMALIES WITHIN THE BASEMENT IN THE TELLURIC 
       MAP OF TRANSDANUBIA

Andras Madarasi

Lorand Eotvos Geophysical Institute, H-1145 Budapest, Kolumbusz u. 17-
23, Hungary
madand@elgi.hu

In 1999 the telluric map of western part of Hungary - Transdanubia 
- was completed using more than 7000 stations measured in 
almost 40 years. Tellurics is a simple method to investigate several 
km deep sedimentary basins. When measurements are carried out 
in the S-interval, i.e. the basement is of high resistivity and the 
period time of the processed pulsations is long enough compared 
to the conductance of sediments, from measurements of the 
electric components of the natural electromagnetic field the 
conductance of sediments filling up the basin can be determined 
related to a base station. We have transformed these relative 
values to telluric conductance by means of magnetotelluric 
soundings performed at the base stations, which refers to the 
middle of the unavoidably narrow period range of telluric 
measurements, to 25 s.  Basement is, however, not resistive 
everywhere in Transdanubia, the telluric conductance significantly 
exceeds at some places the conductance of sediments. We 
intended to present this phenomenon known for a long time in a 
new map. Using the data of magnetotelluric soundings and well-
logging we made a more or less acceptable estimation of 
conductance distribution of the sediments filling up the basin. 
Subtracting these from the telluric values we obtain the stripped 
telluric map, which emphasizes the conductors within the 
basement.




8.27p  REINTERPRETATION OF THE TRNASDANUBIAN CONDUCTANCE 
       ANOMALY USING 2D INVERSION

Geza Varga

Lorand Eotvos Geophysical Institute, H-1145 Budapest, Kolumbusz u. 
17-23, Hungary
varga@elgi.hu

Magnetotelluric measurements carried out by researchers of the 
Miskolc University and the Geodetic and Geophysical Research 
Institute of the Hungarian Academy of Sciences at the end of the 
1960s detected the Transdanubian conductance anomaly in 
Hungary. There are, however, uncertainties in their interpretation 
even more than 30 years after this discovery.  The Eotvos Lorand 
Geophysical Institute of Hungary has been involved in the study of 
the conductance anomaly since the early 1980s and performed MT 
soundings at more than 300 sites. Primarily 1D inversion was used 
and due to the strong anisotropy of curves the results were 
generally of poor quality. To reduce the interpretational 
uncertainties we have reinterpreted our measurements using 2D 
inversion.  To choose the TE and TM mode curves we compared 
the strike direction of tipper and principal directions of the 
horizontal impedance tensor at each site. For an overwhelming 
majority of curves - in many cases in contrast to the earlier 
classification  - maximum curves represent the TE mode. To make 
the structural information more correct all of the studies were 
performed with joint inversion, and we obtained very good fitting 
with the Smooth inversion of the WinGLink program package.  2D 
inversion was carried out along eight profiles crossing the Central 
Range. Inversion provided completely identical structure for all 
profiles: conductive formations appear at the SE side of the 
megastructural line separating the Alpian and Central Range units  
- the so-called Raba Line - and steeply descend below the 
Mesozoic rocks of the Central Range.




8.28p  GEOELECTRIC MODEL OF THE TECTONICS IN THE AREA OF THE 
       BERHIDA EARTHQUAKE (HUNGARY)

A. Adam

Geodetic and Geophysical Research Institute of the Hungarian Academy 
of Sciences, H-9401 Sopron, POB 5, Hungary
adam@ggki.hu
 
A conducting dike has been indicated by magnetotelluric soundings 
near the area of the maximum intensity of the Berhida earthquake - 
August 15, 1985 - supposedly at the crossing of the longitudinal 
and transversal fractures. The character of the dike well fits to the 
general structure of the "Transdanubian Conductivity Anomaly" - 
lying in the area of the Trransdanubian Central Range and in its 
NW foreground. To find the real structure of the dike, different 
modellings - polarization study - have been carried out taking into 
account the relation between the conductors and induction vectors, 
too.  Supposing that in the conducting fractures graphite and fluid 
accumulated and these decrease the bulk resistivity, the high 
seismic attenuation in the area can be explained by the low 
viscosity of these materials. They can play role in the release of the 
stress accumulation in case of the Berhida earthquake, too. 
Destruction appears at the brittle wall of the fracture.




8.29p  PROJECT OF MAGNETOTELLURIC SURVEY IN THE POLISH CARPATHIANS - 
       METHODOLOGY OF DATA ACQUISITION, PROCESSING AND INTERPRETATION, 
       AND SELECTED RESULTS

Michal Stefaniuk(1,2), Tomasz Czerwinski(1), and Wojciech 
Klitynski(1,2)

(1) Geophysical Exploration Company, ul. Jagiellonska 76, 03-301 
    Warsaw, Poland
(2) University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 
    Krakow, Poland
stefan@geolog.geol.agh.edu.pl

In 1998, the Geophysical Exploration Company , Polish State 
Geological Institute, and Polish Oil and Gas Company began wide 
magnetotelluric investigations in Polish Carpathians. Data 
acquisition was made with the use of MT-1 magnetotelluric system 
designed and produced by Electromagnetic Instruments Inc. 
Measurements were made in a frequency range of 300 - 0.0005 Hz 
with remote magnetic reference. As a result of data processing 
amplitude and phase MT sounding curves, impedance polar 
diagrams, and skew were obtained. 1D inversion was made with 
Bostick, Occam and LSQ methods. Based on Bostick 1D inversion, 
pseudo 2D resistivity cross-sections were computed. Results of 1D 
LSQ inversion were used to obtain 2D input resistivity section. 
Results of data interpretation from eight profiles provided new 
elements in recognition of the structure of the flysch orogen and its 
basement and confirmed earlier interpretation of the roof of the 
high-resistivity horizon. A considerable resistivity differentiation was 
observed in the Mesozoic, Paleozoic and Precambrian formations 
in N part of the cross-sections where the high-resistivity platform-
type basement occurs beneath the autochthonous Miocene 
sediments. The zone of low-resistivity rocks, probably connected 
with an alpine compressional structure, occurs in the south-eastern 
part of the study area. Quite different situation is in the western part 
of the study area. Southern parts of profiles are located in the Inner 
Carpathians close to a northern margin of the Tatra Mts massif. 
Results of MT data interpretation confirm the allochthonous 
character of that massif. Resistivities of the flysch orogen are much 
lower than of its basement.




8.30p  MODELLING OF THE CONDUCTIVITY DISTRIBUTION AT THE CONTACT OF THE 
       BOHEMIAN MASSIF WITH THE WESTERN CARPATHIANS

S. Kovacikova, V. Cerv, and O. Praus

Geophysical Institute, Academy of Sciences of the Czech Republic, Bocni 
II/1401, 14131 Praha 4, Czech Republic
SVK@ig.cas.cz

Anomalous magnetic field maps for the eastern part of the 
Bohemian Massif and the Western Carpathians are calculated from 
the single-station GTFs by separating the field into external and 
internal parts. The results are compared with the hypothetical event 
analysis. Anomaly sources are replaced by a thin sheet, and the 
equivalent current systems for the thin sheet in free space and the 
thin sheet in the conductive Earth model are obtained and 
compared. The inversion for the conductance within the thin sheet 
is computed. Minimum gradient support stabilizing functional is 
used in order to obtain more clear and focused image then for 
conventional maximum smoothness functional.




8.31p   MT RESULTS ON RUSSIAN GEOTRANSECTS	
	
I.S. Feldman(1), B.A. Okulessky(1), A.V. Lipilin(2), R.G. Berzin(2), 
V.G. Sibgatullin(2), and I.P. Shpak(2)

(1) Electromagnetic Research Center, 3, Jubileinaya str., 142092, Troitsk, 
    Moscow region, Russia
(2) The Ministry of Natural Resources of the Russian Federation, Russia
emrcingf@mtu-net.ru
novikov@hedric.msk.su

MT-results fulfilled with high-precision equipment (ES-MT) along 
Russian geotransects of total length of 2000 km are discussed 
together with reflection-and refraction-seismic data. The profiles 
cross the Russian platform, the Siberian platform, the Caucasus, 
the Sayany mountains and the Baikal region.  Within basins and 
deepened structures at the plates, including foreplains, the 
geoelectric section remaining relatively conductive up to depths of 
10-15 km. Depending on the depth of erosion, three structural-
tectonic floors (Mz-Kz, Pz,  Pr2-3) manifest themselves more or 
less and each of them has its own structural appearance which is 
in accordance with RFL data. The depth of the basement of high 
resistivity (>500 Ohm.m) varies from a few hundred meters up to a 
few kilometers on shields, anticlines  and large arches to 10-12 km 
in basins. Depth interval of high resistivity corresponds to the 
interval of low reflective power.  Mz - Kz mountain-folded structures 
(MFS) (The Caucasus) to the depth of 30 km are formed of 
relatively low-resistive rocks (~100-300 Ohm.m), characteristic of 
Paleozoic sedimentary formation of folded basement. The 
peculiarity of Pz MFS (The Ural-Mongolian belt) is a lateral 
occurrence of a conductive layer with a resistivity of a few ohm.m 
at depths from a few to 20 km that may be explained only by 
occurrence of electron conductive rocks. In more ancient MFS (the 
Yenisei Mauntain-ridge, Baikal region) the geoelectric section, 
beginning from shallow depths and, sometimes, from the surface to 
the depth of 10-15 km, is formed of abnormally conductive 
formations, which have a quite complicated shape, from steep folds 
to vertical stocks.




8.32p  ELECTRICAL CONDUCTIVITY ANOMALIES OF THE EARTH CRUST AND 
       UPPER MANTLE OF THE UKRAINE

S.N. Kulik and T.K. Burakhovich

Kiev, Ukraine
kulik@sabbo.net

In a relatively small Ukrainian territory there is a diversity of tectonic 
elements which are characterized by numerous anomalies of high 
conductivity (CA) in the earth's crust and upper mantle. Korosten 
CA with summary longitudinal conductivity (S) from 500 to 1000 S 
was discovered at the depth of 15 km within Volynian and Podolian 
blocks of Ukrainian Shield (USh). There is some evidence of a 
conductivity layer with S around 600 S at the depth of 50-70 km in 
the upper mantle.  Gajvoronian and Kirovogradian CA are found in 
the territory of Kirovogradian and the south-western part of 
Peridnieprovian blocks and Golovanian and Western-Ingulec 
suture zones of USh. Kirovograd CA lies at the depth of 25 km and 
has electrical conductivity up to 2000 S. Peridnieprovian block and 
part of Orekhovo-Pavlogradian suture zone of USh are 
characterized by electrical resistivity up to hundreds of thousands 
Ohm.m . The estimation of the thickness of the isolator is 10 km.  
There is a CA in the core of Periazovian massive which partly 
overlaps with the Western-Periazovian regional magnetic anomaly.  
Volyn CA is found on the northern part of the Volyn-Podolian Plate 
(VPP). Its thickness is 2-3 km and the depth is 1-2.5 km . Its 
integral conductivity is 1.08×10**8 S.m. The existence of the 
second anomalous area at the depth of 25-50 km remains an open 
question. Chernovcy CA is found at the depth of 50-100 km in the 
territory of VPP, it has electrical conductivity around 2000 S. There 
is Javorov CA within the Lviv Paleozoic Deep. The conductive part 
has a longitudinal conductivity of 200 S, its width is 60 km at the 
depth of 20-25 km. There were found 10-20 km and 40 km 
conductors with longitudinal conductivity of 100-300 and 400-1000 
S respectively in the earth's crust and the upper mantle of the 
Peridobrogea Deep.  In Dnieper-Donets Deep and Donets folded 
structure in the earth's crust, there was determined a wide area of 
high electrical conductivity that consists of alternating weakly 
(S=1000 S) and highly (10000 S) conductive parts.  There is a 
conductive layer at the depth of 110 km with S=700-800 S, that is 
located in the core of the young Epicimmerian Platform of Crimea. 
Two other conductive layers at the depth of 14 and 60 km with 
S=5000 S were located at Tarkhankut Peninsula in Crimea.  There 
lies CA at the depth of 13 km with electrical resistance of 0.5-1 
Ohm.m in Ukrainian Carpathians Mts. In the upper mantle under 
Pannonian there is a conducting astenosphere at the depth of 70 to 
170 km with resistivity of 25 Ohm×m which wedges out the Folded 
Carpathians in the direction of the East-European Platform. 




8.33p  COMPARISON OF ELECTRICAL STRUCTURE AND SEISMICITY ALONG 
       SEGMENTS OF THE SAN ANDREAS FAULT: HOLLISTER, PARKFIELD, 
       AND CARRIZO PLAIN

Paul Bedrosian

University of Washington, Geophysics Program, Box 351650, ATG 
Building, Seattle, WA, 98195, USA
bedros@geophys.washington.edu

High-resolution MT studies at Carrizo Plain, Parkfield, and most 
recently Hollister, California have imaged the electrical structure of 
the San Andreas fault zone.  These three regions show markedly 
different types of seismicity, ranging from a locked segment 
(Carrizo Plain) which ruptured in 1857 to a creeping segment 
(Hollister) with few events above magnitude 5.  The electrical 
structure will be discussed is the framework of fluids and their role 
within fault zones.  In particular, the relationship between fault zone 
fluids and seismicity as well as their implications for the state of 
stress on the San Andreas fault will be discussed. Additional work 
on the analysis of MT data in the presence of significant cultural 
noise (Hollister) will be discussed.




8.34p  IMAGING PROTEROZOIC FE-METASOMATISM USING A/MT OVER THE 
       TEMAGAMI LAKE MAGNETIC ANOMALY, ONTARIO, CANADA

J.A. Craven, D.E. Boerner, and R.D. Kurtz

Geological Survey of Canada, Canada
craven@cg.nrcan.gc.ca

The Temagami Lake Magnetic Anomaly, TLMA, is located 
northeast of Sudbury, Ontario and is thought to be related to 
exposed iron formations and perhaps a deeper cryptic intrusion 
beneath Proterozoic cover rocks within the Cobalt Embayment.   
This interpretation is problematic due to a number of reasons chief 
of which is the absence of a large positive Bouguer anomaly.  
Responses from nineteen combined audio-magnetotelluric, A/MT, 
sites acquired in 1995 and 1997 to determine the electrical 
structure of the anomaly have recently been modeled and 
demonstrate only weak to moderate conductivity enhancement in 
the region.   A tectonic model for the origin of the magnetic 
anomaly will be presented that is consistent with all the available 
geophysical observations and with the Proterozoic tectonic history 
of the region.




8.35p  SNORCLE - NORTHERN CORDILLERA MT EXPERIMENT: REGIONAL 
       STRUCTURES

Juanjo Ledo(1), Alan G. Jones(1), Ian Ferguson(2), Lisa Wolynec(2), 
Jessica Spratt(1), and Grant Wennberg(2)

(1) Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 
    0E9, Canada
(2) Department of Geological Sciences, University of Manitoba, Winnipeg 
    MB R3T 2N2, Canada
ledo@cg.nrcan.gc.ca

The Slave-Northern Cordillera Lithospheric Evolution -SNORCLE- 
Transect addresses LITHOPROBE's theme of crustal evolution 
through an integrated program of geological and geophysical 
studies across a geologically diverse region and through four billion 
years of Earth's history. Previous work focused on the Archean and 
Proterozoic aspects of SNORCLE, but the new MT data acquisition 
in Autumn 1999 was on the Cordillera. The application of the 
magnetotelluric -MT- method to the study of the northern Cordillera 
has three principal objectives. The first concerns characterization of 
the regional conductivity properties of the northern Cordillera in 
relation to the major tectonic units. The second thrust involves 
upper mantle features: determination of the depth extent and 
internal variation within the lithosphere beneath the Cordillera the 
fate of the vast amount of pacific lithosphere overridden by North 
America during the past. The third specific goal is determination of 
the subsurface geometry and character of the Tintina strike-slip 
fault zone for comparison with the reflection data and a better 
understanding of reasons for conductivity enhancement at faults. 
The MT experiment was the largest ever undertaken by 
LITHOPROBE, and consists of more than two hundred MT sites -
AMT, broad band and long period- covering over 2000 km of 
transect. In this paper we will present the first results characterizing 
the regional conductivity distribution.




8.36p  SNORCLE - NORTHERN CORDILLERA MT EXPERIMENT: ELECTRICAL 
       CONDUCTIVITY DISTRIBUTION OF THE TINTINA FAULT

Juanjo Ledo(1), Alan G. Jones(1), and Ian Ferguson(2)

(1) Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 
    0E9, Canada
(2) Department of Geological Sciences, University of Manitoba, Winnipeg 
    MB R3T 2N2, Canada
ledo@cg.nrcan.gc.ca
   
The northern Rocky Mountain Trench-Tintina strike-slip fault -TTF- 
is a 2000 km-long transcurrent fault along which an estimated 450-
km or more of Cretaceous-Tertiary dextral displacement occurred. 
Rocks to the northeast of the TTF are mainly sedimentary and 
represent the ancient North American Margin. Rocks to the 
southwest of the TTF are mostly young, mainly igneous and 
metamorphic, and represent numerous accreted terranes. Tectonic 
forces caused the block of rocks southwest of the fault to grind up 
against the stable North American block and, during a history of 
innumerable earthquakes, moved the southwestern block 
northwest to Alaska. To determine the subsurface geometry and 
character of the TTF zone, one dense AMT and three regional MT 
surveys have been carried out as part of the SNORCLE transect 
investigations. The study of the fault had been done at three 
different length scales. First of all a high density AMT profile to 
study the local structure of the upper few kilometres of the fault. 
Secondly, three MT broadband profiles crossing the fault at 
different locations, which allows us to obtain the image of the fault 
at crustal and upper mantle scale. Finally the integration of the 
three MT models together with different geological and geophysical 
information to produce a 3D image of the fault.  




8.37p  COMPARISON OF THE MAGNETOTELLURIC AND SEISMIC ANISOTROPY 
       DIRECTIONS ACROSS THE GREAT SLAVE LAKE SHEAR ZONE,
       NORTHWEST TERRITORIES, CANADA

Xianghong Wu(1), Alan Jones(2), Ian Ferguson(1), and David 
Eaton(3)

(1) Department of Geological Sciences, University of Manitoba, Winnipeg, 
    Manitoba, R3T 2N2, Canada
(2) Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 
    0E9, Canada
(3) University of Western Ontario, London, Ontario, Canada
umwux@cc.umanitoba.ca
ajones@cg.nrcan.gc.ca

The Great Slave Lake shear zone, GSLsz, arguably the most 
dominant feature in the aeromagnetic map of Canada, is a 
northeast-trending, N45E, strike-slip dextral fault with 300-700 km 
offset in the northwestern part of the Canadian Shield. It is 
interpreted as a Paleoproterozoic continental transform.  A deep-
probing magnetotelluric and teleseismic study was undertaken 
across it to define its geometry to depth. MT soundings were made 
at 15 sites along a profile across it in 1996, and teleseismic data 
were recorded at thirteen stations across it from mid-May to early 
October, 1999.  Geoelectric Groom-Bailey strike varies with depth. 
High frequency strike to 10 s periodicity, approx. 25 km depth, is 
approx. N30E, whereas at longer periods strike rotates to approx. 
N60E by 100 s, approx. 100 km, and is constant thereafter. The 
high frequency strike we associate with upper and mid-crustal 
structures, whereas the longer period strike is representative of 
lithospheric mantle structures.  We have performed 2D inversions 
for, (A) the whole depth interval simultaneously with the all period 
data at an assumed strike angle of N41E, (B) the upper/mid-crust 
with the data to 10 s at N30E, and (C) the lower-crust to lithosphere 
with the data from 30 - 3000 s at N60E. The resulting models show 
features that infer the GSLsz is a west-dipping fault in the 
uppermost crust, then changes to a vertical fault with a large depth 
extent from the mid-crust to mantle.  The teleseismic SKS analyses 
give an average fast polarization direction for sites away from the 
GSLsz of N57E caused by a single anisotropic layer. In contrast 
the two sites closest to the GSLsz exhibit a complex response 
indicative of two anisotropic layers combining to give an average 
SKS direction of approx. N45E.  The N57E direction is the direction 
of absolute plate motion due to present-day asthenospheric flow, 
and one possible interpretation is that the SKS anisotropy comes 
from the asthenosphere rather than the lithosphere. The 
correspondence in direction between the SKS and MT anisotropies 
suggests though that both are due to lithospheric mantle anisotropy 
that may be coincidentally in the same direction as present day 
plate motion. Alternatively, perhaps present day plate motion 
affects the lower lithospere.




8.38p  IMAGING ELECTRICAL CONDUCTIVITY STRUCTURES AT UPPER MANTLE 
       DEPTHS IN THE ALTO PARANAIBA IGNEOUS PROVINCE (APIP) 
       OF BRAZIL

M.S. Bologna, I. Vitorello, A.L. Padilha, and M.B. Padua

INPE, C.P. 515, 12201-970, S.J. Campos, Brazil
bologna@dge.inpe.br

Electromagnetic investigations from long-period (> 100 s) 
magnetotelluric (MT) soundings have been carried out in 
Neoproterozoic belts of Brasiliano-Pan African age, amalgamated 
around the south-eastern border of the Archean Sao Francisco 
Craton. The focus of this research is the definition of the variations 
in electrical conductivities of the upper mantle, particularly the ones 
associated with the base of the geoelectrical lithosphere.  Situated 
in the SW margin of the craton, a profile composed by 13 MT 
soundings in a 110 km long path crosses several Cretaceous 
intrusives in the Alto Paranaiba Igneous Province (APIP), an 
important site of alkalic magmatism. The presence of a large 
variety of rock types, i.e. carbonatite complex, kimberlite, 
kamafugite, lamproite and volcanics of the Mata da Corda 
Formation, suggests that these rocks were generated at different 
depths within the upper mantle and under distinct mantle 
geotherms, but their geneses, temporal and spatial relationships 
remain obscure. The profile runs from the NE, in the region of the 
Mata da Corda Formation near the proposed border of the Sao 
Francisco Craton, to the SW, on the margin of the Parana Basin. A 
2D inversion of MT data up to 20,000 s indicates an overall 
resistive lithosphere, yet in some degree heterogeneous. In 
particular, the SW portion is more conductive, mainly below 60 km, 
than the central and the NE regions. In the latter, a sharp increase 
in conductivity is observed at about 140 km, possibly associated 
with incipient partial melting, which could be interpreted as the 
base of the geoelectrical lithosphere. Petrochemical data indicate 
for this region the presence of an uplifted asthenosphere with a 
probable link to the source of kamafugitic and lamproitic intrusions 
and the volcanics of the Mata da Corda. Intrusions of deeper 
material with kimberlitic affinities are found in the central part of the 
profile where high resistivity values go deeper than 200 km.




8.39p  MAGNETOTELLURIC SOUNDINGS ACROSS THE TAUBATE BASIN, 
       SOUTHEAST BRAZIL

Paula M.A. Brito, Antonio L. Padilha, and Icaro Vitorello

INPE, C.P. 515, 12201-970 S.J. Campos, Brazil
padilha@dge.inpe.br

Thirteen magnetotelluric soundings were carried out in the Taubate 
Basin region.  They were positioned along a profile cutting across 
the thickest part of the basin, named Tremembe sub-basin, and 
extending over the Serra do Mar and Serra da Mantiqueira 
mountain ranges, respectively to the southeast and the northwest. 
The data were interpreted through a 2-D inversion scheme and the 
results were tested through a forward modelling.  A 10 km deep 
conductive layer (conductance around 20 S) was observed quite 
clearly under the crystalline regions but was undetected under the 
sedimentary rocks. The latter can be probably associated with 
screening effects due to the high conductance (150 to 200 S) of the 
surficial sediments. Studies at different tectonic regions in the world 
have reported the same conductive layer at around the same depth 
but in the present study the layer has an anomalously low 
conductance considering that it is a region which was extensively 
reworked during the Neo-Proterozoic.  These findings are strong 
constraints to any tectonic model suggested to explain regional 
features and indicate that the mechanism related to the basin 
formation did not originate intense deformation at deep parts of the 
lithosphere. The events can be then considered to be shallow, 
mobilizing a small area in the upper crust, a typical characteristic of 
a strike-slip mobile zone.




8.40p  MAGNETOTELLURIC AND GEOMAGNETIC DEPTH SOUNDINGS REVEALING 
       UPPER CRUSTAL CONDUCTORS ALONG THE TORRES SYNCLINE HINGE, 
       SOUTHEAST PARANA BASIN, BRAZIL

Antonio L. Padilha and Icaro Vitorello

INPE, C.P. 515, 12201-970 S.J. Campos, Brazil
padilha@dge.inpe.br
  
Seventeen MT and GDS soundings in the period range of 0.0008-
2048 s have been carried out around the NW plunging Torres 
Syncline in the southeastern border of the Parana Basin, in Brazil.  
Parkinson induction arrows show a NW-trending conductive 
anomaly nearly coincident with the hinge of the syncline and strong 
ocean effects in periods approaching 1000 s at sites near the 
coastline.  2-D inversions of MT data along two profiles parallel to 
the coast and across this anomaly have distinguished two highly 
conductive features: (i) a near-surface zone observed in the profile 
closest to the coast (mainly located over outcrops of pre-volcanic 
sediments); and (ii) a larger electrical conduit, in the central part of 
both profiles, dipping to the NW from a depth of 500 m at the coast 
to 1000 m at a distance of about 100 km further inland. The former 
conductor is probably connected with electronic conduction 
afforded by coal beds underlying the studied region, whereas the 
latter conductor, in the crystalline basement, could be possibly 
ascribed to residual metasomatic fluids concentrated along the 
hinge of the downfolded Torres Syncline, as a fossil record of past 
magmatic activities that affected the Parana Basin and underlying 
lithosphere since the Early Cretaceous.




8.41p  TEM IMAGING OF PLATFORM CARBONATES FROM SERGIPE-ALAGOAS BASIN - 
       BRAZIL: PRELIMINARY RESULTS

Eutair R. Morais and Paulo T.L. Menezes

Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Brazil
eutairmor@starmedia.com

TEM measurements were performed at Sergipe-Alagoas Basin, 
Brazil, in order to study the shallow carbonate bodies onshore, 
which are analogous to several oil reservoirs in the brazilian 
basins. The Riachuelo and Cotinguiba are the Cretaceous 
formations that are related to these bodies. Riachuelo Formation is 
composed of open marine sediments intercalated by clastic 
wedges and shalow platform carbonate banks. Cotinguiba 
Formation is a carbonate section deposited in a ramp-type basin.  
We collected 49 SIROTEM-MK3 100 and 50 m side coincident loop 
stations at three NW profiles with 20 km length each, crossing the 
whole carbonate section. Preliminary analysis based on 1D 
inversions of the data set shows an intercalation of resistive and 
very conductive layers, which was interpreted as intercalations of 
carbonate and clastic units, respectively. These geoelectrical units 
presents good correlation with related well logs.




8.42p  EM INDUCTION MODELLING OF THE ARAGUAINHA IMPACT CRATER USING 
       WALSH FUNCTIONS

P.-A. Schnegg(1) and S. L. Fontes(2)

(1) Geomagnetism Group, Geological Institute, University of Neuchatel, 
    CH2000 Neuchatel, Switzerland
(2) Observatorio Nacional, Rua Gal. Jose Cristino, 77, 20921-400 Rio de 
    Janeiro, Brazil
pierre.schnegg@geol.unine.ch

The Araguainha impact crater is the largest impact site in South 
America with a diameter of 40 km. It is located in sediments at the 
northern tip of the intracratonic Parana Basin. To investigate the 
geometry and the vertical extension of this structure a 40-km profile 
driven in the NW-SE direction across the centre of the crater was 
carried out in 1992 with magnetotelluric equipment. In a previous 
work three-dimensional modelling of the horizontal electric and 
magnetic fields has shown that the bottom of the structure reaches 
a depth of 2 km.  In an attempt to increase the resolution of the 
method we carried out a 3D modelling of the geomagnetic 
induction field, i.e. vertical magnetic field, with an automatic 
scheme based on a radial expansion of Walsh functions, and using 
R. Mackie's forward code. The constraint of a vertical symmetry 
axis is raised by allowing azimutal depth variations  with the same 
type of functions. We also compare the resolving power of 1. 2-
polarization MT, 2. vertical magnetic field induction and 3. both MT 
and vertical field.




8.43p  MAGNETOTELLURIC INVESTIGATION AT A PROFILE BETWEEN VITORIA, 
       STATE OF ESPIRITO SANTO, AND BELO HORIZONTE, STATE OF MINAS 
       GERAIS, BRAZIL: PRELIMINARY RESULTS

Gleide A N. Dias and Irineu Figueiredo

Observatorio Nacional, Rio de Janeiro, Brazil
irineu@on.br

It was performed 20 MT soundings along profile of 280 km, with 
average spacing of 14 km between soundings and frequency range 
covered was generally 0.003 Hz to 300 Hz, crossing  resistive 
crystalline terrains related to Ribeira Belt, Southeast Brazil, 
between the Serra do Mar and Serra da Mantiqueira with a 
direction of the about  West to East. Data from profiles located over 
terrain Pre Cambrian and lithological variations occur from one 
intermontane with, Gnaisse Piedade, Complexo Paraiba do Sul, 
Complexo Pocrane, Grupo Italva, Suite Intrusiva Santa Angelica, 
Complexo Juiz de Fora and Suite Intrusiva Espirito Santo. The 
processing was carried out in the frequency domain using a robust 
code. In this work we used the Groom and Bailey approach, in 
order to obtain the decomposition of the impedance tensor and to 
determine the regional and local geoelectric strike.  Studies in the 
area include 1D and 2D inversion of TE and TM data for the 
improvement and understanding of the North part of Ribeira Belt. 
MT model is supported by the surface geology, represented by the 
geologic units and associated faults, with predominant direction 
East to Northeast. 




8.44p  INVESTIGATIONS IN A EXPLORATORY PROGRAM OF THE CENTRAL PART 
       OF SAO FRANCISCO BASIN - BRAZIL

Ana L. Rodrigues(1), Emin Ulugergerli(2), Sergio L. Fontes(1), and 
Jorge Porsani(3)

(1) Observatorio Nacional, Rio de Janeiro, Brazil
(2) Ankara University, Turkey
(3) IAG-USP, Sao Paulo, Brazil
sergio@on.br

Fourty three magnetotelluric (MT) soundings were undertaken 
along eight profiles at the central portion of the Proterozoic Sao 
Francisco Sedimentary Basin, northern region of Minas Gerais 
State. The study aimed at the mapping of the geoelectrical 
structure of the basin. The MT profiles were disposed along ten 
seismic lines covering 11,0000 km2 of total area. The MT 
soundings were irregularly spaced and covered the frequency 
range from 0.001Hz to 400 Hz, probing the electric structure from 
the surface to a maximum depth of 60 km. The MT data corrupted 
by static shift were corrected by fixing the resistivity value (given by 
the median) of the first observed conductor. Two-dimensional 
modelling showed a relatively conductive zone in the centre of the 
studied area (1-3 Km in depth) which might be associated to a rift 
structure or normal faulting of the basement.  The results show 
good agreement with seismic and gravimetric data. Deep structure 
depicted from the MT data is presently being assessed.




8.45p  PRELIMINARY MAGNETOVARIATIONAL AND MAGNETOTELLURIC RESULTS 
       FROM 9 MT SOUNDINGS ALONG A WE PROFILE AT 33-34 DEG SOUTH 
       IN ARGENTINA

E. Borzotta and M. Mamani

Unidad de Geofisica, IANIGLA, CRICYT, Casilla de Correo 330, 5500 
Mendoza, Argentina
eborzota@lab.cricyt.edu.ar

Preliminary results from 9 MT soundings performed along a W-E 
profile of 600 Km long at 33-34 deg South Lat. in Argentina are 
discussed. H and D magnetograms obtained at each location were 
compared with observations at the Pilar Geomagnetic Observatory 
located next to the profile, and some induction vectors were also 
estimated in order to detect possible conductivity anomalies CA 
and lateral electrical resistivity variations.  The results show a 
general agreement between MV and MT studies, suggesting an 
important CA between 66 and 68 deg West Long. with NW-SE 
general strike. This anomaly would possible run following the 
border between the Pampean and Precordillera terranes 
connecting the Cuyana and Beazley sedimentary basins along the 
Tunuyan lineament.  High electrical resistivities in the lithosphere 
and a good developed conductive layer are also observable at 64 
deg W Long from the Pilar Geomagnetic Observatory location to 
200 Km south, with top at about 200 Km depth.  Analysis of these 
features in the frame of the regional geological context is 
performed.




8.46p  DISTORTIONS ON MT TRANSFER FUNCTION DUE TO PERCOLATION

Alicia Favetto, Matias de la Vega, and Ana Osella

Dpto. de Fisica, Univ. Buenos Aires, Ciudad Universitaria, Pabellón I,1428 
Buenos Aires, Argentina
favetto@df.uba.ar

In the present paper we discuss possible effects that percolation in 
porous media can produce on the MT response curves. As a first 
approach to study this complex system we assume that the 
percolation occurs in a layer of a 1-D structure.  To model this 
layer, we propose a percolation network in which the effective 
conductivity is frequency dependent. We simulate this effect also 
by means of a weak non-linear contribution of the medium.  Finally, 
we calculate the MT response of these models and compare them 
to the non-distorted results.




8.47p  A CASE OF STRONG CURRENT CHANNELLING IN THE SOUTHERN
       CENTRAL ANDES

Pamela Lezaeta and Heinrich Brasse

Freie Universitat Berlin, Germany
pamela@geophysik.fu-berlin.de

Data in the period band 20-20000 s from MT measurements 
carried out in the Andes at 20-21 S latitude show common features. 
In the Coastal Cordillera, several fault and mega-fault systems 
oriented NNW-SSE characterise this region. Induction arrows point 
SE, thus deviated from the expected direction about pointing away 
of the ocean. Dimensionality analysis indicates 2-D regional 
structure with a remarkable strong current channelling. The local 
azimuth - by minimisation of the diagonal tensor elements - ranges 
between -20 and 10 deg. in the Coast, directions which are nearly 
parallel to the coast line - approx. N-S. When the impedance is 
rotated in the local azimuth, the phases of the approx. N-S E-field 
component increase with period reaching values of almost 180 
deg. A tensor decomposition by a complex distortion matrix allows 
to detect the direction of the local structure -if strong current 
channelling is present- as well as the estimation of the regional 
responses in the regional coordinate system, iff telluric and 
magnetic distortion effects are galvanic. In this way, data are 
corrected from distorted phases over 90 deg. and can be 
considered for a 3-D modelling of the whole region, from the ocean 
until the Western Altiplano.  In the centre of the study area, data 
are characterised as 3-D and also phases over 90 after rotating in 
the local azimuth are seen. This distortion is related with inductive 
effects probably associated with known mega-fault systems. In the 
model, these are traced as 3-D conductors reaching depths of 35 
km.




8.48p  THE APPLICATION OF LINEAR AND NONLINEAR SENSITIVITY 
       STUDIES ON A TWO-DIMENSIONAL CONDUCTIVITY MODEL FROM THE
       CENTRAL ANDES

Katrin Schwalenberg

GFZ Potsdam, Telegrafenberg, 14473 Potsdam, Germany
Katrin@gfz-potsdam.de

Due to nonlinearity and ill-posedness the discrete inversion of 
magnetotelluric data is a nonunique instable problem. To stabilize 
the inversion process one has to consider regularization terms in 
the objective function. We use the 2-D inversion code from R. 
Mackie, were an optional first or second order difference operator 
is considered leading to smooth models, i.e. finding a minimum 
structure model. Because of the infinite degree of freedom the 
inverted model can only be one possible solution among others. To 
find alternative models we carried out linear and nonlinear 
sensitivity studies. The former implies the visualization of the 
sensitivity matrix itself. Applying suitable weightings to the 
derivatives the illustrations represent the induction space. This 
approach is only valid in a small environment of the final model. In 
the nonlinear domain, we can check special features in the model 
by forward modeling or incorporate them as a priori information in 
the inversion, where the algorithm is forced to find a solution close 
to the a priori model. So, the combination of inversion, forward 
modeling, inspection of the sensitivity matrix and a priori 
information increases the chance of finding reliable models 
and helps in the geological interpretation.




8.49p  CONDUCTIVITY ANOMALIES AS MARKERS OF CRUSTAL 
       REACTIVATION - AN EXAMPLE FROM THE DAMARA BELT IN NAMIBIA

Oliver Ritter, Ute Weckmann, Tim Vietor, and Volker Haak

GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, 
Germany
oritter@gfz-potsdam.de

We report on results of a study combining surface geological and 
high resolution aeromagnetic investigations with deep 
magnetotelluric (MT) profiling in the Damara Belt of Namibia. The 
Pan-african Damara crust was intruded by mafic dikes and 
intrusive ring complexes prior to continental breakup in the early 
Cretaceous. High-resolution aeromagnetic data reveal a 
concentration of dikes, termed the Hentjies Bay-Outjo Dike swarm 
(HOD) which extends some 300 km NE from the Atlantic to the 
edge of the Congo Craton. Broadband magnetotelluric and GDS 
data were recorded in two field experiments at 107 sites in 1998 
and 1999. This paper is concerned with the interpretation of a 
subset of sites aligned along a 200 km long profile which crosses 
major Damara zone boundaries, the HOD, and the Van Zijl and de 
Beer conductivity belt. We show two-dimensional inversion results, 
discuss the validity of the 2D approach and assess the robustness 
of the derived models. The results indicate a very resistive upper 
crust which is typical for the granites and metasediments of the 
Damara belt. In the area of the HOD, the MT profile unravels a high 
conductive lower crust and two subvertical conductors in the middle 
crust which coincide with the location of fault zones. Rock samples 
found at the faults indicate that subvertical conductors are caused 
by graphite enrichment along shear zones. The correspondence of 
conductivity anomalies and high dike concentration marks a 
tectonically weakened crust with a high concentration of faults 
which in turn controlled the emplacement of melts in the 
Cretaceous.




8.50p  MAGNETOTELLURIC STUDIES ACROSS THE INDUS TSANGPO AND SHYOK 
       SUTURE ZONES IN THE NW HIMALAYAN REGION

S. G. Gokarn, C. K. Rao, C. Selvaraj, and Gautam Gupta

Indian Institute of Geomagnetism, Colaba, Mumbai 400 005 India
gokarn@iig.iigm.res.in

Magnetotelluric studies were conducted across the Indus Tsangpo 
suture and the Shyok suture in the NW Himalayas. Data collected 
at 15 stations in this region indicated a predominantly two 
dimensional structure. The regional strike direction could be 
constrained to N40E using Groom Bailey tensor decomposition and 
is in good agreement with the regional direction of the various 
tectonic elements in this region. The rotated response functions 
were inverted using the Rodi and Mackie two dimensional inversion 
scheme.  The geoelectric structure shows three high resistivity 
bodies in shallow depths, the first on the SW corresponds to the 
exposure of the Tso-Morari Crystalline complexes and extends to 
depths of about 15 km. The second resistive body corresponds to 
the Ladakh batholith and is separated from the Tso-Morari 
Crystalline complexes by the Indus Tsangpo suture. Further NE, 
the third resistive body is located in the region of the Chushul 
Batholith separated from the Ladakh batholith by Shyok suture.  
Both sutures were associated with high conductivity of 20-50 ohm-
m. The Indus Tsangpo suture was mapped as a NE down-dipping 
conductive zone with resistivity of 20-50 ohm-m up to depth of 25 
km. Further NE, it becomes sub-horizontal below the Ladakh and 
Chushul batholiths and seems to correspond to the subducting 
oceanic crust. The deeper signatures of the Shyok suture were not 
clear from the MT studies. 




8.51p  CONDUCTIVITY STRUCTURE OF THE BANGONG SUTURE ZONE, CENTRAL 
       TIBET, FROM INDEPTH-III MAGNETOTELLURIC DATA

Kurt D. Solon(1), A.G. Jones(2), K.D. Nelson(1), and M.J. 
Unsworth(3)

(1) Department of Earth Sciences, 204 Heroy Geology Laboratory, 
    Syracuse University, Syracuse, New York 13244, USA
(2) Geological Survey of Canada, 615 Booth Street, Room 218, Ottawa, 
    Ontario, K1A 0E9 Canada
(3) University of Washington Geophysics Program, Box 351650, Seattle, 
    WA 98195-1650, USA
kdsolon@syr.edu

As part of the collaborative and ongoing efforts of project 
INDEPTH, both broadband and long period magnetotelluric data 
were collected from July to August 1998 along a 400 km north-
south profile in central Tibet.  The profile crosses the Bangong 
Suture, which is the principal terrane boundary within the central 
Tibetan Plateau and coincides with marked south-to-north changes 
in the seismological properties of the plateau lithosphere. Tensor 
distortion analyses were applied to the MT data from the sites 
around the suture in order to determine dimentionality and derive 
regional geoelectric strike direction and regional MT response 
functions.  These analyses suggest that the regional electrical 
structure is two-dimensional to periods of 5,000 seconds, with a 
strike direction of 095.  This strike direction corresponds well with 
the surficial geological strike in this region.  Two-dimensional 
inversions, Mackie 2D, of the MT data show the following first-order 
features.  1 The regional midcrustal conductor sensed previously in 
the southern Tibetan Plateau continues into the central plateau, 
with its top at depths of 15-20 km beneath the suture zone.  2 The 
Bangong Suture coincides with a 40 km wide, sub-vertical, 
conductive zone/s  which extends upward from the regional 
midcrustal conductor to the near surface.  This zone of conductivity 
is spatially coincident with a measured low seismic velocity zone, 
mapped strike-slip faults as well as geothermal activity.  3 Deeper 
in the crust, beneath the suture, a very conductive zone is sensed 
at depth of 30-40  km.




8.52p  DEEP ELECTRIC STRUCTURES IN HUNAN-HUBEI AREA AND ITS 
       TECTONIC IMPLICATIONS

Sun Jie, Jin Guangwen, Wang Jijun, Tang Ji, and Liu Tiesheng

Institute of Geology, China Seismological Bureau, Beijing 100029, P. R. 
China
sj66@eq-igl.ac.cn

The study area is located in the southeastern part of Yangtze Block 
(South China Block). It is tectonically bounded on the north by 
Qinling fold belt and South China fold belt on the south. The area 
has undergone multi-phase tectonic movements and displays 
considerably complex geologic structures.  MT field data collection 
was performed using GMS-05 equipment from Metronics. The 
frequency band is 8000Hz-4096s. In data processing several 
techniques, such as Swift decomposition, Bahr decomposition and 
Robust enhancement method were employed. Smith and Booker's 
fast relaxation method was used in two-dimensional inversion. 
Combining the regional geological and other geophysical data, 
integrative geological and geophysical interpretations for the deep 
structures in the area have been made. This paper deals mainly 
with deep electric structures and their tectonic implications. (1) The 
uppermost part along the measurement profile is the basement of 
upper Proterozoic complex. The shallow electric layer, consisting of 
sedimentary and igneous rocks,  is in good agreement with 
regional geological data. (2) The basement rocks in the study area 
are laterally inhomogeneous. The basement occurs at depth of 3-5 
km in the northern part of the area is highly resistive while in the 
southern part beneath the Wuling uplift, where the basement 
deepens up to 7-10 km, is moderate resistivity and moderate 
conductive. It well reflects that the basement is characterized by a 
transition from deep metamorphic to shallow metamorphic facies. 
In the Jiangnan block-fault zone in the southernmost of the study 
area a highly resistive block with resistivity higher than 2000O m  
occurs obviously. These features are the important discoveries 
from the MT soundings. (3) In Jiangnan block-fault zone, a 1-1.5 
km thick regional low resistive layer is firstly found over the upper 
Proterozoic basement. It rakes from southeast to northwest and 
was inferred to be a weathering layer or the lower part of Sinian 
system on the top of the basement. It may be a tectonic 
decolement formed by the latter tectonism. (4) The deep electric 
structures indicate that two highly conductive layers are 
discontinuously developed in the crust and upper mantle along the 
profile and their occurrence depth is about 21-43 km and 90-100 
km, respectively. They tend to gradually uplift from northwest to 
southeast and vary smoothly. It means that the Yangtze block is a 
relatively stable block. Meanwhile, the variation of the depth of the 
highly conductive layers in the crust and upper mantle beneath the 
Jiangnan block-fault zone indicates the inhomogeneities beneath 
the area.  Our MT study provides a new knowledge on the deep 
structure of Jiangnan block-fault zone, the collision-transition zone 
between two large tectonic blocks, the Yangtze platform and South 
China fold belt. It suggests that the Jiangnan block-fault zone (at 
least in the study area) is not a nappe structure, but a tectonic uplift 
zone formed compositely by multi-phase tectonic movements. 




8.53p  DEEP ELECTRICAL STRUCTURE BENEATH TIANSHAN MOUNTAIN OROGENIC 
       BELT AND THE VICINITY

Guoze Zhao, Yan Zhan, Ji Tang, and Junmeng Zhao

Institute of Geology, China Seismological Bureau, Beijing 100029, China
zhaogz@public.bta.net.cn

Tianshan orogenic zone is one of the youngest with less than 20 
Ma and highest with elevation greater than 7000 m orogenic zones 
in the world. There is a well-known seismotectonic belt. Some 
important mineral resources are developed. Two large 
hydrocarbon-rich basins, Junggar basin and Tarim Basin are 
located to it's north and south respectively.  Magnetotelluric 
measurement as one of important parts of synthetic geologic and 
geophysical investigation has been carried out recently. The MT 
sites distributed along a profile about 700 km and oriented 
approximately N-S in western Xinjiang Autonomous Region, which 
stretched from the middle of Tianshan Mountain in the south, 
through the Junggar basin , ending in the Artai mountains at the 
north end. Another profile is located to the south of Tianshan 
mountain.  Impedance tensor decomposition and 2-D inversion 
methods have been used for data interpretation. Electrical structure 
of the crust and lithosphere beneath two profiles are obtained. The 
thickness of lithosphere for Tianshan fold system is about 160 km 
that is greater than those in both sides to north and to south. A 
zone with a marked conductivity gradient inclined to the north 
appears at a depth of about 50 km at the south end of the first 
profile, which can be compared with the electrical structure beneath 
southern profile. 




8.54p  THE ELECTRICAL STRUCTURE BENEATH WUDALIANCHI VOLCANIC 
       CLUSTER AND ITS COMPARISON WITH THAT BENEATH TIANCHI VOLCANO 
       IN NORTHEAST CHINA

Yan Zhan, Guoze Zhao, Fei Xuan, Ji Tang, and Denghai Bai

Institute of Geology, China Seismological Bureau, Beijing 100029, China
zhaogz@public.bta.net.cn

Wudalianchi volcanic cluster in northern part of North-East China 
consists of 14 cones. The last eruption of the volcanoes was in the 
year of 1719 to 1721. MT measurement are recently carried out at 
71 sites along 3 EW profiles and 4 NS profiles in the volcanic 
cluster and the vicinity to study the magma chamber and deep 
structure. The distribution of real part of magnetic induction arrows 
at most of MT sites shows that there is a resistive body  beneath 
the cluster in the upper crust and  a lower resistivity body in the 
deeper depth in upper mantle. The 2-D inversion results by RRI 
method for EW profiles show that there is a high resistivity body 
like rivet form from surface to about 20 km depth. The width of the 
body in EW direction for depth less than 5 km is about 20 km. The 
width for the lower part of the body is less than 10 km. The tip of 
the rivet body extends continuosly down to several tens kilometers 
and resistivity decreased with increasing depth. It is postulated that 
a high resistivity body is a cooling magma chamber. A passage of 
magma exited in the deeper depth.  The crust electrical structure 
beneath Tianchi volcano in eastern margin of Jilin province has 
been also studied recently. It is found that there is low resistivity 
body appeared at depth of about 15 km, which is suggested as 
magma chamber. 




8.55p  MAGNETOTELLURIC INVESTIGATION IN A SHALLOW SEISMICALLY ACTIVE 
       AREA: A CASE STUDY IN THE NORTHERN MIYAGI PREFECTURE, JAPAN

Yuji Mituhata(1), Yasuo Ogawa(2), Masaaki Mishina(3), and 
Toshihiro Uchida(2)

(1) University of British Columbia
(2) Geological Survey of Japan
(3) Tohoku University
yuji@geop.ubc.ca

Seismic activity in inland is almost confined to the upper crust, 
which can damage heavily.  This confined distribution of 
earthquakes may be explained by the existence of active faults and 
related free water, and the brittle-ductile transition. We believe a 
resistivity structure provided by magnetotelluric investigation will be 
useful to interpret the existence and relationship of them in shallow 
seismically active area.  The northern part of Miyagi Prefecture is 
one of most seismically active areas. In this area, the 1962 
Northern Miyagi Earthquake(M6.5) occurred. Many shallow 
earthquakes occur in and around the area since it. We carried out 
magnetotelluric surveys in 1993, 1998 and 2000 in this area.  
Results of the 1993 survey show that there is a large resistive block 
on the west side of the area and we can see a resistivity 
discontinuity under it. The distribution of the resistive block is 
consistent with a high gravity anomalous zone. Besides, the 
resistivity discontinuity almost agrees with the hypocenter 
distribution of microearthquakes. The data collected in 1998 and 
2000 support this resistivity structure and provide a more detailed 
and reliable resistivity structure.




8.56p  INTERPRETATION OF THE RESISTIVITY STRUCTURE OF THE ASO 
       CALDERA, CENTRAL KYUSHU, JAPAN, INFERRED FROM A 
       MAGNETOTELLURIC SURVEY

Shinichi Takakura(1), Takeshi Hashimoto(2), Katsuaki Koike(3), 
and Yasuo Ogawa(1)

(1) Geological Survey of Japan, Japan
(2) Kyoto University, Japan
(3) Kumamoto University, Japan
shin@gsj.go.jp

Magnetotelluric survey was carried out along two NNE-SSW 
profiles traversing at the Aso caldera with a number of central 
cones. The purpose of this survey was to investigate the deep 
hydrothermal system including the geothermal source in the 
caldera. The analyzed resistivity sections of the two lines were 
almost same and were consistent with the resistivity logging data of 
the nearby exploratory test wells which were drilled to deeper than 
1 km. 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 hot spring. The low 
resistivity may be due to hot water or clay minerals that were 
produced by the geothermal alteration. The high resistivity layer 
correlates well with the basement interpreted from gravity, which 
reflects pre-Tertiary basement rocks or granitic rocks. However the 
upper part of the basement is relatively conductive around the 
central cones, indicating the fractures with geothermal fluid are 
present in the basement. An upheaval of the resistive layer like a 
resurgent dome is shown beneath the central cones. There are no 
conductors that correspond to magma at least to a depth of 10 km. 




8.57p  MAGNETOTELLURIC IMAGING OF DEEP STRUCTURE OF ACTIVE FAULTS 
       IN NE JAPAN

Yasuo Ogawa(1), Masaaki Mishina(2), Tadanori Goto(3), Hideyuki 
Satoh(4), 
Naoto Oshiman(5), Yukie Takahashi(6), Tadashi Nishitani(6), 
Makoto Uyeshima(7), Yoshimori Honkura(8), and Masaki 
Matsushima(8)

(1) Geological Survey of Japan, Japan
(2) Tohoku University, Aoba, Sendai, Japan
(3) Aichi Univ. Educ., Kariya, Aichi, Japan
(4) Hokkaido Univ., Sapporo, Japan
(5) DRRI, Kyoto Univ., Uji, Kyoto, Japan
(6) Akita Univ., Akita, Japan
(7) ERI, Univ. of Tokyo, Tokyo,  Japan
(8) Tokyo Institute of Technology, Tokyo, Japan
oga@gsj.go.jp

Deep resistivity structure of active faults around the backbone 
ranges of NE Japan was investigated by wide-band 
magnetotellurics. NE Japan is a typical island arc and the study 
area is currently under compression tectonics. The thrust fault 
systems on both sides of the backbone ranges are believed to help 
pop-up the mountain block.  We had 40 MT sites in total which 
were aligned on a 90 km E-W profile running across the mountain 
range. We had up to 11 simultaneous measurements using GPS 
synchronization. Two-dimensionality with regional strike was 
supported by tensor decompositions in the period range 1-1000 s. 
After removing telluric distortions, two-dimensional inversion with 
static shift was carried out. A model using TM mode showed the 
following features. Dipping conductive zones corresponding to the 
extension of the known thrust faults. Resistive zones which have 
high seismicity, and conductive anomalies at the root of the active 
faults. 




8.58p  SENSITIVITY OF MT FOR DETECTING 3D ANOMALY INDUCED BY 
       SALT-DOME UNDER THE OIL FIELDS

Tateyuki Negi and Hitoshi Okamura

Japan National Oil Corporation, 2-2, Hamada 1Chome Mihama-ku Chiba-
shi, Chiba 261-0025, Japan
negi-t@jnoc.co.jp

Sensitivity study of MT was conducted in order to detect 3D 
resistivity anomaly induced by salt-dome under oil fields. Salt-dome 
is assumed as super high resistivity anomalous body located in low 
resistivity carbonate sediment layers. 3D MT forward modelings 
were carried out for several resistivity background layer conditions 
and shapes and depths of salt-dome.  The background conditions 
were homogeneous and 2 layer structures. The models of salt-
dome, rising up from the basin bottom, was conditioned into 3 
cases of width, 3 cases of depth and 3 cases of resistivity models.  
As a result, in the case of homogeneous background, the salt-
dome is detectable when the depth of salt-dome is shallower than 
the twice of its width. In the case of two layer conditions, the 
detectability of salt-dome anomaly decreases when the resistivity of 
shallower layer is relatively lower than that of deeper layer. But, it 
increases when the resistivity of shallower layer is relatively higher 
than that of deeper layer. Furthermore, the detectability does not so 
depend on the resistivity contrast between the layers and the salt-
dome, but between each of the layers.




8.59p  AN ELECTRICAL RESISTIVITY STRUCTURE OBTAINED BY STATIONARY 
       WIDE-BAND MT SYSTEM

Kiyoshi Fuji-ta(1) and Satoshi Fujiwara(2)

(1) Department of Earth and Planetary Sciences, Faculty of Science, 
    Kobe University, Japan
(2) Mizusawa Geodetic Observatories, Geographical Survey Institute, 
    Japan
fuji-ta@kobe-u.ac.jp

A stationary magnetotelluric system is settled at the Mizusawa 
Geodetic Observatories of Geographical Survey Institute in NE-
Japan.  Since 1996, continuous of 24 hours resistivity and phase 
data have been acquired by wide band MT measurement devices.  
In order to get accurate data, remote reference station, Esashi 
observatories which is located about 20 km away from the 
Mizusawa is utilized.  Both of data are processed by the remote 
reference technique to minimize the effect of local noises.  We also 
stacked a large number of cross power spectrum and reprocessed.  
Although data quality at Mizusawa is sometimes poor, the above 
mentioned treatments improved MT data.  As the result, extremely 
stable apparent resistivity and phase curves were obtained.  Not 
only we evaluated the stability of apparent resistivity and phase 
data, but also the resistivity structure of NE-Japan was estimated 
using various kinds of modelling procedures.  Modelling suggests 
anomalous layers which may represent an anisotropic or regional 
discontinuous structure around NE-Japan.    




8.60p  INVERSION OF THE COPROD-2S SYNTHETIC DATA SETS

Ivan Varentsov and Natalia Baglaenko

Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, 
Russia
igemi1@pop.transit.ru

We present geoelectric models resolved for two COPROD-2S data 
sets. The first data set consists of noise-free apparent resistivity 
and phase responses for both modes and magnetic tipper 
estimates in the model with a set of descending conductive bricks. 
The second one is simulated for the flexure-type structure with the 
same data components, but now with different noise effects, 
including strong outliers and static shifts.  The robust inversion 
scheme for piece-wise continuous media (Golubev, Varentsov, 
1998; Varentsov, 1999) is applied in 1D and 2D implementations. 
The inversion solution includes the following steps: (i) the 
determination of the normal (background) section, containing left 
and right 1D models with fixed number of layers and thicknesses; 
(ii) the location of windows with anomalous resistivity structure; (iii) 
the joint optimization of the anomalous structure parameters and 
the normal section resistivities.  Primarily, we apply 1D inversion in 
the multi-site mode for the apparent resistivity and phase data at 
the profile edges. The same approach gives the very first idea of 
the anomalous structure. The main tool to select proper windows 
with anomalous conductivity is the 2D inversion with vertical 1D 
finite function (FF) parametrization. At the last stage, at first, we 
continue to use 2D inversion with FF parametrization, but with 2D 
functions. This is the right moment for the static shift correction, 
based on the formal selection of sites with the systematic apparent 
resistivity misfit in the whole period range.  Then, the most detailed 
parametrization scheme with pseudo-correlated resistivity cells is 
applied to resolve details. Geophysicists dealing with the MT 
inversion problems in 2D formulation are invited to verify their tools 
using COPROD-2S data sets. The project materials are available 
at the MTNET and at http:\\user.transit.ru\~igemi\c_2s_p0.htm




8.61p  RESULTS OF THE COMDAT COMPARATIVE STUDY OF MT PROCESSING 
       TECHNIQUES

Elena Sokolova and Ivan Varentsov

Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, 
Russia
igemi3@pop.transit.ru

We give the status information on the developments in the 
COMDAT project, organized to compare MT processing tools on 
the basis of specially designed synthetic time series with known 
impedance structure (Sokolova, Varentsov, 1998). The examples 
of successful processing of two COMDAT data sets (one noise-free 
and another with sufficient noise level) are presented. Our own 
results for these tests are discussed in details. Geophysicists 
dealing with the MT processing  problems  are  invited  to  verify  
their  tools 
using COMDAT data sets. The project materials are available at 
http:\\user.transit.ru\~igemi\cmdt_p0.htm.