Case Studies of E and EM Surveys in Natural Resources Chairpersons: T. Grant Caldwell and B. Tezkan 6.1 GEOELECTROMAGNETIC CHARACTERISATION OF NATURAL RESOURCE TARGETS: GENETIC MODELS, CORRESPONDENCE PRINCIPLES AND CASE STUDIES (INVITED REVIEW PAPER) Maxwell A. Meju Department of Geology, University of Leicester, Leicester LE1 7RH, England mxw@le.ac.uk 6.2 OKAK BAY: A LESSON IN DIMENSIONALITY FOR US ALL Alan G. Jones and Xavier Garcia Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 0E9, Canada ajones@cg.nrcan.gc.ca A large aeromagnetic anomaly at Okak Bay, north of Voisey's Bay, Labrador, prompted exploration activity to determine if there existed another nickel body like Voisey's Bay. Exploration activities included undertaking audio-magnetotelluric, AMT, data acquisition at almost 50 sites with one third centred on the magnetic high. The AMT dataset was interpreted in terms of a small shallow conductor at 600 m and a major deeper conductor at 1-3 km. Drilling intersected a body of sub-economic grade ore at 454-470 m, consistent with 1D inversion of the distortion-corrected data, but failed to intersect any ore down to base of drilling at 1.5 km. Examination of the responses showed a non-inductive effect at frequencies less than 10 Hz. We undertook 2D inversion of the data from all profiles in the restricted frequency range of 1,000 - 10 Hz. A pseudo-3D model constructed from the 2D models infers the geometry of the body sufficient information for exploration purposes. The response below 10 Hz we modelled in 3D, using the extended Born approximation of Habashy et al. We found that the Atlantic Ocean, some 25-50 km distant, interacts with the shallow body to produce a galvanic response that distorts the apparent resistivity curves at the sites close to the body, but not the phase curves. Including these distorted rho_a curves in an inversion leads to a mis-interpretation of the deep conductivity below the shallow body. 6.3 MAGNETOTELLURIC INVESTIGATION AT THE BAJAWA GEOTHERMAL FIELD IN FLORES ISLAND, EASTERN INDONESIA Toshihiro Uchida(1), Mitsuru Honda(2), Ashari(3), and Achmad Andan(3) (1) Geological Survey of Japan, Japan (2) West Japan Engineering Consultants, Inc., Japan (3) Volcanological Survey of Indonesia, Indonesia uch@gsj.go.jp A magnetotelluric survey with dense station coverage was conducted at the Bajawa geothermal field in Flores Island, eastern Indonesia, in September 1999, under an Indonesia-Japan joint research project for exploration of small-scale geothermal resources. MT results were used, in conjunction with geological and geochemical data, for construction of a preliminary geothermal model of the area and selection of a potential zone for the first pilot drilling. Forty-one MT stations were arranged on a grid array to cover the Mataloko surface manifestation zone with a final goal of 3-D interpretation. Station interval is approximately 500 meters. Additional 12 stations were arranged along a line crossing another manifestation zone. Data quality was generally very good because of low-level cultural noises. However, even so, remote reference data was necessary to remove local noises caused by winds and human activities in nearby villages. Preliminary 2-D inversion results indicate the following. The 3-D survey area is entirely underlain by a low resistivity layer in the shallow zone and a large high-resistivity layer in deep. Resistivity of the low-resistivity layer is the lowest, as low as 1 ohm-m, and its thickness is the smallest, approximately 500 meters, near the manifestation zone. The low- resistivity layer is interpreted as a clay-rich cap layer of the geothermal reservoir, while the high-resistivity layer below as a high-temperature water circulation zone. Later geological data from a 100m-deep drillhole suggested existence of montmorillonite in the low-resistivity zone. 6.4 LONG-OFFSET TRANSIENT ELECTROMAGNETIC MEASUREMENTS AT THE WAIRAKEI GEOTHERMAL FIELD T. Grant Caldwell(1), S.L. Bennie(2), D.E. Keen(2), and D.J. Graham(2) (1) Institute of Geological and Nuclear Sciences, Wellington, New Zealand Presently at:National University of Ireland, Galway (2) Institute of Geological and Nuclear Sciences, Wairakei, New Zealand g.caldwell@gns.cri.nz Transient electric field measurements were made in a line crossing the northwestern boundary of the Wairakei geothermal field. Three grounded bipoles were used as current sources enabling the apparent resistivity tensors to be determined at each measurement site. Source-receiver separations ranged between 7 and 10 km. Despite high levels of electrical noise instantaneous apparent resistivity tensors could be determined reliably between 0.1 and 4 s. The apparent resistivity image derived from these data shows that, beneath the southern part of the Wairakei geothermal field, higher resistivities underlie a layer of much lower resistivity about 1 km thick. 6.5 REGIONAL-SCALE MAGNETOTELLURIC SURVEY OF THE YELLOWKNIFE FAULT Xavier Garcia and Alan G. Jones Geological Survey of Canada, 615 Booth St., Ottawa, Ontario, K1A 0E9, Canada garcia@cg.NRCan.gc.ca During the 1996 Lithoprobe magnetotelluric, MT, survey measurements were made at sites from west of Yellowknife - Northwest Territories, Canada- across the Yellowknife fault to the east. The 2D interpretation of this profile showed the most anomalous response at the site located north of Yellowknife virtually on the Yellowknife fault. We undertook two other profiles across the fault as part of an initiative to extend knowledge of the gold mines in the area. The first MT study carried out during winter 2000 was located 10 km north of the previous profile, on a profile extending 20 km with variable station spacing from the Yellowknife fault. The main objective of this study is the study of the Yellowknife fault as a regional feature and the possible relation of the enhanced conductivity with the gold deposits. The second MT study realized in the area consisted of two short AMT profiles located south of the city of Yellowknife, close to the Kamex islands. The first profile consists on 8 stations and is located over a shear zone that contains a gold deposit. The second one consists on 5 stations and is located south of the first profile over the same shear zone where no mineral has been found. Different geophysical studies realized in the area failed to locate the mineral deposit. The main objective of this study is whether the MT method is valid to locate gold deposits in the Yellowknife area and an understanding of this shear zone and its relation with the Yellowknife fault. In this work we present the 2D modeling of these three profiles, and come to conclusions regarding the 3D nature of the Yellowknife fault, which may have controlled ore deposition. 6.6p STUDY OF THE GEOTHERMAL FIELD AT HAMAM FARAUN HOT SPRING USING 3D RESISTIVITY MODELING G. El-Qady(1), K. Spitzer(2), and K. Ushijima(1) (1) Earth Resources and System Eng. Dept, Kyushu Univ., Hakozaki, 6-10-1, Fukuoka 812-8581, Japan (2) Joint Geoscientific Research Institute, Stilleweg 2, 30655, Hannover, Germany Gad@mine.kyushu-u.ac.jp The present work deals with the 3D forward modeling of the resistivity data set carried out at Hamam Faraun area where the hottest hot spring (72 C) in Egypt is located. Seventeen Schlumberger vertical electrical soundings were measured with maximum AB/2 = 1000 m, in a successive steps, as a part of geophysical study for the geothermal reservoir at the study area. The main objective of the 3D modeling was to incorporate the partial knowledge obtained from the previous 1D and 2D interpretation into a geoelectric model. A 3D finite difference algorithm, that utilizes the singularity removal, had been applied to acquire information about the subsurface structure and to explain the observed field data. The 3D model was derived in a trial and error way to detect the conductive zones associated with the hydrothermal circulation. The latest form of the model depicts the form of the fault structure and spatial low resistivity zones that might be interpreted as a part of the geothermal reservoir. However, the 3D model exhibits several characteristics different from those resulted from the previous 1D and 2D interpretations. Using the resultant 3D model, we could give explanation for the hypothesis of the hot water origin and its circulation system in the area. 6.7p A NEW MAGNETOTELLURIC EXPERIMENT IN SOUTHERN TUSCANY: AN ELECTROSTRATIGRAPHIC RECONSTRUCTION OF A WELL KNOWN GEOTHERMAL AREA Adele Manzella(1), Gianni Volpi(2), Nicola Praticelli(3), and Annalisa Zaja(3) (1) International Institute for Geothermal Research, National Research Council, Italy (2) IIRG-CNR, Via Alfieri 1, 56010 Ghezzano- Pisa, Italy (3) Dept. Geology, Paleontology and Geophysics, University of Padua, Italy manzella@iirg.pi.cnr.it javol@tin.it The application of MagnetoTelluric method in the geothermal fields of southern Tuscany in Italy have been a challenge in the last decades. The complex geology of this tectonically active area, the strong electromagnetic noise, the absence of a strong resistivity contrast of alteration minerals, check the limits of applicability of MT in defining the geothermal features. To define the potentiality of this method in this particular context a MT survey was carried out in an area where geothermal features were known quite well by direct investigation with wells and a number of geophysical data such as seismic and gravimetry data, Schlumberger soundings. Electric and magnetic fields were recorded at 10 main sites spaced 1 km along a SW-NE profile. The electric field only was recorded at 40 sites, called telluric sites, spaced 200 m along continuous dipoles located between the main sites. Synchronous recording of two adjacent main sites, the intermediate telluric sites, and a remote site located on an electrically quiet island allowed an accurate data processing with robust remote reference tecniques. Examples will be given on the quality achieved with different processing algorithms. Processed data were 2D modelled and the results interpreted in view of the geothermal targets. A nearby MT profile previously recorded allowed a 3D modelling and the reconstruction of the main geoelectrical features. 6.8p THE GEOTHERMAL AQUIFERS AT THE TACO RALO ZONE, TUCUMAN PROVINCE, ARGENTINA Claudia Sainato(1), Cristina Pomposiello(2), Griselda Galindo(3) and Ricardo Picicelli(4) (1) Fac. Agronomia, University of Buenos Aires, Argentina (2) CIRGEO, CONICET, Argentina (3) FCEN, University of Buenos Aires-CONICET, Argentina (4) FCEN, University of Buenos Aires, Argentina csainato@ciudad.com.ar The study zone, a sedimentary basin between the Aconquija and the Guasayan range, is a geothermal area. However, the knowledge of the resource is limited to well data. Within the Tertiary sequence, it may be recognized the Guasayan Formation with red and green siltstones with interbedded gypsum, whose aquifers are extremely saline and, overlaying it, there is a sequence of red to brownish-red clay siltstones interbedded with quarziferous sandstones, Las Caņas Formation, with thermal aquifers with better quality water. The Quaternary sediments exceeds 400 m of thickness in some sectors with good aquifers. Previous Audio Magnetotelluric soundings showed the geothermal aquifer below 300 m depth at some places and at approximately 100 m at other ones. Ten AMT soundings were performed in this work, in the range 10-1000 Hz, to have a better description of the geothermal resource. The rho plus algorithm of Parker and Booker was used to determine the offset in gain at certain bands at some sites in order to predict the apparent resistivities from values outside these bands. Fixed resistivities were chosen comparing with previous AMT data. The two modes curves almost coincide at some sites and some evidence of 2D behaviour with a NS strike is shown at others. First, a 1 D modeling was attempted to have the initial step for a 2D inversion. Sands from Las Caņas Fomation evidence variable resistivity showing good quality water at depths between less than 100 m at some sites up to more than 400 m at others. 6.9p A MAGNETOTELLURIC STUDY IN THE TERMAS DE IBERA DISTRICT, NORTHWEST OF SAO PAULO STATE, BRAZIL Cristina Pomposiello(1), Alicia Favetto(2), Claudia Sainato(3), and Carlos Mendonca(4) (1) CIRGEO, Ramirez de Velasco 847, 1414, Buenos Aires, Argentina (2) Depto de Fisica, Univ. Buenos Aires, Ciudad Universitaria, Pabellon I, 1428 Buenos Aires, Argentina (3) Catedra de Fisica, Fac. Agronomia, Univ. Buenos Aires, Av. San Martin 4453, 1417, Buenos Aires, Argentina (4) Depto. de Geofisica, IAG/USP, Brazil cpomposi@mail.retina.ar The Bauru Basin is a depression developed in the south central part of the South American Platform in the Late Cretaceous, following the rupture of the Gondwane continent and opening the Atlantic Ocean. The Upper Cretaceous (inland basin) are formed essentially by sandy sedimentary siliciclastic sequence, 300 m thick. This sequence lies upon volcanic rocks of the Serra Geral Formation (early Cretaceous), formed mainly by basalt. Hydrochemical regional anomalies seem to indicate the existence of deep fractures through the basalts of the Serra Geral formation, which allow hidrothermal fluids flow from deeper zones (Botucatu Formation). At the Termas de Ibera District, groundwater flows by two basic aquifer systems, the most shallow through the Adamantina Formation (Bauru Group) and the most deep through the Serra Geral Formation. A Magnetotelluric study was carried out in the Termas de Ibera District (northwest of Sao Paulo State) in order to image the upper part of the crust. Data were collected at seven sites along a profile of 30 km, which crosses normally a fracture. Two-dimensional (2D) inversion of the data with electric field polarized perpendicular to strike (the TM mode) reveals a shallow conductive layer approximately 200 m deep, which correlates with the Adamantina Formation. This ones overlies a much more resistive layer with a thickness of 400 m, which correlates with Serra Geral Formation. A shallow and narrow resistive anomaly is present which is correlated with the fracture. 6.10p USING MT AND SEISMIC DATA TO MODEL THE SEDIMENTARY BASIN OF THE TUCUMAN PLAIN, ARGENTINA Alicia Favetto(1), M. Cristina Pomposiello(2), and Eduardo Rossello(3) (1) Dpto. de Fisica, Univ. Buenos Aires, Ciudad Universitaria, Pabellon I,1428 Buenos Aires, Argentina (2) CIRGEO, Ramirez de Velasco 847, 1414, Buenos Aires, Argentina (3) Dpto. de Geologia, Univ. Buenos Aires, Ciudad Universitaria, Pabellon II,1428 Buenos Aires, Argentina favetto@df.uba.ar In 1998, we collected 8 wideband MT sites along 27§ 39 S at the geothermal district at the northern end of the Pampean Ranges, Argentina. The traverse extends 80 km across a widespread, economically important, low-temperature(< 60C) geothermal reservoir. A previous analysis of MT data was performed in order to describe the sedimentary basin and to define the permeable horizons hosting aquifers. Due to the presence of a very conductive shallow structure, the electromagnetic fields are very distorted. In this paper we present a new interpretation using seismic reflection data coincident to the profile. The 2-D model was obtained using as constrain the geometry of the interface between Precambrian basement and sedimentary rocks. The model shows at this interface a sharp resistivity contrast. 6.11p ELECTROMAGNETIC INVESTIGATION OF THE SETE CIDADES VOLCANO (S. MIGUEL ISLAND-AZORES) Jandyr Travassos(1), Fernando M. Santos(2), Paulo T.L. Menezes(3), and Eugenio Almeida(2,4) (1) Observatorio Nacional, Brasil (2) FCUL/C.G.U.L., Portugal (3) DGAP/FGEL/UERJ, Brasil (4) IPT, Portugal jandyr@on.br This work presents the preliminary results from an electromagnetic survey of the Sete Cidades Volcano, located at the western end of Sao Miguel Island, in the Azores. The Sete Cidades is an active volcano, with a circular caldera displaying pumice cones, maars and domes. Sao Miguel is a young ((4 My) volcanic island with three volcanoes running along its axis, the largest of the Azores Islands. The Terceira Rift cuts the caldera along a NW-SE trend, and controls the location of several scoria cones. The Mosterior graben is located on the NW flank of the volcano. That is a sub- aereal segment of the Rift. Another main alignment displays a NE- SW trend. MT data were collected at 7 sites distributed inside and outside the caldera spaced a couple of kilometers. Fieldwork was carried out using three types of equipment and field strategies. Long period data ((10 s) was collected at 1 site. Medium period data ((1 s) at two sites was collected using a single-site strategy. Broadband remote-referenced data (0.01-1000 s) was collected at 4 sites. Controlled-source high-frequency (10-104 Hz) was collected employing an EMAP field strategy. High-frequency data was collected at each broadband MT location. Those three data ranges will be used in a complementary fashion in the modelling stage. Only the results of the high-frequency data will be used to illustrate the shallow structure of the volcano. 6.12p 3D MODELLING AND COMPARISON OF TEM AND MT FOR GEOTHERMAL EXPLORATION Anja Kreutzmann(1) and Knutur Arnason(2) (1) Institute for Meteorology and Geophysics, University of Frankfurt, Feldbergstrasse 47, D-60323 Frankfurt, Germany (2) Orkustofnun, Grensasvegur 9, Reykjavik, Iceland akreutz@geophysik.uni-frankfurt.de Resistivity exploration at shallow (< 1 km) depths in geothermal fields in Iceland has provided strong evidence that high- temperature geothermal systems in basaltic rocks have a similar resistivity structure. Beneath cold and unaltered rocks with high resistivity a low resistivity cap (1-10 Ohmm) is found on the outer margins of the reservoir, overlying a more resistive inner core. In the volcanic zones of Iceland central volcanos with high- temperature geothermal systems are found at approximately 20 km intervals. A common set of 3D models for TEM and MT modelling was developed to compare different EM techniques on detecting a possible geothermal connection between existing geothermal systems. A finite difference code (TEMDDD) was used for the TEM model calculations. Calculations were made for 25 different source locations and three types of sources (perpendicular dipoles and a loop). Five transient signals (Ex, Ey, dBx/dt, dBy/dt, and dBz/dt) were calculated at 81 receiver sites on a regular grid between the geothermal systems. The difference in the response of connected and non-connected models varies for different source- receiver combinations. For the MT model calculations Mackie's code was used and apparent resistivities, phases, and induction arrows for the different models were compared. These transfer functions show differences for connected and non-connected systems subject to the lateral and vertical size of the geothermal reservoir. The significance of the differences between the models is evaluated in order to estimate the detectability and depth of exploration. 6.13p TEM-MT STUDY OF THE DEEP STRUCTURE OF THE NORTHERN MARGIN OF PARNAIBA BASIN: TECTONIC AND HYDROGEOLOGICAL IMPLICATIONS Adel K. Mohamed(1), Maxwell A. Meju(1), and Sergio L. Fontes(2) (1) Department of Geology, University of Leicester, Leicester LE1 7RH, UK (2) Observatorio Nacional-MCT, Rio de Janeiro, Brazil Akm6@le.ac.uk The combined TEM-MT methods have been applied to the determination of the resistivity structure of the northern margin of Parnaiba basin. The exact position of the basin edge is poorly defined on existing geological maps. A discontinuous E-W fault found to the west of the study area may be a possible basin- bounding fault. TEM-MT data were collected in early 1999 along a 95 km long N-S line extending from the coast across the projected subcrop position of the fault. The MT data were processed to yield the TE and TM mode responses and then corrected for static shift using central-loop and single-loop TEM data respectively. The result of 2-D regularised inversion of MT data clearly shows the presence of major basement faulting at the expected position of the concealed E-W fault (75 km from the coast). We interpret it as marking the main basin margin and suggest that it may exert controls on groundwater flow in the area. The results of TEM inversion detect a conductive horizon (120 m maximum depth) extending 23 km from the coast. This might be a clayey formation or intruded seawater. 3-D modelling will be undertaken in future to improve our model of the resistivity structure of the region. 6.14p INFLUENCE OF SHALLOW AQUIFERS ON THE RESOLUTION OF SUB-TRAPPEAN SEDIMENTS Saurabh K. Verma, Hemant K., and Ajay Manglik National Geophysical Research Institute, Hyderabad 500 007, India skvngri@hotmail.com The Deccan Traps flood basalts occupy a vast region in peninsular India and cover some of the promising sedimentary horizons with potential for hydrocarbons along the western margin of the country. Due to high velocity in Traps, seismic delineation of underlying low velocity sediments becomes a difficult proposition. As an alternative, MT and DC soundings have been carried out extensively in recent years in the Saurashtra region to map the sub-Trappean sediments exploiting the contrasts in the electrical properties of Traps, sediments and the underlying basement. Basaltic cover generally comprises a number of flows - some of which may contain mesh work of holes and tubes formed by escaping gas bubbles in the top regions. Below massive basalts, a thin layer of this fractured material is often present providing suitable locale for the formation of an aquifer. The present study reveals that an aquifer in the Traps, even if located at a shallow depth, can cause significant changes in the MT and DC soundings curves. The shallow part of the DC sounding curve is affected more severely in comparison to the MT curve. Thus the parameter estimates of the sedimentary layer are considerably modified. Individual and joint inversions of the MT and DC data sets are studied for synthetic noisy data considering a 3 layer standard model (basalts, sediments and basement) and a 5 layer model (basalt layer with additional shallow aquifer). For the standard model the DC data resolves the basalts well and yields equivalent solution for the sediments. MT data provide better estimates of the sediments and the joint-inversion improves the results further. For 5 layer situation the DC data provides good estimates of the shallower information while the MT data still provides better resolution for sediments. Finally when the shallow information from the DC data was jointly inverted with the MT data good estimates were obtained for the basalts while consistent solutions were obtained for the sedimentary layer. 6.15p ELECTROMAGNETIC IMAGES OF MAJOR STRUCTURES CONTROLLING GROUNDWATER DISTRIBUTION IN THE SOUTHEAST MARGIN OF PARNAIBA BASIN, BRAZIL Emin U. Ulugergerli(1), Sergio L. Fontes(2), Emanuele F. La Terra(2), Maxwell A. Meju(3), and C.R. Germano(2) (1) Ankara University, Turkey, Presently Visiting Fellow at Observatorio Nacional, Brazil (2) Observatorio Nacional, Brazil (3) University of Leceister, UK sergio@on.br Transient electromagnetic and magnetotelluric methods have been jointly applied to the determination of possible structural controls on the pattern of occurrence of groundwater near the contact between basement and sedimentary cover rocks in Sao Raimundo Nonato area in southeastern Parnaiba basin. The main problem is how to identify and delineate graben-like zones with thick clastic sequences where significant quantities of good quality water can be found and not too distant from most populated towns in the region. Six SE-NW trending TEM-MT profiles were recorded across the aeromagnetically-inferred trend of a possible graben-like feature. The MT data were corrected for static shifts using TEM constraints and then inverted to reveal the subsurface resistivity structure underneath each survey line. A notable zone of discontinuity is present in all the 2-D models in the region where there are prolific groundwater wells and is interpreted as a graben with increased thickness of sediments. Two confirmatory boreholes drilled within the inferred geoelectrical graben-like structures in the Serra da Capivara National Park (953 m deep) and Mocambo (750 m deep) are productive and did not penetrate crystalline basement. It is concluded that the observed pattern of groundwater distribution is controlled by the Sao Raimundo Nonato graben. Based on the MT models, we suggest that the productive zone is of regional extent and can be developed to minimize the water shortage problem experienced in this region. 6.16p GROUNDWATER PROSPECTING USING TRANSIENT ELECTROMAGNETIC GEOPHYSICAL METHOD IN CAMPOS DOS GOYTACAZES CITY, NORTH OF RIO DE JANEIRO STATE, BRAZIL A. Carrasquilla(1), E. Ulugergerli(2,3), C.A. Gonįalves(1), and C.R. Germano(2) (1) Laboratorio de Engenharia e Exploracao de Petroleo, Universidade Estadual do Norte Fluminense (LENEP/UENF), Macae, RJ, Brazil (2) Observatorio Nacional(ON/CNPq), Sao Cristovao, Rio de Janeiro, RJ, Brazil (3) University of Ankara, Ankara, Turkey abel@lenep.uenf.br LENEP/UENF and ON/CNPq developed a field work utilizing transient electromagnetic (TEM) geophysical method with the objective to prospect groundwater in the periphery of Campos dos Goytacazes City, North of Rio de Janeiro State, Brazil. We performed geophysical surveys in three areas near the fluvial plain of Paraiba do Sul River, which aproximately crosses this city in the west-east direction. The prospected areas are located in the edge of continental portion of Campos Basin, which is formed by tertiary and quaternary sediments (Barreiras and Sao Tome Formations, with red alluvial continental and shaly-sand sediments, respectively) over a gnaissic basement. TEM method was used through 231 central-loop array soundings with 20 or 50 meter grids. The main goal was to map the conductive or resistive underground zones, which are related with absence or presence of groundwater. Thus, in all the propected areas, the interpretation of the geophysical data showed that this method is able to detect very conductive sedimentary packages (shaly-sand geological formations), with an average thickness of 50 meter and 10 ohm.m of resistivities, and resistives zones (wheathering or fractured cristalline rocks) in depths more than 60 meters and 100 ohm.m of resistivities. Because the sediments (shaly-sand formations) have iron rich clays which pollute groundwater, fractures (which cross the sediments and achieve the basement), are considered the main aquifers of these areas. 6.17p GEOELECTRICAL STUDIES IN THE BRAZILIAN LAKE DISTRICT REGION - RIO DE JANEIRO STATE, BRAZIL: STRUCTURAL FRAMEWORK AND GROUNDWATER Alan F. Machado, Sergio L. Fontes, Jefferson V. de Miranda, and Jean-M. Flexor Observatorio Nacional, Rio de Janeiro, Brazil sergio@on.br As part of a multi-disciplinary geophysical study for groundwater, a preliminary geoelectrical survey was carried out in the Brazilian Lake District in Rio de Janeiro State, along with a semi-detailed gravity survey and a reinterpretation of existing aeromagnetic data. The region has touristic potential and experiences shortage of water supply during summertime, when the population increases drastically. Pre-Cambrian basement units dominate the geology of the study area with quaternary sediments cover. The regional faults in the area trend in the NE-SW direction and are associated mainly with the opening of the Atlantic ocean. As a preliminary attempt to map the geoelectric structure, fourteen MT and TEM stations were made along a profile transversal to the regional fault system. TEM data were used for static shift correction and a robust processing code for MT data was applied to account for the noise present in this resistive environment. Geoelectrical section from 2-D inversion of MT depicts main faults and show good correlation with known geology as well as with ground and aerogeophysical data. 6.18p A HYDROGEOLOGICAL STUDY USING MT, EM AND SP F. Monteiro Santos(1), Rita P. Castro(1), Liliana Matos(1)(2), Eugenio Almeida(1)(2), Rita Nolasco(1), Luis Trindade(1), and Luis Mendes Victor(1) (1) Centro de Geofisica da Universidade de Lisboa, Portugal (2) Instituto Politecnico de Tomar, Portugal dfams@fc.ul.pt Mineral waters emerging in the Vilarelho da Raia basin have been known since roman times and are one of the external manifestations of the mineral water systems occurring on the Portuguese mainland. The geomorphology of the area is dominated by the Vilarelho da Raia basin that has a NNE-SSW trend and is bounded by granitic formations (Hercynian). The main faults that control the structure belong to the Penacova-Chaves- Verin strike-slip fault system originated during Hercynian times and reactivated in the Quaternary. The geophysical study included electromagnetic (EM), self-potential (SP) and magnetotelluric (MT) surveys. The EM and SP surveys allow to define the uppermost structure of the area. The results show a NE more conductive overburden. The main conductivity gradients were associated with the shallow faults affecting the sedimentary filling of the basin. The MT survey allows to define the deep structure in the area. The sedimentary formations were represented by conductive structures. High resistivity bodies representing the granitic formations were revealed at middle crust. The geophysical surveys detected features related to the preferential deep and superficial water-flow paths. 6.19p INTERPRETATION OF SPECTRAL ELECTROMAGNETIC DATA FOR COPPER AND GOLD EXPLORATION IN NORTHEASTERN BAHIA, BRAZIL Edson E. S. Sampaio and Hedison K. Sato CPGG/UFBA, Salvador, Brazil edson@cpgg.ufba.br Multi-separation and multi-frequency electromagnetic surveys employing large transmitting loops provide a combination of both parametric and geometric induction sounding and profiling. Furthermore they may prospect deep ore targets and indicate the presence of induced electrical polarization. We interpret FDEM data from two areas located in the northeastern part of Bahia State, Brazil by employing pseudo-sections of apparent conductivity and apparent polarizability. Each pseudo-section results from the inversion of PER and HCP configurations of FDEM data for 12 values of frequency versus 14 values of distance. The inversion consists of scanning the matrix abacus for a polarizable and homogeneous half-space for those two configurations. The two abacus consist of the apparent induction number versus the apparent polarizability parameter for the synthetic data of both horizontal coplanar-HCP and perpendicular-PER configurations. The interpretation delineates regions of the subsurface containning both conductive and polarizable bodies in two regions of economic interest, respectively for copper and gold exploration. They are important either in the definition of a drilling program or as basis for more sophisticated inverse modeling. 6.20p APPLICATION OF INDUCTION WELL LOGGING WHEN SEARCH OF KIMBERLITE PIPES OF SIBERIA Alexander Saraev(1), Mikhail Garat(2), and Vladimir Phomin(2) (1) St. Petersburg State University, Universitetskaya nab., 7/9, St. Petersburg, 199034, Russia (2) ALROSA Co Ltd, Chernyshevskoe shosse, 7, Mirny, Yakutia, 678170, Russia elmag@elmag.ecri.pu.ru The searches of kimberlite pipes of Siberia (Yakutia diamond bearing province) are carried out with wide use of drilling. The drilling in conditions of spreading of frozen rocks are usually carried out without fluids and an electrical logging method cannot be applied in the dry boreholes. In these conditions, a significant efficiency has the use of induction logging. The IKRS-1 instrument of induction logging with frequency of 50 kHz and probe diameter of 42 mm was created in St. Petersburg State University. The influence of a near zone is reduced using of a focused probe. Range of measurement of the instrument 1... 3000 mS/m. Very important problem, was solved using of induction logging when search of kimberlite pipes of Siberia, is the study of geological nature of electrical methods anomalies. The tuffs and dolerites differentiation, the changed traps identification, the carbonate kimberlite-hosting and sedimentary kibberlite overlaying rocks differentiation and correlation, the changed zones distinguishing in the top of kimberlite-hosting rocks, the parametrical conductivity measurements for creation of models of search objects are too carried out using of induction logging. 6.21p DETECTION OF LOW VELOCITY, HIGH CONDUCTIVITY SEDIMENTS BY A JOINT-INVERSION APPROACH Saurabh K. Verma, Ajay Manglik, and Hemant K. National Geophysical Research Institute, Hyderabad 500 007, India skvngri@hotmail.com High velocity flood basalts occupy a vast area exceeding 500,000 sq km in the western and central part of peninsular India. Geological considerations suggest that these basalts, also known as Deccan Traps, cover some of the most promising sedimentary horizons with potential for hydrocarbons. This scenario represents a typical problem of delineating a low velocity layer below a high velocity layer. As an alternative, MT and deep resistivity surveys have been conducted in certain regions to obtain independent estimates of basaltic and sedimentary thickness. In a qualitative sense, the electrical information has been also used to constrain the seismic interpretation. This paper presents results for a versatile joint-inversion scheme that considers seismic reflection and refraction, MT and DC data in any desired combination to resolve difficult structures. Efficacy of the approach is established by considering a thin sedimentary horizon below thick basalts. This model is either resolved poorly or equivalent solutions are obtained by the inversion of individual data sets. Even the joint- inversions of seismic reflection and refraction, DC and MT, or other data set pairs yield equivalent solutions. Joint-inversion of all the four data sets on the other hand yields a good resolution of the structure even when vital data from vital regions is sparse or missing.