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3rd International Congress of the Brazilian Geophysical Society
- Conference date: 07 Nov 1993 - 11 Nov 1993
- Location: Rio de Janeiro, Brazil
- Published: 07 November 1993
81 - 100 of 308 results
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Potencial Elétrico Devido ŕ Uma Fonte de Corrente no Interior de um Semi-Espaço Heterogęneo
More LessÉ notório o fato dos estratos geológicos sedimentares apresentarem variaçőes verticais nas suas propriedades físicas, entre elas, a condutividade elétrica. Assim, diversos trabalhos tęm estudado o problema elétrico associado a uma fonte de corrente contínua na superfície de um semi-espaço simples ou estratificado horizontalmente, mas que apresentam variaçőes verticais na condutividade elétrica, proporcional a funçőes exponenciais, ou, de potęncias reais de uma dependęncia linear da profundidade.
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Electrical Resistivity Imaging
Authors P. Jackson, M.A. Lovell and P. HarveyAn extension of resistivity measurements in both the laboratory and field situations is demonstrated. Using high density computer-controlled measurements images of the electrical resistivity distribution in a core, or over an area of ground are produced. At both scales the variability in electrical properties over relatively small distances is evident. In a sediment-core the electrical image shows high resolution detail which complements a conventional photo, thus enhancing the usual interpretation. The electrical resistivity imaging is then used to monitor the passage of a saline intrusion through a volume of rock. Firstly this is demonstrated for a large volume of ground, and then separately for a core sample, This technique can be used to zone areas of investigation, assigning different characteristics to each zone, or to monitor the movement of fluids with time. Its application to both core and site investigations includes hydrocarbon and geotechnical characterisation, as well as aquifer and pollution monitoring.
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Three-Dimensional Transient Electromagnetic Modeling of Complex Conductive Earth
By L. RijoThe aim of this work is to explore the tremendous computer power of the new generation of workstations to model efficiently 3- D transient electromagnetic responses of complex conductive earth by finite element method. The transient solution is obtained via inverse cosine (sine) transform of the frequency domain solutions through very short digital filter integration. This takes about 20 frequency-domain solutions for each time-domain response. To minimize CPU time and memory (RAM and Disk) for each frequency-domain finite element solution, we use the preconditioned bi-conjugate gradient algorithm to solve the system of linear equations. With this technique, only the nonzero entries of the linear system are stored in the memory, a great benefit for such huge problem. It takes about 10 min for each frequency-domain solution for a fairly complex model energized by a grounded wire carrying a step function current. The complete transient response takes approximately 200 min. Comparison with known integral equation results shows that our algorithm is working properly. Cost-effective transient electromagnetic modelling is vital for electrical geophysical prospecting of conductive terrains in many parts of the world, in particular in the Amazon, Brazil.
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Joint TEM/AMT Feasibility Studies in Parnaíba Basin, Brazil: Geoelectrostratigraphy and Groundwater Resource Evaluation in Piauí State
Authors M.A. Meju, S. L. Fontes and M. F. B. OliveiraThe utility of the joint transient electromagnetic (TEM) and audiofrequency magneto-telluric (AMT) methods in stratigraphic and hydrogeological mapping has been evaluated in the southeastern part of the Parnaiba basin. The field experiments employed central-induction and coincident-loop TEM and tensorial AMT depth sounding techniques at several observational stations along a 400 Km traverse. The processed data are in excellent agreement with borehole data and known geology of the area. The results show that all the main stratigraphic formations can be distinguished on the basis of their resistivity characteristics and also allow proposing a zone for the Transbrasiliano lineament. A preliminary groundwater resource evaluation study of the Piaui State transect showed that the prolific aquifers in the basin are easily detectable using the TEM/AMT methods due to the favourable juxtaposition of electrically-contrasting permeable and relatively impermeable geological formations forming confined aquifers of regional extent. It is contended that these regional groundwater reservoir resources may be developed by machine-drilled boreholes to meet the water needs of the region but further detailed geophysical evaluation will be required for a cost-effective borehole siting program. Suggestions are also offered to guide any future detailed exploration for the more localized and shallower reservoir occurrences and resource development possibilities in the basement areas in the southeastern margin of the basin.
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Sondeos Electromagneticos en San Jose, Provode Entre Rios, Argentina.
Authors M. Mananí, J. Venencia, A. Maidana, B. Castiglione and E. BorzottaSe realizaron 5 sondeos Magnetotelúricos en la localidad de San José, Prov. de Entre Ríos (58°12' Long. W; 32°12' Lat. S) con el objetivo de investigar el perfil estratigráfico hasta el basamento cristalino, para el aprovechamiento de aguas termales. Por las cercanías de los sitios de la realización del trabajo, se utilizó el método de agrupamientos de sondeos para definir una sola curva promedio donde se realizó el modelado 1D. Los resultados muestran dos capas conductoras intercaladas dentro del estrato sedimentario y otra en corteza. La primera de elIas coincide con SEV ya realizados y la segunda, el contacto con el basamento cristalino.
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Magnetotelluric Investigations in the Săo Francisco Basin, Minas Gerais, Brazil
Authors J.L. Porsani and S.L. FontesA geoelectric image of a portion of the Sao Fransisco sedimentary basin (Minas Gerais) was obtained based on magnetotelluric (MT) data from 43 MT soundings (400 Hz - 0.001 Hz). The soundings were carried out along 10 seismic lines. covering an area of 11.000 km˛ approximately. The distances between soundings were irregular and varied from 2.5 km to 20 km. Static shift corrections were applied to the data using the median resistivity of the first conductor. One dimensional (1D) Occam geoelectrical sections were obtained for all profiles. A sequence of conductors and resistors was found in the sedimentary package. followed by a geoelectrical basement. The MT sections show good agreement with both borehole information and seismic sections.
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Bacia do Săo Francisco: Estilo Estrutural e Perspectivas Exploratórias
More LessA Bacia do Săo Francisco tem uma área de aproximadamente 300.000 km2. Os primeiros trabalhos de sismica de reflexăo foram realizados pela Shell Exploration Services (contrato de risco) em 1981, que levantou 137 km na parte norte da bacia.
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Estudo Audiomagnetotelúrico na Jazida Santa Maria, Minas do Camaquă (RS)
Authors A.L. Padilha and Í. VitorelloPela sua potencialidade em discriminar camadas condutoras próximas ŕ superfície, o método audiomagnetotelúrico (AMT) vem sendo utilizado com sucesso na prospecçăo de depósitos maciços (Strangway e Koziar, 1979; Lakanen, 1986). A possibilidade de seu emgrego em estudos de sulfetos disseminados parece, porém, limitada a casos específicos nos quais a concentraçăo desses sulfetos se correlacione a algum outro parâmetro geológico que possa ser mapeado por levantamentos AMT.
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Levantamento MT na Área de Fartura, SP
Authors P. De Tarso Menezes and J. De Menezes TravassosO método magnetotelúrico (MT) descrito primeiramente pelo soviético Tikhonov (1950) e pelo francęs Cagniard (1953), consiste na determinaçăo da condutividade elétrica de subsuperfície através da medida das componentes horizontais dos campos elétrico e magnético naturais, sobre a superfície da Terra. Vários autores (Berdichevski et al., 1980; Ranganayaki, 1984; Park e Livelibrooks, 1989) tem utilizado inversőes 1D para analisar situaçőes geológicas 2D, ou mesmo 3D. Com o objetivo de obter a estrutura geoelétrica na regiăo de Fartura foi utilizado o algorítmo de inversăo 1D Occam (Constable et al., 19987) para o tensor MT. O efeito estático nos dados MT provocados por mudanças bruscas de resistividades de camadas próximas ŕ superfície (Larsen, 197; Jones, 1988) foi considerado no processo de interpretaçăo.
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Fonte Sísmica para Levantamentos VSP Marítimos
By M.A. GallotiAtualmente no Brasil a fonte sísmica para levantamentos VSP ( vertical seismic profiles) é o airgun. O pulso de um airgun possui duas deficięncias : baixa potęncia e o efeito causado pela oscilaçăo da bolha que gera pulsos secundários apos a emissăo do pulso primário (Ziolkowski, 1970; Schulze-Gatterrnann, 1972; Giles & Johnston, 1973 e outros).
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Cross-Well Electromagnetic Tomography
Authors C. Torres-Verdin and Tarek M. HabashyWe present an efficient approach to cross-well electromagnetic tomography based on a new nonlinear scattering approximation. The approximation uses a source-independent scattering tensor whose projection onto the background electric field (i.e., the electric field excited in the absence of conductivity variations) is an approximaton to the electric field inside the region of anomalous conductivity. The scattering tensor adjusts the background electne field by way of amplitude, phase and cross-polarization corrections that result from frequency-dependent mutual coupling among scatterers. Numerical simulations and comparisons with a 2.5-D finite:differene.e code show that the new approximation accurately describes scattered fields even with large contrasts in electrical conducrivity and large scatterer dimensions within the frequency range qf -a crass-well electromagnetic experiment. In our inversion. we implement a Gauss-Newton search technique to minimize a quadratic cost function with penalty on the spatial derivatives of the sought model. We derive an approximate form of the Jacobian matrix directly from the nonlinear scattering approximation. A conductivity model is found by repeated linear inversion steps within range constraints that help reduce nonuniqueness in the minimization of the cost function. Examples of inversion are shown with both numerically simulated data as well as data from an electromagnetic cross-well field experiment acquired by a university research consortium.
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MT as an Exploration Technology: A Review
By D. BeamishThis paper is a review of recent developments in the magnetotelluric method. Although there are many application areas to which MT may be applied, the increasing use of MT as a 'standard' survey method is emphasized. The review briefly considers the directions being taken to enable MT to 'become more useful' to the exploration community. A number of developments in the areas of data acquisition, response function estimation and modelling/inversion are discussed. Such developments should ultimately lead to MT surveys which consistently 'add-value' in the solution of exploration problems.
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Re-lnterpretation on MT Data from Antinaco-Los Colorados Valley Through a Decomposition Technique
Authors C. Pomposiello and A.M. OsellaWe performed a geophysical studv in the Antinaco-Los Colorados Valley in order to determinate the thickness of the sedimentary cover and the sequence of the more superficial layers. Nine magnetotelluric and four audiomagnetotelluric soundings were established along an east-west profile. The analysis of the results, using the traditional Swifts diagonal minimization method, indicates a deep basin and a very conductive layer is detected at around 400m, coincident with the depth of drillings, which may be associated to the presence of aquifers. In this paper we present a re-interpretetion of the magnetotelluric impedence tensor using the decomposition techniques developed by Bahr.
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A Magnetotelluric Survey in Southern Portugal
Authors F. W. Jones and A. CorreiaIn the summer of 1990 a magnetotelluric (MT) survey was performed in southern Portugal to try to delineate the geoelectrical structure of a geothermal anomaly reported to exist there. Although the geological and electrical characteristics of the region of the MT study are apparently three-dimensional, a twodimensional modelling approach is being used for an initial interpretation of the data. The objective of this approximation is twofold. First, it allows a preliminary geoelectrical interpretation for the region; second, it will provide bounds for the construction of a threedimensional model for detailed interpretation and which will be closer to the actual geological character exhibited in geologic and tectonic maps of the area. The main features seen in the results of the numerical models are also seen in the results of the measured data. Low electrical resistivity zones which coincide with fault structures that cross the region are evident in both the model and data pseudosections.
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On Features of Transient Electromagnetic Sounding of Horizontally-Inhomogeneus Media (Based on Physical Modelling Results).
Authors A.N. Kuznetsov and T.N. PetrovaCertain features of electromagnetic transient exhibitions excited by controlled source (transient sounding - TS) are considered. The investigations are conducted by physical modelling method. Field anamalously strong distortions associated with near surface and deep inhomogeneities are modelled. Two types of distortions are revealed: Sign inversion of the field measured near the high conductive quasi-2D inhomogeneities and variation of the magnetic field transition when the measuring loop is near the edge of inhomogeneity of any conductivity. Application of electromagnetic sounding systems with ceasured value averaging and relating the results in some cases to a receiver loop and in other cases - to the center of the set (depending on the inhomogeneity type) allows to substantially attenuate anomalously strong distortions of the electromagnetic field.
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Arquitetura da Bacia do Recôncavo
More LessA Bacia do Recôncavo situa-se no nordeste brasileiro, entre os paralelos 11°30 'S e 13°S, ocupando uma área de aproximadamente 11.000 km2. Em superfície ocorrem afloramentos que caracterizam tanto a borda flexural (a oeste) quanto a falhada (a leste), o que a torna especial em relaçăo aos demais riftes conhecidos, onde afloramentos desse tipo săo raros.
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Lithospheric Studies along the 27.50°S Transect
Authors A.M. Osella, C. Pomposiello, A. Favelto and P. MartinélliWe performed a geophysical study of a geothermal anomaly in the eastern border of i the Aconquija range. at the north-east of Argentina. An east-west MT profile was two-dimensionally analyzed in order to determine the thickness of the sedimentary basin and the depth of the highly conducting layer associated to the thermal anomaly. Also, a model for the basin is obtained from the analysis of the gravity data. These results, together with the ones previously obtained from MT and AMT data, were correlated with the geologic data and tectonic features, in order to give a better description of the geothermal reservoir.
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Estimation of the Transfer Function from Magnetic Variations of the Equatorial Electrojet Measured in Brazil
By A. StoerzelThe Z/H-method uses the ratio of the vertical to the horizontal magnetic component of a non-uniform source field to determine a transfer function which equals the magnetotelluric (MT) impedance. At mid latidudes the Z/H-method can only be applied to the Sq-field with periods from 4 to 24 hours and to the On-field with even higher periods. In the vicinity of the dip equator the Equatorial Electrojet (EEJ) generates a non-uniform source field with periods from 100s to 10000s. Thus the transfer functions of the Z/Hmethod estimated from magnetic variations of the EEJ can be compared directly with the MT impedances. In this study the application of the Z/H-method to the EEJ-field and the comparison of the resulting transfer functions to MT impedances is investigated in numerical models and using electromagnetic data from the Northeast of Brazil.
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Efeitos do Eletrojato Equatorial nos Dados Magnetotelúricos Tridimensionais
Authors A. Carrasquilla and L. RijoA premisa básica do método magnetotelúrico (MT) estabelece que as fontes indutoras se localizam fora do planeta, estando as suas flutuaçőes periódicas e transientes relacionadas com as variaçőes diurnas do campo magnético terrestre. Estas variaçőes săo causadas pela emissăo solar, a qual tem uma influęncia direta nas correntes da ionosfera, que induzem, pela sua vez, as correntes telúricas.
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The Effects of the Onwumechillian Equatorial Electrojet Model on MT Soundings
By L. RijoThe equatorial electrojet practically does not effect MT soundings at frequency higher than 10-2 Hz. However, below 10-2 Hz its influence can be very intense, increasing as frequencies decrease. Such influence can be felt at hundred of kilometers from either sides of the magnetic equator. The erratic behavior of the equatorial electro jet currents in the ionosphere and not its intensity seems to be the principal cause of such big effects. Since the electrojet currents are changing constant and erratically, the effects 011 the soundings can be very large and unforeseeable. To reach these conclutions we used in our numerical simulation the Onwumechillian model which is the simplest realistic model available for equatorial electrojet.
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