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4th International Congress of the Brazilian Geophysical Society
- Conference date: 20 Aug 1995 - 24 Aug 1995
- Location: Rio de Janero, Brazil
- Published: 20 August 1995
101 - 120 of 330 results
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Resultados de Sondeos MT en Precordillera Oriental (Matagusanos-La Laja-Angaco) Provincia de San Juan, Argentina
Authors F. Ruiz, M. Marmani, N. Ponti, B. Castiglione, D. Trad and J. VenenciaEn la Privincia de San Juan, sobre una línea de 35 km de extensión, a la latitud de Bańos La Laja (31· 20' sur y 68· 30' de longitud oeste), se efectuaron cinco sondeos magnetotelúricos (MT), apoyados con Sondeos EIéctricos Verticales. Abarca desde el Valle de Matagusanos al oeste, transpone la Sierra de Villicum y Ilega al Valle de Tulum al este, límite entre Precordillera y Sierras Pampeanas. Hay dos sondeos geoeléctricos diferenciados, el del oeste. La Laja 5 (LL 5) es más resistivo que el ubicado al este (LL 1), esta condición permite mayor penetración en el primero, según los sensores utilizados. En los tres sondeos interiores de la linea (LL 2, LL 3 y LL 4). las resistividades de los estratos superiores son menores y con mayores variaciones. Los resultados de su interpretación confirman la complejidad estructural del área dada por la geologla regional, donde Precordillera Oriental puede considerarse la zona de transición entre dos unidades tectonoestratigráficas distintas, Precordillera y Sierras Pampeanas.
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Resultados de MT en Cuenca Sedimentaria Profunda Y Area de Contacto de Provincias Geologicas, Mendoza-Argentina
Authors M. Marmani, D. Trad, C. Moyano and B. CastiglioneSe realizaron tres sondeos magnetotelúricos en la cuenca sedimentaria triásica en el norte de la provincia de Mendoza, Argentina (33"Sur - 69'W). Los resuItados muestran el espesor y la resistividad de estratos del subsuelo donde podria explotarse agua subterranea (cuartario), subyacente a éstos, se encuentra el de edad terciaria, del orden de 2000 mt y de baja resistividad. Un estrato conductor determinado a la profundidad promedio de 9.5 km, indicaría la zona de sobrecorrimiento entre las provincias geológicas de la Precordillera y las Sierras Pampeanas determinada en las cercanias por sismica profunda. Este resultado permite la búsqueda de esta capa conductora intercalada a lo largo del frente orogénico de Precordillera hacia ambos sentidos (Norte y Sur) de los sitios estudiados.
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Three-Dimensional EMAP Modelling
Authors C.R. Teixeira Régis and L. RijoEMAP data is modelled in a 3D environment via integral equation. We simulate a two layer earth with one body in the first layer. The presence of this body creates static shift on the data that distorts the response of the embasement. Using the EMAP filter we eliminate the static distortions getting a better evaluation of the basement depth. Different body sizes and dipole line positions were modelled.
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Multidimensional Inversion of Magnetotelluric Data
Authors J. Gouvęa Luiz and L. RijoOne-dimensional (ID) inversion of two-dimensional (2D) magnetotelluric (MT) data was used to find a priori information for 2D inversion. The result reduced both ambiguity and computation time, since only part of the 2D model needed to be really inverted. The technique can be valuable when doing tri-dimensional (3D) inversion, now considered economically impractical because of the large computation time demanded.
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Magnetotelluric Response of I-D Inhomogeneous Earth
By L. RijoTwo simple strategies for computing the one-dimensional magnetotelluric response due to an inhomogeneous conductive (or resistive) earth is implemented in this work. Both schemes use piecewise linear approximation for the earth conductivity profile. One is numerical, based on the finite element method, the other is an analytical approach. They perform egually well as shown by some examples.
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A MT Regional Profile in the Ponta Grossa Arch, Parana Basin, Brazil
Authors P. de Tarso Luiz Menezes, J. de Menezes Travassos and D. BeamishOne of the most important structure in Paraná Basin is the Ponta Grossa Arch. It has been intensely studied, but very little is known about it crustal structure. Two hypothesis are considered. The first one is crustal thinning, and the second one, crustal thickening with intrusion of mantle derived material into the crust. A MT regional profile was carried out to obtain a geoeletric model to the Arch, in order to acess if the region had undergone crustal thinning or thickening. I-D inversion indicate that the crustal thickness in the region is around 40 km.
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Magnetotelluric evidence for a crustal conductor beneath the Serra do Mar - RJ, Brazil
Authors I. Figueiredo and S.L. FontesIn order to understand the tectonic evolution of the Brazilian Southeastern region, it is certainly relevant to study its deep crustal structure. An important parameter in that case is the poorly understood electrical structure of the region. As the first attempt in that matter, a broadband (0.003 - 300Hz) magnetotelluric (MT) field survey has been carried out with the objective of mapping the geoelectric structure along a profile about 70 Km long crossing transversely the 'Serra do Mar' mountain range in Rio de Janeiro State, Brazil. The decomposition of the magnetotelluric impedance tensor (Groom and Bailey, 1989) was applied to the magnetotelluric data to remove existent local galvanic distortion. A new definition of apparent resistivity introduced by Basokur (1994) was used. Preliminary one-dimensional (1-0) modelling results suggest a conductor around 10 Km deep along the entire profile.
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A Simple Approximate Method of TEM Data Analysis
By M.A. MejuAlthough the field instrumentation for the transient electromagnetic (lEM) method Is rapidly evolving as simple. high resolution systems for recording accurate subsurface signatures. the development of matching simple Interpretation schemes for field data has lagged behind and is one reason why the uninitiated finds it more convenient to fall back on alternative geotechnologlcal tools with robust Infield data analysis capabllities. An effective practical method of lEM apparent resistivity data analysis is described In this paper and leans on a recently developed heuristic lEM inversion scheme that can be implemented easily on a hand-held calculator. However. unlike the previous scheme. it has the advantage of enhancing the resistive as well as the conductive parts of the reconstructed resistivity-depth profile leading to improved model identification. A comparison of the proposed scheme and that in current use is made using hydrogeophysical (TEM and VES) survey data recorded in the vicinity of a validation borehole.
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The Treatment of Noise in Electromagnetic Induction Studies - A Review
By A. JungeConductivity models of the earth derived from observations of electromagnetic field variations require reliable data. Thus the treatment of noise is essential. Various different types and sources of noise exist (e.g. Szarka, 1988; Nicols and Morrison, 1988; Adam et al.: 1986), which sometimes are difficult to identify and to separate. The estimation of relations between simultaneous time series of field components observed at different field sites either yield estimates of the conductivity distribution or serve to test the validity of hypothetical conductivity models. The algorithm for deriving the estimate from a given data set is called the estimator. Estimates are generally characterized by a systematic deviation from the true value (bias) and by a random deviation. The optimum choice of the estimator depends on the assumed physical and statistical properties of the observed quantities. Both properties may vary with time (non-stationarity). The dilemma of all processing schemes is not the intrinsic uncertainty of randomly distributed data but the problem of choosing the right estimator. As the processing of the data starts right after their collection visual inspection and subsequent selection of subsets of data is in a wider sense part of the estimation procedure. Time variations in the data which are obviously not related to the assumed induction process may be of various origin. Some aspects of noise characterization will be given, however, the main emphasis is put on the treatment of noise by numerical processing algorithms.
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Time Domain Electromagnetic Modeling in the Media with Smoothed Boundaries
Authors M.B. Rabinovich, L.A. Tabarovsky and K.-M. StrackThe transient electromagnetic (TEM) method is becoming increasingly important in the surface geophysics as well as in airborne and borehole applications. The required quality, resolution and accuracy of interpretation cannot be achieved without considerable modification of modeling concepts. In particular. the simulation of complex geology is largely based on the assumption that transition between different rocks occurs abruptly. Models with sharp boundaries only roughly represent smooth sedimentation processes. They are also inaccurate in simulating transition zones that appear between a borehole and a virgin formation. . Here we present a new time domain forward modeling algorithm where smooth transition zones substitute sharp boundaries. Since the conductivity becomes a continuous function. no boundary conditions are required for differential equations. This considerably simplifies the algorithm, makes it uniform and more stable. Use of the alternating direction implicit finite-difference scheme allows for solving the equations sequentially. Comparison against the existing 2.5-0 solutions with sharp boundaries in the time and frequency domains validates the approach. Improvement of the algorithm stability allows for modeling high conductivity contrasts. A numerical example illustrates modeling in the medium with a gradient conductivity distribution.
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Business Performance and Value of Exploration 3D Seismic
By B. AylorThis paper supplements a similar paper titled "Business Performance and Value of Exploitation 3D Seismic," which was presented at Workshop S, Highlights of the 1994 SEG Development & Production Forum, at the 1994 International SEG Convention in Los Angeles, CA. In that paper the incremental value that 3D seismic adds to Exploitation Ventures was shown to mainly derive from its ability to detect new development locations, locations with higher than normal Initial Flow Rates and improvements in cycle time. This paper analyzes the very extensive data base Amoco has collected which documents 3D seismic's features and its ability to impact exploration activities.
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Ciclicidade de Alta Frequencia (Escala de Milankovitch) e Halocinese em Sedimentos Albianos de Agua Rasa - Plataforma de Regencia do Espirito Santo
Authors I. Riveron de Gruber, J. Moreno and M. CorderoO Albiano (Grupo Barra Nova) na Plataforma de Regęncia, Bacia do Espírito Santo, é representado pela alternância, de caráter cíclico, entre sedimentos siliciclásticos e carbonáticos, depositados em ambiente que varia de continental transicional a marinho raso (plataforma). As evidęncias obtidas até o presente (em parte selecionadas a partir da interpretaçăo de seçőes sísmicas e perfis de raios gama), favorecem a hipótese de geraçăo alocíclica para esta alternância, causada fundamentaImente por oscileçőes eustáticas de alta frequencia (4a e 5a ordens). Nesta escala de alta frequęncia as variaçőes eustáticas tęm sido consideradas, ultimamente, como as principais geradoras de ciclicidade, pois nem a tectono-eustasia, nem os movimentos tectônicos do assoalho da Bacia, atuam em escala tao pequena de tempo (Plint et al., (1992).
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Exploration Drilling in the Madre de Dios Basin, Bolivia: Balancing Potential and Risk in the Amazon Rain Forest
Authors K. T. Conrad and I. SeguraThe Nueva Victoria-Xl wildcat well, located in the Bolivian portion of the Madre de Dios basin, is an excellent example of Mobil's ability to operate successfully in a cost effective manner in the remote, environmentally-sensitive Amazon rain forest. Although commercially unsuccessful, the well was important -- because it 1) tested our applied seismic techniques of seismic inversion and amplitude strength analysis for sand detection and 2) helped accelerate our evaluation of the area. This discussion will review the Nueva Victoria prospect from concept through post-drill analysis with a focus on key technologies that unpacted the interpretation.
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Fast and Stable Two-dimensional Inversion of Magnetotelluric Data
Authors M.S. Zhdanov, P.P de Lugao and O. PortniaguineThe two-dimensional inverse problem in magnetotellurics has been addressed by several authors, the most well-known approaches being developed by de Groot-Hedlin and Constable (1990) and by Smith and Booker (1991) in the search for a smooth model. However, a smooth model can not describe well the real model of the earth, which contains sharp contrasts at the boundaries of structures with different conductivities. In this paper, we present an approach that deals with the model of an arbitrary structure. This approach is based on regularization theory and the quasianalytic calculation of the Frechet derivatives. For the forward solution we use a fast and efficient finite difference formulation to the solution of the Helmholtz equation based on the balance method (Berdichevsky and Zhdanov, 1984). One of the most computationally expensive tasks in the solution of an inverse problem, the calculation of the Frechet derivative matrix is obtained as a solution to simple forward and back substitution of the LU decomposed matrix of coefficients from the forward problem with a different right hand term. The method of steepest descent is utilized to minimize the parametric functional in the search for a stable solution of the two-dimensional magnetotelluric inversion problem. Preliminary results for synthetic data of a conductive body in a resistive host are presented. The method can be applied for the inversion of MT and CSAMT data.
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Magnetotelluric Investigation of the Serra da Cangalha Impact Structure, Brazil
Authors W. Masero, S.L. Fontes and P.A. SchneggIn order to know more about the structural characteristics underneath impact crater a series of magnetotelluric (MT) soundings were carried out in the Serra da Cangalha in Northeast Brazil. The MT data indicate in general a one-dimensional conductivity distribution of the subsurface. Nevertheless, anisotropic MT responses in the period range between 0.01 and 0.1 sec are observed at a few sites, all located in the vicinity of the centre. They reflect a fractured zone, which reaches a depth of a few hundred metres and has an horizontal extension smaller than the total diameter of the structure. Additionally, the identification at all MT sites of a good conducting layer at a depth around 1100 metres, provides an accurate maximum depth for the observed disturbances related to the impact event.
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Geoelectromagnetic investigations in the Central Andes within the Framework of an Integrated Geophysics Project
By H. BrasseDuring the last ten years, electromagnetic investigations have been carried out in the Central Andes to study the conductivity structure of the active subduction zone at the South American western margin. They are part of an integrated geological and geophysical project, which also comprises seismic refraction, reflection and gravity studies, The working area is located north and south of the tropic of Capricorn, between the Chilean town of Antofagasta to the Andean foreland in Bolivia and Argentina.
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Distorçőes em Sondagens Magnetotelúricas na Regiăo da Anomalia Magnétics do Atâintico Sul
Authors A.L. Padilha, S.L.G. Dutra, Í. Vitorello, N.B. Trivedi and J.M. da CostaEfeitos de fonte em estudos MT foram antecipados em latitudes aurorais e equatoriais e tem sido extensivamente discutidos na literatura de forma teórica. Como conclusăo desses estudos foi demostrado que quando esses efeitos săo significativos, a relaçăo entre os campos vetoriais elétrico e magnético é dominada pelas características da fonte.
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Regiăo do Espaço que Mais Influencia em Medidas Eletromagnéticas no Domínio da Frequęncia, Para uma Linha de Corrente Sobre um Semi-Espaço Condutor: Modelo Bidimensional
Authors L. Peixoto de Brito and C.A. DiasQuando se mede uma grandeza física, que corresponde a uma propriedade de um meio, tem-se em geral dificuldade em identificar a porçăo do espaço que é a principal responsavel pelas medidas observadas. Sabe-se que cada porçăo do espaço influęncia diferentemente os resultados observados e que existe uma determinada regiăo, relativamente pequena, responsável pela maior parte do sinal medido.
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Levantamento CSAMT/MT na Regiao do Graben de Baquirivu-Guacu (Aeroporto de Guarulhos)
Authors H. Araújo Franco, J. de Menezes Travassos and J.M.F. MiccolisA regiăo do Aeroporto de Guarulhos está inserida na Bacia Sedimentar de Săo Paulo, no Graben do Baquirivu-Guaçu. Nesta bacia, foi realizado urn perfil CSAMT/MT com o intuito de mapear, em subsuperficie, estruturas associadas ao sistema graben. O Graben do Baquirivu-Guaçu, na regiăo do aeroporto, desenvolveu-se nos xistos do Grupo Săo Roque e Conjunto Paranapiacaba. Além disso, na área, a bacia de Săo Paulo está preenchida pelos sedimentos terciários inconsolidados das formaçőes Resende e Itaquaquecetuba, e por depósitos aluvionares recentes. Os dados obtidos com o método CSAMT/MT demonstram grande potencialidade no mapeamento de estruturas em subsuperficie com fins de prospecçăo de aguas subterrâneas.
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SPAM MK-III Sistema de Aquisiçăo, Processamento e Análise de Dados para Aplicaçăo em Geofisica
Authors D. Nascimento and G. DawesO Departamento de Geologia e Geofísica da Universidade de Edinburgh, Escócia, desenvolve instrumentaçăo eletrônica para aplicaçăo em Metodos Magnetoteluricos desde 1980. Desde a sua última versăo, novos componentes eletrônicos foram desenvolvidos e outros tiveram suas características de ruído, consumo de energia e dimensőes físicas bastante implementadas. Alem disto, novos conceitos em aplicaçőes computacionais se tornaram possíveis. Em funçăo disto, em Outubro/94 uma nova geraçăo desta série de equipamentos foi entregue ŕ comunidade científica e seu empréstimo vem sendo requisitado continuamente através do pool de equipamentos do NERC-National Environment Research Council.
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