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5th International Congress of the Brazilian Geophysical Society
- Conference date: 28 Sep 1997 - 03 Oct 1997
- Location: São Paulo, Brazil
- Published: 07 November 1997
101 - 150 of 416 results
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Integraçăo dos Dados Magnetométricos das Bacias de Campos e Santos
More LessEste trabalho apresenta uma analise e integraçăo dos dados magnéticos (aéreos e marítimos) abrangendo a regiăo das Bacias de Campos, Santos e áreas vizinhas, tendo em vista futuras interpretaçőes estruturais a nível regional para pesquisa mineral e/ou de hidrocarbonetos. Săo feitas também consideraçőes sobre as características dos levantamentos e quaJidade dos dados obtidos nos diferentes projetos, tres dos quais, da própria PETROBRAS, dois do Projeto LEPLAC e dois do Departamento Nacional da Produçăo Mineral. Limitaçőes e possibilidades interpretativas mais detalhadas a partir dos dados já disponíveis, săo também comentadas.
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Contribuiçőes Magneéticas para a Carta Geológico-Metalogenética da Folha Jaguaribe (SB.24-V) Estado do Ceará
Authors José Farias de Oliveira and Jose Carlos RodriguesOs exames aerornagnéticos da Folha Jaguaribe-NO localizado numa latitude magnética baixa e limitada pelas coordenadas 39 graus e 42 graus de long. WGr e 4 graus e 6 graus de latitude suI, forarn executados para subsidiar a elaboraçăo da Carta Geologico-Metalogenética, envolvendo as estruturas tectônicas e diferenciaçőes litológicas e seus contatos. Nos estudos do processamento dos dados aeromagnéticos da regiăo, foram utilizados o MPS software da geosoft, para desenvolvimento de interpretaçăo e modelagem.
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Imagens Geofísicas do K, U e Th da Província Uranífera de Lagoa Real (BA), com Base no Levantamento Aerogamaespectométrico do Projeto Săo Timóteo
More LessConsiderada uma das maiores jazidas de uranio do país, a Província Uranífera de Lagoa Real (BA), foi descoberta na década de 70, através de levantamentos aerogamaespectrométricos realizados na porçăo centro-sul do estado da Bahia. Com a perspectiva de início da explotaçăo do urânio na regiăo para os próximos anos, os dados aerogamaespectrométricos estăo sendo reestudados, gerando mapas com anomalias radiométricas pra a determinaçăo de novas áreas alvos para a exploraçăo de urânio. Deste modo, este trabalho apresenta os principais processos aplicados para 0 tratamento dos dados e os resultados "semi-quantitativos" obtidos.
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Geophysical Signatures Of Mineral Deposits In South And Western Australia: Their Compilation And Exploration Significance
By Mike DentithIt is comparatively common for Australian mining companies to allow publication of case history papers describing the geophysical signatures of mineral deposits. Most of these the case histories have been in publications associated with the Australian Society of Exploration Geophysicists, notably the society's journal, Exploration Geophysics. The society has actively encouraged such publications, for example by organising relevant symposia at its national conference. More recently the ASEG has collaborated with the Centre for Strategic Mineral Deposits at The University of Western Australia and Mines and Energy South Australia to produce special publications on the geophysical signatures of mineral deposits in Western and South Australia. Deposits described in various publications include virtually all the main styles and types in Australia. For example, the base metal mineralisation in the Mount Isa region Queensland, the giant copper-uranium-gold deposit at Olympic Dam, South Australia, the Argyle diamond deposit in Western Australia, numerous gold deposits in the Archaean greenstone belts of Western Australia and supergene iron-ore deposits in the Hamersley Province, Western Australia. The compilation of geophysical signatures has been further assisted by "exploration initiatives" funded by the State and Federal governments. These have made available large quantities of cheap, easily accessible, aeromagnetic data. Geological structure is a major control on most of the mineral deposits in Australia and these data can be interpreted to identify the structural setting of known deposits. Equivalent structural scenarios can then be sort elsewhere. The extensive database of geophysical signatures of Australian mineral deposits that has thus been created is a useful resource for companies exploring both in Australia and also the rest of the world.
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The Detection of Potassic Alteration by Gamma Ray Spectrometry: Recognition of Alterations Related to Mineralization
Authors R.B.K. Shives, B.W. Charbonneau and K.L. FordCase histories developed by the Geological Survey of Canada since 1970 clearly demonstrate the successful application of airborne and ground gamma ray spectrometry to map potassium alteration associated with different styles of mineralization. These include: volcanic hosted massive sulphide (Cu-Pb-Zn) mineralization; polymetallic (Au-Co-Cu-Bi-W-As), magmatic-hydrothermal mineralization; and porphyry (Cu-Au-Ilvlo) mineralization. Alteration associated with mineralization may produce potassium anomalies which can be distinguished from the normal lithologic signature by characteristic eTh/K ratio lows. Interpretations incorporating airborne and ground spectrometry, surficial and bedrock geochemistry and petrology show that gamma ray spectrometric patterns provide powerful guides to mineralization. This information compliments magnetic, electromagnetic, geological and conventional geochemical data commonly gathered during mineral exploration programs. In addition to applications to mineral exploration, airborne and ground gamma ray spectrometry provides valuable information useful for regional and detailed bedrock and surficial geological mapping and environmental radiation monitoring. Worldwide, increased levels of airborne gamma ray spectrometric surveying will foster increased ground spectrometric surveying and improvements to data processing, interpretation, case history development and presentation. More effective communication of these results will lead to better understanding and acceptance of the currently under-utilized gamma ray spectrometric method.
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Caracterizaçăo Geológico-Geofísico das Mineralizaçőes Auríferas do Greenstone Belt Rio das Velhas
More LessO Aerolevantamento Geofísico de Detalhe do Projeto Rio das Velhas, realizado pelo Convęnio DNPM/Consórcio de Empresas de Mineraçăo (DNPM, 1992) e 0 Mapeamento Geológico do Greenstone Belt Rio das Velhas (DNPM/CPRM, 1994/1996) proporcionaram urn grande avanço científicotecnólogico, fomentando acentuadamente a prospeçăo e pesquisa mineral na regiăo do Quadrilátero Ferrífero, notoriamente uma das principais províncias minerais do país.
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Simulated Annealing: A Tool for Estimating the Macro-Model for Prestack Migration Velocity Analysis and AVO
Authors Paul L. Stoffa, Mrinal K. Sen and Ganyuan XiaSeismic waveform inversion problems are naturally decomposed into two parts: determining the background velocity and the contrasts in material properties described by the high-frequency variations in impedance and Poisson's ratio. Seismic traveltimes are determined by the background velocity while the high-frequency variations in impedance and Poisson's ratio determine the reflection amplitudes. AVO studies which exploit changes in reflection amplitude as a function of incidence angle require as a prerequisite knowledge of the background velocity so that the correct reflection phases can be compared. For example, depending on the method employed either the offset vs. traveltime, ray parameter vs. intercept time, or depth vs. incidence angle trajectory must be known or taken into account through dynamic corrections to the data.
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Practical Approaches to Uncertainty Estimation in Geophysical Inversion
Authors Mrinal K. Sen and P.L. StoffaGeophysical inversion is essentially a two-step procedure: the first is to find at least one model that fits the data and the second step is to find all possible models that explain the data. The latter is caused by the fact that most geophysical inverse problems have nonunique solutions. An optimization approach that searches for a minimum of a suitably defined error function (that measures the misfit between observed and synthetic data) is often applied to select a candidate solution. A global optimization method such as simulated annealing (SA) or a genetic algorithm (GA) can be used to address the first part of the problem since these methods are often independent of the starting solution. However, at this stage, the solution to our inverse problem is far from being complete. To address the nonuniqueness aspect of the problem we require some measures of the uncertainty in our solution.
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Developing Accurate Depth-imaging Velocity Models, Rapidly and Efficiently, for Gulf of Mexico Style Subsalt Exploration.
Authors Uzi Egozi and Stuart FaginTo build depth-imaging velocity models in the Gulf of Mexico accurately and rapidly, one must take account of, and make use of, the distinctive geologic conditions which prevail there. While the water column and salt layers have relatively invariant velocities, velocities in the sedimentary section are compaction-driven and therefore mainly a function of depth below mudline. A successful model building strategy must handie both the layer-based velocities for the water column and salt and grid based velocities for the sedimentary section. The purpose of this article is to present a series of techniques which show how this sort of model building strategy may be applied.
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Interval Velocities Inversion from Seismic Reflection Traveltime in 2-D Earth
By J.C.R. CruzA new inversion procedure to estimate the interval velocities model and the normal-incidence-point (NIP) wavefront attributes - the radius of curvature R, and the emergence angle Bo - is presented in this paper. Although we make use of the same formulas of the Common-Reflecting-Element (CRE) Method, in this new approach the searched-for NIP wavefront attributes are determined with help of the common-offset (CO) isochrone. In contrast, the original CRE method obtains the same parameters by using an auxiliary model of one a priori known near-surface constant velocity. The final result of this procedure is one full intervalar velocities model and as by-product the optimal R, and Bu for each two-way reflection time in zero-offset (ZO) section.
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Reciprocity, Elastic GRT Inversion, Caustics, Multipathing
More LessLinearized inversion can be derived from the reciprocity theorems of the time-convolution and the time-correlation types. The convolution-type theorem defines a mapping from medium contrast to scattered field, while the correlation type theorem defines its adjoint, viz. a mapping from scattered field to a quantity that relates to medium contrast. The time-correlation provides an error criterion for the data misfit as well as an imaging condition. The image is nothing else then the gradient of the error function (Tarantola 1984).
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Evolution of Passive Continental Margins with Special Reference to the U.S. East Coast from Seismic and Potential Field Data
By M. TalwaniThe structure and evolution of continental margins, especially those that contain large thickness of sediments, are difficult to resolve. Seismic reflection data acquired by the EDGE project off the US East Coast employing a 6 km streamer and large capacity air guns, provided the best data across this margin (Sheridan et aI., 1993). These data were supplemented by wide angle reflection and refraction data collected by ocean bottom hydrophones and ocean bottom seismometers (Holbrook et aI., 1994). In addition to resolving the structural problems, there are several interesting geophysical features associated with this margin which, in some cases, are in common with similar features associated with other margins.
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The Mid-Atlantic Ridge south of Kane Fracture Zone: A Synthesis of the Geophysical Knowledge
More LessThe Mid-Atlantic Ridge south of the Kane Fracture Zone (between 200 and 240 N) was surveyed by the NO "L'Atalante" SEADMA cruise in 1991 out to 10 Ma. This data set allowed the study of the temporal evolution of the ridge segmentation in an area away from the influence of the Azores hot spot. Five segments were identified from the bathymetry, showing complex rhomboedric shapes, suggesting a fast evolution of the spreading configuration during the last 10 Ma. These segments display the characteristic bull's eye Mantle Bouguer Anomalies in a clear three dimensional pattern. Segments centres are associated with circular negative minima, while segments borders, or discontinuities, are marked by less negative values (Gente et al., 1995). Residual gravity inversion yielded a crustal pattern similar to the patterns already observed elsewhere along the Mid Atlantic Ridge (Lin et al., 1990; Rommevaux et al., 1993; Pariso et al., 1995), i.e., thick crust at the segments centres and thin crust at their edges.
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Inversăo Integrada de Dados Sísmicos Utilizando Tomografia de Difraçăo
Authors Antônio A. Rocha Filho, Jerry M. Harris and Amin BassreiA tomografia de difraçăo permite identificar mínimas variaçőes de velocidade. Este trabalho visa abordar urn novo conceito de inversăo, através da tomografia de difraçăo, fazendo uso das interfaces interpretadas de seçőes sísrnicas migradas. A forrnulaçăo matricial rnultifrequęncia utilizada, se por urn lado utiliza os conceitos fundamentals na descriçăo do fenômeno, por outro busca urn equacionamento muito semelhante ŕ tomografia de raio. Define-se uma parametrizaçăo para o campo de velocidades dividindo-se a área imageada em urn número finito de blocos com velocidade constante. Desta forma é possível calcular a contribuiçăo para o campo espalhado de cada bloco, em cada frequencia, para cada par fonte/receptor. A utilizaçăo de arranjos de fontes e de receptores e de várias frequęncias permite montar urn sistema linear solucionado sob a ótica dos minimos quadrados visando uma soluçăo com estrutura mínima. Uma das principais vantagens desta abordagem é a possibilidade de se empregar quaisquer combinaçőes de fontes/receptores, bern eomo informaçőes a priori tais como interfaces interpretadas a partir de uma seçăo migrada. O modelamento e inversăo foram feitos com dados acúisticos 2-D sintéticos a partir de model os com pequenos contrastes de velocidade.
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An Iterative Method for the Solution of Linear Inverse Problems in Geophysics
By Amin BassreiIn this work we present a new numerical technique for the solution of geophysical ill-posed inverse problems, in the case of discrete data and discrete model parameters. The Landweber's (1951) algorithm is applied to invert synthetic tomographic data corrupted by noise. The algorithm is given by the simple formula mk+1 = m' - ACT(Gm" - d) , where 0 < A < 2/ a MAX' in order to guarantee the algorithm convergence, and aMAX is the maximum eigenvalue of the matrix C . When using an iterative algorithm one has to investigate: the existence of a solution, the uniqueness of this solution, the speed of convergence, and the properties of the solution. At this stage we are more interested in the speed of convergence. This "new" iterative method is in general faster than the Algebraic Reconstruction Technique (ART), as showed in the numerical simulations.
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Electromagnetic Inverse Problems
More LessInversion of geophysical data is complicated by the fact that the data are invariably contaminated by noise and are acquired at a limited number of observation points. Moreover, mathematical models are usually complicated and yet are also simplifications of the true geophysical phenomena. As a result, solutions are ambiguous and error-prone. The principal questions arising in geophysical inverse problems are the existence, uniqueness and stability of the solution. The solutions can be based on linearized and nonlinear inverse techniques, and can include different approaches, such as least-squares, gradient type (including steepest descent and conjugate gradient), and others. A central point of this talk is the application of the so-called "regularizing" algorithms for the solution of ill-posed inverse problem in electromagnetics. These algorithms can use a priori geologic and geophysical information about the earth's subsurface to reduce the ambiguity and increase the stability of the solution.
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A New Regularization Technique in Magnetotelluric Inversion
Authors Fernando Manuel Ramos and Haroldo Fraga de Campos VelhoThe use of electromagnetic fields induced by natural sources in the ionosphere and magnetosphere to determine the electrical conductivity of the earth's subsurface has a wide range of applications in geophysics. Appearing in various areas such as petroleum prospection, rrumng and search for groundwater, this inverse problem, also known as inversion of magnetotelluric (MT) data, has special relevance in the exploration of regions which are difficult to probe with conventional seismic methods. These areas usually involve either near-surface basalt layers, which cause very strong reflections, or regions where tectonic events have disrupted the sedimentary layer geometries and greatly complicated the seismic signature (Madden and Mackie, 1989).
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Inversion of 2D Magnetotelluric Data Using The Complex Singular Value Decomposition Method
Authors Emin U. Ulugergerli and Max A. MejuThe numerical solution to the two dimensional (2D) magnetotelluric (MT) forward problem can be obtained in many ways involving the solution of integral or differential equations and the inverse problem is generally solved using regularised iterative inversion techniques. However, the existing inverse schemes effect matrix computations in real domain due to operational simplicity. A common problem encountered when dealing with real matrices is their huge size in 2D regularised inversion. To partly overcome this problem, a new inversion strategy using complex singular value decomposition (CSVD) techniques have been developed. The use of analytical partial derivatives and a variety of problem regularization measures ensure that the scheme is stable and rapidly convergent. In this method, instead of using the Cagniard apparent resistivity and phase, the frequency normalised impedance (FNI) is adopted as the interpretative data functions to improve model resolution. Applications to synthetic and field data from Parnaiba Basin in Brazil are presented and it is shown that the complex form of the data-space and parameter-space eigenvectors contain information on parameter resolution.
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Inversăo de Sondagem Elétrica Vertical Utilizando Algoritmos Genéticos Combinado com Inversăo Linearizada
Authors Niraldo Roberto Ferreira and Milton José PorsaniInversion of DC resistivity sounding is a nonlinear problem. Local or global optimization methods are commonly used to solve it. Local methods are fast but, require that the start model be close to the true solution and may be trapped in local minimum. Global methods are robust, but computationally expensive since the space is usually very large. Here we combine the genetic algorithm (AG) with the linearized inversion method, Gauss-Newton (GN), to overcome their limitations and explore the advantages of the two methods. The algorithm was tested with a I-D Schlumberger resistivity sounding data and its performance was compared with pure AG. The joint operation improves the convergence even when using a reduced population of models.
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Aplicaçăo das Redes Neurais Artificiais na Inversăo de Sondagem Elétrica Vertical
Authors Telesson Neves Teles and Niraldo Roberto FerreiraO objetivo da sondagem elétrica vertical (SEV) é, a partir de medidas elétricas efetuadas na superfície da Terra, investigar a variaçăo da resistividade da formaçăo com a profundidade. A técnica assume estratificaçăo horizontal do meio, cujos parâmetros relevantes săo as resistividades 'e espessuras das camadas. Estes parâmetres săo estimados através do ajuste ótimo entre as resistividades aparentes observadas e calculadas. 0 ajuste pode ser obtido de vários modos. Tradicionalmente săo empregadas técnicas de inversăo linearizada que fazem uma pesquisa local no espaço dos modelos, procurando melhorar uma estimativa inicial no sentido dos mínimos quadrados. A técnica é eficiente e bastante empregada, porém a dependęncia de um modelo de partida pode produzir soluçăo insatisfatória. Como alternativa, métodos de otimizaçăo global tęm sido investigados. Săo exemplos o "simulated annealing" e os algoritmos géneticos. Estas técnicas săo robustas e năo dependem de um bom modelo inicial, porém săo de custo computacional considerável. Recentemente, as redes neurais artificiais começaram a ser investigadas/aplicadas na soluçăo de problemas de inversăo. Ver,.por exemplo, Macias e Sen (1993).
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Inversăo Gravimétrica Estável da Base de urn Corpo Arbitrário
O problema linear inverso em gravimetria , isto é, a estimaçăo da distribuiçăo de densidade em subsuperfície em termos da componente vertical do campo gravimétrico observado em pontos situ ados na superfície do terreno, é um problema mal-posto no sentido de Hadamard (1902), conduzindo a soluçőes năo-únicas e/ou instáveis.
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Boundary Control Method in Inverse Problem of Geophysics
Authors Alexander P. Kachalov and Milton José PorsaniThis paper present a new method for solution of inverse problem in geophysics. The method requires the boundary measurements only and is called as Boundary Control method. No parameters information about the medium are required and a direct procedure for solution of the inverse problem is derived. Numeric examples for ID seismic velocity inversion are presented.
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Direct and Inverse Problems of Electromagnetoelasticity
Authors A.V. Avdeev, V.I. Priimenko, E.V. Goruynov and D.V. ZvyaginOne of the possible statements of inverse problems connected with electrodynamics of vibrating elastic media is considered. The motion of an elastic conductive medium in the electromagnetic field is described by two sets of equations: that of elasticity and that of electrodynamics. The waves arising in the result of this interaction are usually referred to as electromagnetoelastic. These waves contain information about both electromagnetic and elastic parameters of a medium. The electromagnetic wave rides the "back" of the seismic wave, that is, the induced electromagnetic wave is "frozen" into the seismic wave and propagates either with P- or with S- seismic wave velocity, depending on the type of waves. The dominant frequency and the velocity of the induced seismo-magnetic wave is equal to the frequency and velocity of the seismic wave. L. Knopoff [1] studied the influence of electromagnetic fields on the propagation of elastic waves and arrived at the conclusion that in the class of geophysical problems the effect of electromagnetic phenomena on the process of elastic waves propagation is negligible, at least in the case of not too large electromagnetic disturbances.
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Anomalous Mantle at 45°N on the Mid-Atlantic Ridge?
Authors S. Mello, J.R. Cann and C.M.R. FowlerThe Mid-Atlantic Ridge between 45°N and 46°N is part of a 1300 km long section of mid-ocean ridge lacking major transform faults, between the Azores at 39°N and the Charles-Gibbs Transform at 51°N, Within this area, the floor of the axial valley is from 9 to 18 km wide, and strikes at 019° : water depths to the median valley floor are between 3000 and 3500 m. Bathymetry allows the 80 km of spreading axis in the survey area to be divided into three spreading segments. The two southernmost are well defined by the existence of axial volcanic ridges, while the third is characterised by a 3500 m deep valley, which is almost blocked by two mountains that encroach on the valley from east and west. The ridge crest morphology in the region is quite similar to that of other sections of the MAR to the south of the Azores, especially in terms of sea floor depths, fault style, segmentation and volcanology.
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Planejamento de Levantamentos Magnetométricos Usando a Teoria dos Problemas Inversos
Authors Renato Cordani and Carlos Alberto MondençaNormalmente uma campanha de prospecçăo mineral emprega magnetometria terrestre para confirmar e detalhar feiçőes geológicas previamente reconhecidas em levantamentos aerotransportados. Levantamentos terrestres nessa fase, denominados de follow-up, procuram tarnbém determinar as bordas e as profundidades do topo das fontes magnetizadas para orientar a prospeçăo por sondagem que geralmente sucede a prospecçăo geoffsica.
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A New Criterion for Result Evaluation in Geophysical Inversion
By Amin BassreiIn the solution of inverse problems we are never sure about the validity of the results. In the case of seismic tomography for instance, we cannot guarantee our 2-D or 3-D reconstructed images. We can only be certain in the so called artificial or synthetic examples, where the true model (or image), gi ven by the vector of model parameters m, is known. Based on Barbieri (1974), we have the alternative approach for evaluating the inversion process where the final step is a constant or in fact pseudo-constant image where we can check and observe in what regions of the model the inversion was not successful. We show some simulations with synthetic data in traveltime tomography.
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A New Simulated Annealing Technique for Non-Linear Optimization
Authors K.C. Mundim, T.J. Lemaire and A. BassreiThe search of a global extremum of a cost function f is, in general, a very difficult task that appears frequently in the most different fields of the knowledge. Physicists, engineers, geophysicists and economists are particularly concerned with constrained optimization, where a priori limitations are imposed on the allowed values of the variables to be optimized. There are different methods to attack this problem. The most usual technique is based on gradient methods principally if the cost function fis convex, i.e. has a single minimum. But if f has multiple extrema (nonconvex function), more sophisticated methods must be used, in order to not get trapped in one of the local minima. This kind of method has the dvantage to map local and global minima.
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1-D Inversion of Anisotropic Magnetotelluric Data
Authors Cícero Régis and Luiz RijoAn inversion scheme, using the Marquardt method, is applied to 1-D anisotropic magnetotelluric data, generated by numerical modeling. The results presented here will lead to the study of more complex models.
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Impedance Layering Through 3D Stratigraphic Inversion of Post Stack Seismic Data
More LessThe new program we propose converts 3D zero-offset migrated seismic data into acoustic impedance by inverse modeling where the impedance model updating results from volumetric impedance perturbations and strata interface deformations. The end-product of the program consists of a 3D stratified impedance volume from which maps of the space-variant acoustic impedance of the strata together with the geometry of the strata interfaces can be extracted. The method can contribute to reservoir characterisation owing to its good resolution resulting from the sparse parameterization coupled with the volumetric seismic data handling.
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Mapa Gravimétrico do Nordeste Setentrional do Brasil e Margem Continental Adjacente
More LessO Nordeste Setentrional Brasileiro está inserido geologicamente na Província Borborema. As rochas precambrian as desta regiăo apresentam uma complexa evoluçăo geodinâmica, dominada por trends estruturais com direçăo principal NE, além de um importante sistema de zonas de cisalhamento E-W e NE-SW (Almeida e Hasui, 1984). As rochas sedirnentares das bacias costeiras e interiores do Nordeste Setentrional, bem como os sedimentos da Bacia do Parnaíba, representam as coberturas fanerozóicas nao deformadas na Província Borborema.
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Relevo da Interface Crosta-Manto no Nordeste Setentrional do Brasil: Comparaçăo entre os Vínculos de Isostasia e Suavidade
Authors David L. Castro, Valéria C.F. Barbosa, Joăo B.C. Silva and Walter E. MedeirosUtilizando-se dados topográficos e gravimétricos, pode-se estimar o relevo da interface crostamanto através de, pelo menos, duas abordagens independentes. A primeira abordagem (amplamente utilizada) emprega a hipótese de que a regiăo em estudo encontra- se isostaticamente compensada (e.g. Banks et aI., 1977). Por outro lado, a segunda abordagem (menos utilizada) efetua previamente a separaçăo regional-residual do campo gravimétrico total e estima o relevo da interface crosta-manto através da inversăo da coinponente regional do campo (e.g. Beltrăo, 1989). Por ser urn problema mal-posto (cuja soluçăo é instável), esta inversăo requer a introduçăo de vínculos para fomecer soluçőes estáveis. Um dos vínculos mais utilizados é o da suavidade do relevo da interface crostamanto.
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Deconvoluçăo de Euler Aplicada ŕ Prospecçăo de Esmeralda: Campo Formoso, Bahia
Thompson (1982) apresentou uma técnica automatica de interpretaçăo magnética para estimar a posiçăo horizontal e a profundidade da fonte anômala, que resolve urn sistema linear sobredeterminado nas incógnitas: posiçăo (x0 ) e profundidade (zo ) da fonte, e urn nfvel de base (b) para cada posiçăo da janela móvel de dados e para alguns valores de n, estabelecidos a priori. Como este procedimento produz uma "nuvem" de soluçőes, Thompson (1982) estabeleceu urn critério de aceitaçăo baseado no desvio padrăo da estimativa de Z0 de cada soluçăo, levando a uma reduçăo do número de solucçős. Adicionalmente, Thompson (1982) apresentou urn critério empírico para estimar 11, aceitando, como a melhor estimativa, o valor usado para n que produz uma menor dispersăo na nuvem de soluçőes, Embora produza resultados insatisfatórios, os critérios propostos por Thompson (1982) vern sendo comumente empregados, apenas com ligeiras modificaçőes,
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Ivis 3D and Igmas: Tools for Interactive 3D-Visaulization and Modelling of Potential Field Models VIS
Authors Christian Klesper, Sabine Schmidt and Hans-Jürgen GötzeEDV-based interactive modelling and visualization methods have become an essential part in the process of construction and analysis of three-dimensional models in Earth sciences. However, dealing with complex geoscience models, existing modeling systems can satisfy the user requirements only inadequately. The modelling program IGMAS (Interactive Gravity and Magnetic Application System) [Gatze et al, 1988] and the visualization program IVIS-3D (IGMAS Visualization in 3D) [Klesper, 1997] were designed to ease interactive modelling projects. The interactive modelling program IGMAS provides numerical simulation by forward modelling, to build and analyse 3D potential field models (gravity, geoid & magnetics). Various functions, an intuitive and easy to use enviroment and the combination of measured and interpreted data allows the flexible creation of different model versions. Up to now missing functionality of interactive 3D-visualization was compensated by a patchwork of different programs or the user had to handle complex visualization systems with a large overhead of functionality. For this reason we decided to create a program which provides enhanced visualization methods within the modelling process. IVIS3D now adapts the capabilities of interactive 3D-visualization with an easy to use graphical user interface to the existing modelling program IGMAS.
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Assinatura Gravimétrica da Sutura de Alterosa nos Arredores de Alfenas (MG)
Authors Sérgio Henrique S. Almeida and Hans Dirk EbertA estruturaçăo tectonica regional do sudeste brasileiro é configurada pela articulaçăo de tręs blocos crustais: Săo Paulo, Vitória e Brasilia (Haralyi & Hasui 1982), do Arqueano ao Paleoproterozóico, ou paleoplacas da "Gondwanaland" que lembram junçőes tríplices. Os blocos Săo Paulo e Vitória compreendem terrenos granulfticos, interpretados como crosta inferior, que se encontram justapostos a terrenos granito-greenstone e supracrustais, ambos representando o Bloco Brasília. A conexăo entre esses blocos e marcada pelas suturas de Alterosa (blocos Sao Paulo e Brasilia) e Abre-Campo (blocos Sao Paulo/Vitória e Brasília), identificadas por cinturőes de cisalhamento, forte anomalia gravimétrica positiva e por rochas granulíticas (Ebert & Hasui 1997).
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EGM96 Geopotential Model and Geophysics
Authors D. Blitzkow and S. MelloGeopotential models have been rapidly developed in the last twenty years. They are now very accurate and can be useful in many fields. This paper aims to synthesize the evolution of the models and exemplify one of its possible use in geophysics.
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Interpretaçăo Aeromagnetométrica do Alto Estrutural Juruena Teles Pires
Authors Paulo Parro and Augusto C.B. PiresO alto estrutural Juruena Teles Pires localizase na porçăo centro-sui do escudo Brasil Central, e é parte da Província Tapajós de Almeida et al. (1977). Embora esta regiăo seja uma província aurifera bastante conhecida, na qual localizam-se as reservas garimpeiras de Peixoto de Azevedo, Zé Vermelho, Cabeça e Bacia do Rio Juruena, o conhecimento geológico sobre a mesma e escasso. As unidades geológicas aflorantes na regiăo estao representadas predominantemente pelo Complexo Xingu (indiviso), de idade arqueana, que constitui o embasamento polimetamórfico, composto principalmente por rochas granitóides deformadas. Rochas supracrustais estăo representadas pel a seqüęncia vulcano-sedimentar Comemoraçăo (possível greenstone belt), Formaçăo Iriri (vulcânicas ácidas a intermediárias, associadas a sedimentos), granitos anorogęnicos (tipo Teles Pires, Serra da Providęncia e Rondonianos) e sedimentos continentais e/ou marinhos epicontinentais (Grupo Beneficente, formaçőes Dardanelos, Pacaás Novos e Pi menta Bueno). Derrames básico-alcalinos estăo representados pelas formaçőes Nova Floresta e Arinos.
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La Correccion Isostatica Aplicada a la Exploracion de Petroleo
More LessEn este trabajo se hace un analisis de la información gravimétrica respecto a las elevaciones topográficas, tanto en la localidad donde se realizó la exploración así como en sus alrededores, de este analisis se concluyó que en el área existe el problema isostático, por tal motivo se procedió a efectuar la corrección con el propósito de eliminar las anomalías profundas. Se utilizó el modele de Airy-Heiskanen para diferentes profundidades del parámetro T, encontrándose que la más apropiada para esta área fue de 20 Km.; después de aplicar la corrección por isostasia se observó que el mapa residual isostático conserva y resalta algunas anomalfas que son de interés para los objetivos exploratorios.
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O Problema da Representaçăo do Campo de Gravidade na Regiăo Amazônica: Avaliaçăo de Alternativas
More LessA regiăo amazônica e um grande segmento Sul-americano que contém importantes unidades geológicas e tectônicas, entre elas, as bacias sedimentares do Acre, Parecis-Alto Xingu, Alto Tapajós, Amazonas, Marajó, e Boa Vista, e os crátons Rio Branco e Arnazônico. Apesar do grande interesse científico que essa regiăo desperta e das suas potencialidades para a exploraçăo de petróleo e rninério, pouco se conhece nela em termos de Geofisica. A grande extensăo e as peculiaridades geográficas da regiăo dificultam os levantamentos geofisicos básicos, Atualmente, os levantamentos aerogeofísicos e as missőes geofísicas por satélite, estăo iniciando uma nova fase na coleta de dados dos campos gravitacional e magnético da Terra. A análise das alternativas para a representaçăo do campo de gravidade na amazônia, apresentada neste trabalho, sugere a combinaçăo da gradiometria por satélite com a aerogravimetria, numa estratégia de médic a longo prazo.
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Mapa de Anomalias Bouguer e Modelagem Gravimétrica do SuI de Minas GeraisAP A
O Instituto de Geociencias e Cięncias Exatas (IGCE) da UNESP, iniciou, a partir de 1991, um levantamento integrado de gravimetria e geologia estrutural no sul de Minas Gerais numa área de aproximadamente 250 km x 200 km, delimitada pelos paralelos 21° 00' S e 23° 00' S e pelos meridianos 44° 30' W e 46° 30' W, com os seguintes objetivos principais:
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Modelagem da Espessura Crustal Oceânica no Atlântico SuI Ocidental entre 18°S E 40°S
More LessObjetivando uma melhor compreensăo dos processos de formacao e evoluçăo da crosta oceânica adjacente a margem continental sul sudeste brasileira, procedemos a modelagem tridimensional da estrutura crustal oceânica para essa regiăo. Na área de estudo se encontra uma das maiores cadeias assísrnicas do Oceano Atlântico, a Elevaçăo do Rio Grande. Acreditase que essa cadeia assísmica tenha gęnese associada a um hotspot localizado sobre a cordilheira Meso-Atlântica a 84 Ma.
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Digital 5' x5' Gravity Maps of the Săo Francisco Craton, the Marginal Fold/Thrust Belts and Contiguous Continental Margin/Oceanic Basins
Authors Eder Cassola Molina, Naomi Ussami and Yára Regina MarangoniDigital 5'x 5' Free-air, Bouguer and residual anomaly maps are presented based on over 45,000 gravity stations collected by different institutions and stored at IAG-USP gravity data bank. The area defined by 54-32°W and 8-24°S covers the Săo Francisco craton and its marginal fold/thrust belts, the conterminous eastern continental margin and oceanic basins. The gridded map was obtained using the least-squares collocation technique. The nominal resolution of the present maps is 5' in areas with good data coverage. In the oceanic region, conventional marine gravity data were processed with satellite altimetric data obtained by GEOSAT Exact Repeat Mission. The gridded anomaly maps, together with the stations istribution are made available in digital form. An improvement in data coverage is expected as new data are collected and contributed to the data-bank, so that continuous update of the present model will be possible. For obtaining these data and more information, connect http://www.iag.usp.br/geofisica/sfcraton.html.
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Modelagem Gravimétrica Direta e Inversa Aplicada no Cálculo da Espessura Sedimentar da Sub-Bacia de Cedro e do Testemunho Sedimentar de Fátima, Nordeste do Brasil
A Sub-bacia de Cedro e o Testemunho Sedimentar de Fátima fazem parte do conjunto de pequenas bacias interiores do Nordeste do Brasil (Cordani et al. 1984) (Fig. 1), que implantou-se regional mente no contexto de rochas pré-cambrianas pertencentes a Província Borborema. O embasamento é composto por xistos, gnaisses e granitos das faixas Piancó-Alto Brígida e Pajeú-Paraíba, entre os lineamentos de Patos e Pernambuco, no Domínio da Zona Transversal. Litoestratigraficamente as áreas sedimentares possuem uma coluna estratigráfica simples constituída no máximo por tręs formaçőes exceto as grandes bacias Araripe, Potiguar, Recôncavo-Tucano-Jatobá, nas quais está preservada a coluna estratigráfica completa.
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Modelagem Gravimétrica "2 D" através do Complexo Granítico Quixadá-Quixeramobim, Regiăo de Algodőes e Uruquę, Ceará
Authors R. Mariano, G. Castelo Branco and Mauro Lisboa SouzaA regiăo em estudo localiza-se na porçăo central do Estado do Ceará sendo caracterizada por uma unidade geotectônica do Proterozóico Superior (Complexo Granítico) encaixada em rochas do Proterozóico Inferior (Complexo Gnáissico-Migmatítico) e Proterozóico Médio (supracrustais do Grupo Ceará). A regiăo e de geologia extremamente interessante sendo caracterizada por seis Fácies graníticos (Muxuré Velho II, Uruquę, Água Doce, Muxuré Velho I, Serra Branca, Muxuré Novo) e por estar delimitada por duas zonas de cizalhamento dúcteis em íntima relaçăo com o mecanismo de posicionamento das rochas graníticas (Torquato et al., 1989). A regiăo do Complexo Granítico envolve uma área de aproximadamente 1.800 Km2.
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Inversăo Gravimétrica 3D para Superfície entre Dois Meios Homogęneos, com Aplicaçăo ŕ Depressăo Circular de Colônia (Săo Paulo/SP)
Authors Gino Francisco dos Passos and Wladimir ShukowskyEm gravimetria, a determinaçăo da distribuiçăo de densidade de massa em subsuperfície correspondente a uma dada anomalia observada é um problema inverso contínuo mal posto, uma vez que existem infinitas distribuiçőes de densidade de massa correspondentes ao efeito observável em superfície.
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Gravity Signature of the Morungaba Granitoids
Authors Wladimir Shukowsky, Silvio R.F. Vlach and Kellerman A. NovaesThe analysis of a gravity survey of the Morungaba granitoids (eastern Săo Paulo State) is presented here as a performance test of the use of a fast and inexpensive gravity surveying technique in the assessment of structural characteristics of granitoids. It is shown that by the use of adequate data processing applied to relatively low precision gravity data acquired over a rugged terrain it is possible to identify anomalies with a few milliGal amplitude, which are suitable for structural interpretation.
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Compartimentaçăo Tectônica do Sudeste do Brasil e suas Relaçőes com as Zonas de Fraturas Oceânicas
Authors E.C. Alves, M. Sperle, S.L.M. Mello and S.E. SichelA compreensăo da interrelaçăo das estruturas geológicas do oceano profundo com as províncias geológicas localizadas na borda continental emersa do sudeste do Brasil e o significado tectôni co de sua topografia, tęm que estar integrado a geologia da margem continental e da bacia oceânica adjacente. O estudo das zonas de fraturas oceanicas (ZFO'S) desde a cordilheira mesoceânica até a margem continental tęm revelado que estas estruturas desempenham um importante papel no estudo do condicionamento tectônico da borda continental emersa e da margem continental adjacente. Neste contexto será feita uma análise dos principais estudos realizados na bacia oceânica do Sudeste do Brasil, onde procurar-se-á mostrar possíveis relaçőes entre estruturas do bordo continental e as ZFO'S na compartimentaçăo tectônica do sudeste brasileiro.
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Mapas Gravimetricos de Goias e Distrito Federal
Authors Yára R. Marangoni, Miguel D. Gago and Maria Cristina LobiancoOs anos 90 estăo sendo marcados pela comunicaçăo eletrônica, Revistas, jomais, artigos, mensagens, arte, tudo está na rede de computadores e tudo pode ser visto por milhóes de pessoas. Assim, as Geocięncias năo podem mais se limitar a produzir mapas coloridos em papel a urn custo alto e produçăo limitada, devendo caminhar rapidamente para a cornunicaçăo digital. Boa parte dos mapas ja săo confeccionados eletronicamente, mas a sua veiculaçăo ainda é fortemente dependente do papel. O que se pretende neste projeto é produzir mapas de anomalias gravimétricas do estado de Goias e do Distrito Federal, a serem distribuídos tanto por via disquete, como atraves da Internet.
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Métodos Potenciais na Bacia do Paraná: Alguns Aspectos a Considerar na sua Interpretaçăo.
More LessCom os trabalhos de prospecçăo de petróleo desenvolvidos pela Petrobras na última década na Bacia do Paraná, novos dados de métodos potenciais foram incorporados ao acervo da Bacia, destacando-se os levantamentos aeromagnetométricos (fig.OI), gravimétricos e perfis de magnetometria e gravimetria ao longo de linhas sísmicas.
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Interpretaçăo Preliminar de Dados Gravimétricos da Bacia de Săo Paulo (Brasil) e Regiăo Circunvizinha
More LessNesse trabalho săo apresentados os resultados preliminares da análise de dados gravimétricos no quadrilátero situado entre as longitudes de 47°09'W e 45°58'W e as latitudes de 23°08'S e 23°57'S, abrangendo uma área total de cerca de 11 mil Km2 na qual se situa a Bacia de Sao Paulo e regiăo circunvizinha.
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Gravity Imaging the Continental Margin of Brazil
Authors Luiz F.S. Braga, J.A. Bach de Oliveira, J.D. Fairhead and D.C. MantonPetrobras has systematically investigated the continental margin of Brazil using gravity surveys since the early 80's. These surveys include sea bottom gravity measurements, detailed 2D surveys and more regional surveys extending into deep water. This effort resulted in a near total coverage of the margin.
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