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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 - 120 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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