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2nd International Congress of the Brazilian Geophysical Society
- Conference date: 27 Oct 1991 - 01 Nov 1991
- Location: Bahia, Brazil
- Published: 28 October 1991
1 - 50 of 203 results
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and Vertical Resolution
Authors P. Andrieux, K. Vozoff, K.M. Strack and A. HordtDuring the past decade electromagnetic techniques have been applied and evaluated for exploration under basalt cover around the world. We show several case histories from the US, Europe and Australia to illustrate a new exploration concept which involves several different electromagnetic techniques and seismic for an optimum resolution of the subsurface structure hidden by near surface basalt covers and / or carbonates.
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Elemental Composition of Aerosols from the Antarctic Peninsula
Authors E.B. Pereira, A.L.M. Loureiro and M.B.V. VasconcellosAerosols collected during 1986/87 in Ferraz station (620 S, 58°W), Antarctic Peninsula were analysed by NAA techniques. Sodium and clarine derived from sea spray were the predominant component in the aerosols. Soil dust components AI and Mn were about 30 times larger than at the South Pole due probably to a combination of the proximity to the S. America and to local sources. A marked decrease in concentration followed by the same change in the atmospheric radon and wind velocity from 1986 to 1987 were also observed. Zn and Sb presented a clear minimum concentration during winter while Br and V displayed a steadily decrease with time. Fe, Ca, Au, La, and Sc did not present any clear trend during the year.
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The April 1991 Iraçuba (CE) Seismic Event and its Aftershocks
Authors E.P. Fernandes, R.K. Ribeiro, M.L.B. Blum, M.I.R. Cunha, A.E. Neves and G.A. MelloA sequence of seismic events occurred near Taperuaba, district of Sobral-CE, following the main event of April 19 that reached magnitude 4.8 mb, the second largest ever recorded in this state. A cooperation effort among the Seismological Observatory, the Special Secretary of Civil Defense and the Municipality of Sabral was done to study these events within the period April 20 to 24. Macrosseismic survey was also realized and the maximum intensity found was VI (Modified Mercalli Scale). The data recorded in four local stations were analyzed and appointed a low activity confined in a narrow zone with NW-SE linear trend, and depths between 7 and 11 km. A composite focal mechanism, using the main event and its aftershocks, indicates a dextral strike-slip motion with small component of normal faulting. The NW-SE P axis is inconsistent with the roughly E-W stress field orientation for the northeastern region and may indicate local process related to the Potiguar Basin.
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Os Sismos de Monsuaba (RJ), Ocorridos em Dez./88 a Fev./89
Authors J. Berrocal, C.M. Fernandes, A.A. Bueno, N. Seixas and A. BassiniDurante as meses de dezembro de 1988, janeiro e fevereiro de 1989, ocorreu uma série de microtremores na localidade de Monsuaba, distrito de Angra dos Reis no Estado do Rio de Janeiro. Esses eventos causaram certa preocupaçăo pela proximidade da zona epicentral com os depósitos de petróleo e outras instalaçőes que a Petrobrás tem nessa área e com a Usina Nuclear de Angra dos Rais, localizada aproximadamente 24km a oeste de Monsuaba.
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The Goias Seismic Zone - A New Approach
Authors E.P. Fernandes, M.L.B. Blum and R.K. RibeiroA seismic zone in Goias state was described by several authors, trying to relate it with geological features, although unsuccessfully due to the lack of informations. We use here the first event with mechanism solution occurred in Porangatu in March 1991. The focal mechanism showed a 30° dextral slip fault with P axis orientation ENE -WSW. This orientation and the strike-slip motion accords to the Transbrasiliano lineaments associated to collisional process occurred during Late Proterozoic. Association with gravimetric and magnetic survey is, also analyzed.
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The Rio Grande do Sul Offshore Earthquake, February 12th 1990
Authors E.P. Fernandes, M. Assumpçăo and J.A.V. VelosoThe occurremce of earthquakes offshore the Rio Grande ela SuI state, in Brazil, are uncommon. This last one, from February 12, 1991, reached magnitude 5.2 mb and was recorded by several stations world-wide. A focal depth of 28 km was determined using pP and sP phases from digitized seismograms from Europ,Africa and North America. A focal mechanism solution, using P-wave first motion and the negative polarity of pP phases to determine the focal planes, showed a 32° reverse faulting, 40° dip, and P axis oriented WNW-ESE, normal to coastal alignement. The depth and this compression direction suggest a legional spreading stress due to the different densities of continental and oceanic lithosphere that can cause reverse faults resulting from the combination of integrated plate driving forces, "ridge-push" and basal drag.
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Determinaçăo de Magnitudes de Sismos Regionais Registrados na Estaçăo RDJ no Intervalo 1923 - 1932
By A. BuenoA magnitude sísmica e um parâmetro útil nos estudos de sismicidade, que permite avaliar o nível e as características da atividade sísmica associados aos processos geodinâmicos de uma regiăo. Os estudos de sismicidade requerem o conhecimento completo da série histórica de sismos ocorridos numa regiăo. No caso da America do Sul, o catálogo é incompleto, principalmente para as tręs primeiras decadas deste seculo, pois contem somente as sismos de maior magnitude que eram detectados pela rede internacional de sismógrafos, composta de cinco estaçőes neste continente (Gutenberg & Richter, 1949 e Presgrave el; at, 1985). Com a finalidade de reduzir a falta de dados de magnitude dos católogos sismológicos suI-americanos, foram calculadas, neste trabalho, as magnitudes Ms (onda; superficiais) e mb (ondas intemas), utilizando as leituras de amplitude e período de sismogramas e de boletins sismológicos correspondente ao intervalo 1923-1932, de sismos que ocorreram na regiăo delimitada pelos paralelos 30°N e 70°S e pelos meridianas 0° a 120°W..
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Sismicidad Superlicial en Consata - Mapiri, Altiplano Norte de La paz
By A.J. Vega B.El Altiplano de Bolivia con una altura en promedio de 3800 metros se extiende entre las Cordilleras Occidental y Oriental de Los Andes, figura 1, formando un gráven delimitado al Oeste por la falla-lineamiento Huancarani y al Este por la falla Uyuni-Poopo, y una serie de fallas paralelas de dirección Andina (Claure, 1983). En esta plataforma de un espesor considerable, 15 km de materiales del cretácico y terciario (Newell, 1949), la actividad sismica es notoriamente de profundidad intermedia (70 a 300 km) relacionada con la subducción de la Placa de Nazca.
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Estudio de Q Coda para Sismos Locales en San Juan, Argentina
Authors L. Giroldi, M.H. Millán and F. MingoranceEl factor de calidad de la atenuación Q está estrechamente relacionado con propiedades geológicas y geofísicas de la corteza tales como vulcanismo, edad de la actividad tectónica, variación de flujo de calor, conductividad eléctrica y velocidad de las ondas sísmicas. Los diversos estudios del factor de calidad de la atenuación obtenidos a partir del decaimiento de la coda sísmica (Qc), muestran que valores altos de Qc corresponden a zonas tectónicamente estables, mientras que valores bajos de Qc están relacionados con zonas tectónicamente activas (Singh y Herrmann, 1983). Esto muestra que Qc es un parámetro que refleja la actividad tectónica de una región.
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Seismotectonics of Venezuelan Andes
Authors R. Esteves and C. SchubertThe geology, tectonics and seismicity of the Venezuelan Andes are Briefly reviewed. The possible origin and distribution of rocks of major geologic time intervals are discussed. Most of the rocks described are marine sediments, which have been metamorphosed and ocasionally intruded or extruded by granitic and basaltic rocks during periods of orogeny. During the Quaternary the region was affected by glaciation in its higher parts, while in the lower parts uplift- and climatic changes discccted alluvial sed.ir:nents to form. spectacular terraces.
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O Brasil e a Nova Lei do Mar
Authors A.T.M. Albuquerque and J.A.R. MarquesA Convençăo das Naçőes Unidas sobre o Direito do Mar (CNUDM), resultado de trabalho profícuo e extenso da comunidade internacional, e que contou com participaçăo efetiva e atuante do Brasil, foi aberta ŕ assinatura em 10 de dezembro de 1982, em Montego Bay, Jamaica. Nosso país assinou a CNUDM naquela mesma data, junto com outros 118 países. Mais recentemente, em 22 de dezembro de 1988, o Brasil depositou seu instrumento de ratificaçăo, e hoje a CNUDM já conta com um total de 45 ratificaçőes das 60 necessárias ŕ sua entrada em vigor (Tabela I).
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A Participaçăo da PETROBRÁS no Plano de Levantamento da Plataforma Continental Brasileira (LEPLAC)
By J.M. SouzaA Convençăo das Naçőes Unidas sobre o Direito do Mar (CNUDM), em seu Artigo 76, ao mesmo tempo que garante ao Estado costeiro direitos de soberania sobre a exploraçăo dos recursos naturais - vivos e năo-vivos - até a distânciade 200 milhas náuticas (370 km) a partir das linhas de base, faculta, ao Estado costeiro, a extensăo dos direitos de exploraçăo dos recursos minerais até o limite exterior da sua Plataforma Continental - na sua conotaçăo jurídica e năo geológica -, desde que satisfeitos os critérios e condiçőes estabelecidas na citada Convençăo.
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Seismic Attributes in Borehole Full Waveform Sonic Logging
By S. KnizeIntegration of surface seismic and borehole sonic data processing and display techniques provides a means for a detailed description of rock properties such as dipping boundaries, fractures, and permeability. The seismic attributes analysis and the color-coded displays which are adapted for investigation of full wave sonic (FWS) responses of formations reveal additional FWS characteristics helpful in describing the formation near the borehole. In this application the seismic attributes have distinct geophysical meanings Which are specifically pertinent to FWS logging and are called Instantaneous Waveform Characteristics, or IWC. The color-coded displays are called colorgrams. The IWC displayed in the colorgrams are also quantified, plotted as log curves, and interrelated to give useful geophysical parameters such as the quality factor (Q) of the formation. The application of the IWC analytical method is demonstrated with colorgrams from four different wells. All these examples illustrate how the IWC method, enhanced with colorgrams, broadens our analytical understanding of FWS well log data and assists in geological interpretation. It provides valuable information useful for the integration of borehole sonic and seismic data as well as for the development of cross-borehole tomography.
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Projeto LEPLAC: Oficina de Manutençăo e Reparo de Cabos Sismográficos dos Nocs. Alte. Câmara e Álvaro Alberto
More LessUma oficina para reparo e manutençăo de cabos sismográficos foi montada nas instalaçőes da Diretoria de Hidrografia e Navegaçăo (DHN), na Ponta da Armaçăo, em Niterói-RJ. O projeto, que foi desenvolvido por técnicos, do Grupo Especial da Antártica e Delimitaçăo da Plataforma Continental Brasileira (GEACON) , do Departamento de Exploraçăo (DEPEX) da PETROBRÁS, contou com recursos financeiros da Secretaria da Comissăo Interministerial para os Recursos do Mar (SECIRM) e tem par objetivo reparar problemas existentes nos cabos sismográficos dos Navios Oceanográficos Almirante Câmara, e Almirante Álvaro Alberto, utilizados nos levantamentos geofísicos de Plataforma Continental Brasileira (Projeto LEPLAC) e em levantamentos no Continente Antártico, decorrentes de convęnio firmado entre a PETROBRÁS e a MARINHA DO BRASIL.
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Projeto LEPLAC: Interpretaçăo Preliminar dos Dados Sismicos e Gravimétricos do Prospecto LEPLAC-I
Authors M. do Carmo, G. Severino and B.S. GomesO levantamento geofísico do Prospecto LEPLAC-I situa-se entre as latitudes 21°S e 31°S e as longitudes 33°W e 42°W (Fig. I) e abrange, principalmente, as regiőes da elevar;ao continental e da planície abissal da margem continental sudeste brasileira. Nesta etapa do trabalho procurou-se mapear as principais feiçőes regionais e dimensionar a espessura do pacote sedimentar. A modelagem gravimétrica de algumas linhas visou esclarecer a problemática do limite crosta continental/oceânica nesta regiăo.
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Regional Setting of the Bransfield Basin - Antarctic
Authors L.A.P. Gambôa, S.L.S. Barrocas and P.R. MaldonadoAs part of the Brazilian Antarctic Program (PROANTAR) , a team of PETROBRÁS geologists and geophysicists conducted two marine geophysical surveys and a geologic field season aiming to contribute to the understanding of the geologic evolution of the region off the northern tip of the Antarctic Peninsula. The broad structure of the region was investigated by two marine geophysical expeditions held in the austral summers of 1987 and 1988, between Adelaide and Elephant islands (Gamboa et aI, 1990). The geological field work took place in January of 1989 in the Melville Peninsula, at the northern tip of King George Island, South Shetland Islands, where outcrop sediments probably deposited during the early stages of the formation of the Bransfield Basin (Moby Oick Group) (Barrocas et aI, 1990; Figure 1 and 2).
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A Geologia Marinha no Departamento de Exploraçăo da PETROBRÁS: Aplicaçăo Năo Convencional da Sísmica Multicanal Digital
Authors C.H.B. Oliveira, D.D. Castro and M.A.G. GuimarăesOs primeiros estudos da plataforma continental brasileira visando a pesquisa de hidrocarbonetos ocorreram em 1957. Porém, a exploraçăo na porçăo marítima das bacias costeiras somente pode ser iniciada na década de 60 com a chegada de equípamentos adequados. Desde entăo, grande quantidade de dados exploratórios (sismica de reflexăo, gravimetria, magnetometria, amostras de calha, testemunhos, perfis elétricos de poços) tęm sido coletada, hoje integrando o acervo do Departamento de Exploraçăo da Petrobras (DEPEX).
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Análise Sismoestratigráfica no Terciário da Bacia de Campos
Authors J.A. Rici and M.R. BeckerA Bacia de Campos localiza-se na margem da coasta brasileira, no litoral do Estado do Rio de Janeiro, abrangendo uma área de aproximadamente 100.000 km2 até a cota batimétrico de 3.400 m (fig 1). O Esforço exploratório da PETROBRÁS nesta bacia iniciou nofinal da década de 60. Em 1974, ocorreu a primeira descoberta (Campo de Garoupa) e desde entăo tem sido alvo de intensos levantamentos sísmicos, na busca constante de novas acumulaçőes. Já foram perfurados em torno de 500 poços exploratórios e cerca de 300 poços de desenvolvimento.
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Deep Seismic Survey of Brazilian Passive Margin Basins: The Southeastern Region
Authors W.U. Mohriak and M.A.L. LatgéA few deep seismic reflection profiles, using conventional acquisition equipment, were shot in Campos Basin in the last decade. These lines (Seismic Survey 203) were processed to 10 s TWT (two-way travel time) using standard processing techniques. Early in 1990, Petrobrás approved an extensive program of deep seismic reflection lines offshore the Brazilian Atlantic margin (Seismic Survey 239). The lines of this program were shot in the first semester of 1990 using special equipment onboard the seismic ship GECO ALPHA. In the Southeastern region of Brazil (from Espírito Santo to Rio Grande do Sul), the total coverage amounts to 4884 km. Briefly and in a preliminary form, some of the main features to be investigated by this survey will be discussed. Processing, which includes migration to 18 s TWT, was initiated in the second semester of 1990 and will be completed by 1991.
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Esboço Estrutural Brasil - África
By E. LiandratO poster apresentado é um esboço estrutural da África e da América do SuI (com a exclusăo dos países andinos do Peru até a Venezuela), com estes dois continentes colocados em suas supostas posiçőes relativas originals.
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Variation in the Thermal Gradient, Degree and Melt Segregation Pressure and Axial Depth Along the South Atlantic Ridge
Authors S.E. Sichel and H. SigurdssonSeveral lines of evidence suggest a cooler upper mantle southward beneath the South Atlantic Ridge. Among them is the distribution of hotspots in relation to the ridge, the decrease of eruption temperature southward, and behavior of trace elements wich are incompatible in basaltic melt. Further evidence includes the deepening of the ridge axis, and general decrease in geoid anomaly (Kane and Hayes, 1989) which could be a consequence of a higher subsidence rate in this direction due to a cooler mantle. The modeling of upper mantle cooling toward the south could be a consequence of influence of hotspots. It is assumed here, based on the fact that a linear relation between hotspots and their distance from the ridge axis only exists for those hotspots located at less than 800knl from the ridge.
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Reconhecimento Tectônico e Estratigráfico da Bacia Sergipe-Alagoas em Águas Profundas
Authors C. Pontes, F.C.C. Castro, J.J.G. Rodrigues, R.R.P. Alves, R.T. Castellani, S.F. Santos and M.B. MonisNos últimos dois anos, os levantamentos sísmicos realizados na Bacia Sergipe-Alagoas concentraram mais da metade de seus esforços em aguas profundas. Paralelamente, foram também levantados dados gravimétricos e magnetométricos, gerando uma malha de razoável densidade, cobrindo toda a extensăo da bacia em lâminas d'água profunda (até aproximadamente 4000 metros). A interpretaçăo lntegrada apoiada nesses novos dados, caracterizados por uma qualidade sismica superior aos de águas rasas tem permitido reconhecer os contornos do arcabouço estrutural e as relaçőes estratigráficas da área. A luz desses dados, é realizada uma avaliaçăo geológica e do potencial expioratório desta porçăo da bacia, destacando o reconhecimento e delimitaçăo de leques turbidíticos e diápiros salinos, além de corpos vulcânicos intrusivos em zona de transiçăo crustal correlativas ŕs feiçőes tectônicas globais.
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Analise lsostática na Regiăo dos Montes Submarinos de Pernambuco
Authors M. Sperle, S.L.M. Mello and L.R. GomesA margem continental nordeste brasileira e fundo oceânico adjacente, entre os estados da Bahia e Pernambuco, vem sendo nos últimos anos amplamente investigada por pesquisadores do Laboratório de Geologia Marinha da Universidade Federal Fluminense (LAGEMAR/UFF). Estes estudos tęm sido desenvolvidos com base em dados oceanográficos coletados durante o Projeto REMAC (DNPM, CPRM, PETROBRÁS, DHN, CNPq) e oProjeto CENTRATLAN (Naval Research Laboratory, DHN, LAGEMAR), visando sobretudo o entendimento da evoluçăo deste setor da margem continental e do assoalbo oceânico adjacente.
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Interpretation of Borehole Gravity in the Presence of Anomalous Vertical Gradients
Authors F.S. Moraes, C.A. Gonçalves, D.P. O‘Brien, H.B. Evans and F.G. ClutsonThe first borehole gravity surveys to be carried out in Brazil were done as part of a joint program between the Federal University of Pará, the USGS and Petrobras (CENPES / DEPEX / DENEST) in the state of Sergipe, in parts of the Sergipe-Alagoas Basin, during October and November of 1990 (Gonçalves, 1991). A total of three producing wells were surveyed to investigate the formation density properties of a producing basal conglomerate zone of Cretaceous age overlying a fractured pre-Cambrian Basement. The basement is shallow, ranging from about 500 to 800 meters below sea level. The interpretation of the borehole data is complicated by the fact that the basement complex, and an overlying evaporite sequence of Cretaceous age, is highly folded and faulted. As the borehole gravimeter actually penetrated both the evaporites and the basement, gravitational effects caused by the differential relief of the basement terrain and evaporite layers contributed a significant amount to the downhole gravity signal. The irregular surfaces of the both the basement and evaporite sequences were modelled using a tesellation of triangular facets and a L1 norm on the borehole gravity data. Computation of the gravitational field of each facet employed an algorithm for a polyhedral body (Pohanka, 1988). The tesselation was automatically generated (Watson, 1982). The resultant three dimensional model was used to compute the borehole Bouguer field, and this was subtracted from the observed borehole data, to yield the anomalous Bouguer field which was used to correct densities estimates.
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Sismo-Estratigrafia da Porçăo da Bacia Oceânica do Brasil Adjacente ao Platô de Pernambuco e sua Relaçőes com o Plato e a Bacia Pernambuco - Paraíba
Authors M.P.A. Costa, E.C. Alves and E.A. CostaOjeda (1976), com base em estudos de integraçăo litorânea na bacia Pernambuco- Paraíba, previu a presença, costa a fora de sequęncias sedimentares basais cretácicas, que, embora ausentes nesta regiăo, ocorrem na plataforma continental atual na maioria das bacias marginais brasileiras. Kowsmann e Costa (1976) tentaram uma correlavăo preliminar entre as sequęncias da bacia e do plató de Pernambuco, que toda via ficou prejudicada devido a escassez de seőes sísmicas sobre o plata.
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Maré Gravimétrica e Tectônica na IIha de Fernando de Noronha: Resultados Preliminares
Authors S.R.C. Freitas, M.S.M. Mantovani, W. Shukowsky, P. Melchior, B. Ducarme and M.V. RuymbekeA formaçăo do arquipélago de Fernando de Noronha é relativamente recente e a compreensăo dos possíveis processos tectônicos associados com sua origem, săo importantes para o estudo da evoluçăo tectônica a nível global. As evidęncias de sua formaçăo intraplaca pela passagem de um "hotspot" que possivelmente esteve associado com o processo de abertura do Oceano Atlântico e a sua locaçăo em litosfera oce6anica em regiăo de fluxo térmico anômalo sem vulcanismo recente, săo aspectos que tęm motivado estudos geofísicos na regiăo.
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Interpretation of a Deep Seismic Reflection Profile in the Pará Maranhăo Basin
By R.P. AzevedoDeep seismic profiling has been a powerful source of geophysical data which has added to improve our understanding of how basins develop. The geological interpretation of a deep seismic profile integrated with gravity data has allowed to suggest. that the development of the Pará-Maranhăo Basin was the result of heterogenous thinning of the lithosphere.
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Aspectos Estruturais e Sismoestratigráficos da Secăo Neo-Cretácea e Terciária da Bacia de Barreirinhas - Aguas Profundas
Authors J.L. Caldeira, L.F.C. Coutinho and M.F.B. MoraesA Bacia de Barreirinhas localiza-se na regiăo central da Margem Equatorial Brasileira. A área estudada compreende as porçőes de talude e bacinal, entre as cotas batimétricas de 100 ate 3000 m (fig.l). Neste trabalho propőe-se um modele tectono-sedimentar para a seçăo do Cretáceo superior e Terciário da Bacia de Barreirinhas com implicaçőes na pesquisa de hidrocarbonetos. Na execusăo do trabalho utilizaram-se cerca de 6000 km de linhas sísmicas levantadas pelas equipes 73, 94, 143, e 239 e os dados de poços perfurados na plataforma. A partir da amarraçăo dos sismogramas sintéticos as linhas sísmicas, associados aos dados de paleontologia disponíveis, foram identificadas e rastreadas as principais reflexőes até a regiăo de águas profundas.
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Detecçăo e Quantificaçăo de Parâmetros do Tipo Montes Submarinos
More LessVários autores, relacionam a forma geométrica de montes submarinos, com princípios hidráulicos. Lacey (1981) e outros propuseram que a forma dos vulcőes é determinada pela resistęncia hidráulica do fluxo de magma pelo edifício vulcânico. Esse edificio é formado por uma superfície de potencial hidráulico constante. Se o edificio fica muito largo, em sua base, erupçőes no topo văo ocorrer para aumentar sua altura, se fica muito alto, erupçőes no flanco văo alargá-lo. Pela razăo de se admitir que os edifícios vulcânicos tem uma permeabilidade uniforme, uma soluçăo semelhante foi obtida para a forma dos vulcőes.
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Como Otimizar os Levantamentos Sísmicos 3D Terrestres
More LessOs levantamentos sísmicos tridimensionais-3D terrestres em programas menores que 100 km2 săo, em sua grande maioria, mais baratos ou com custos equivalentes ao preço de um poço exploratório. Este fato permite duas análises. Primeira, por apresentarem melhor resoluçăo do que os levantamentos sísmicos bidimensionais-2D, os programas 3D permitirăo confeccionar mapas sismicos/geológicos mais confiáveis, aumentando, conseqüenternente, a possibilidade de elevaçăo dos índices de sucesso exploratório e explotatorio. Segunda, por envolverem valores da ordem de centenas de milhares ou milhőes de dólares, diretamente decorrentes dos parâmetros de aquisiçăo, os programas 3D devem ser analisados criteriosamente simulando-se várias possibilidades de parametros de aquisiçăo, no sentido de otimizar·se a relaçăo custo/benefício compatibilizando-a com a finalidade exploratória do programa 3D. Neste trabalho, procura-se estabelecer as principais etapas que normalmente săo seguidas na determinaçăo dos parâmetros de campo nos programas 3D terrestres, enfocando alguns procedimentos para otimizá-Ios sem comprometer a qualidade dos dados. Salienta-se que alguns destes procedimentos Ievam em consideraçăo o processamento sísmica utilizado atualmente, podendo portanto, no futuro, serem reanalisados.
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Algebraic Reconstruction for 3D Seismic Quality Control
By F. BaixasDerived from medical applications, algebraic reconstruction techniques (ART) were first implemented by geophysicists for crosswell tomography (Bois et al., 1972; Bishop et aI, 1985; Paulson- et aI, 1985; Ivansson, 1985). 2D refraction cases were recently studied (Krajewski et aI, 1989; Ruhl and Luschen, 1989), using a particular ART variant known as Simultaneous Iterative Reconstruction Technique (or SIRT), whereby each model bin slowness is modified when all raypaths pertaining to that bin have been taken into account.
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Levantamento Sísmico 3D de Alta Resoluçăo - Mina de Postássio de Taquari - Vassouras (SE), Brasil
Authors C. Pontes, P.R.S. Johann, P.R. Camargo and R.T. CastellaniO levantamento sísmico tridimensional (3D) na mina de potássio de Taquari-Vassouras, Bacia de Sergipe-Alagoas, foi o primeiro trabalho de aquisiçăo sísmica 3D terrestre de alta resoluçăo realizado pela PETROBRÁS. Os dados foram levantados por equipes da própria empresa, ES-26 e Es-27, em Jan/1989. Anteriormente, em 1977, um programa 2D foi levantado com a mesma finalidade, pela equipe contratada Es-49. O objetivo pioneiro constituia verificar a contribuiçăo de um 3D na interpretaçăo da sequęncia evaporítica, da zona mineralizada, utilizando-a como suporte no desenvolvimento da lavra dos sais de potássio. Informaçőes como presença de aquíferos, falhas, atitudes, profundidade do horizonte porfador do minério, sua espessura, definiçăo de zonas de colapso de sal e estruturas internas, săo fundamentais na conduçăo da lavra, tanto no aspecto econômico, quanto no aspecto de segurança. Na área do 3D existem informaçőes de sondagem e galerias de lavra.
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Análise Comparativa dos 10 Primeiros 3D Terrestres Realizados na PETROBRÁS
Authors P.R.S. Johann and J.F. RosalbaDesde 1988, com o 3D Lagoa dos Martins, Bacia do Espírito Santo, marco inicial dos programas sísmicos tridimensionais - 3D terrestres na PETROBRÁS, ja foram realizados e/ou programados os seguintes programas: - Bacía do Espírito Santo: Lagoa dos Martins (LM). - Bacia Solimőes: RUC/LUC (RL), Igarapé Marta (IMT) e Aeroporto (AE); - Bacia Potiguar: Poço Xavier (PX), Riacho da Forquilha (RFQ) e Falha de Baixa Grande (FBG); - Bacia do Recôncavo: Miranga Norte (MN); - Bacia Sergipe-Alagoa: Taquari-Vassouras (TV); - Bacia de Mucuri: Caravelas (CA); Com a experięcia adquirida no pIanejamento, execuçăo, processamento e interpretaçăo de alguns desses primeiros levantamentos pretende-se, neste trabalho, sumarizar suas características e registrar os aspectos peculiares de cada um, com a finalidade de servir de base de informaçőes para futuros programas 3D terrestres a serem realizados pela PETROBRAS.
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Technique to Obtain High Resolution with Conventional Deep Penetration Multichannel Seismic Survey Equipment
Authors M.A.G. Guimarăes, C.H.B. Oliveira and R.L. LenglerThis paper describes the design and results of a survey made to obtain high resolution seismic data in Marlim and AIbacora fields, continental slope of Campos basin, Brazil. The purpose of the survey was to keep the benefits of conventional digital marine seismic data acquisition and replace some field parameters to improve resolution in the upper part of the section. Processing techniques which are important to enhanced resolution such as deconvolution and migration have been applied to the data. Comparisons with high resolution analog data showed good quality and a significant improvement of lateral and vertical resolution obtained by using these procedures. Consequently, a wealth of knowledge concerning the stratigraphic and structural setting of the area was gained.
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Levantamento Gravimétrico de Poço na Bacia Sergípe/Alagoas
Authors C.A. Gonçalves, F.S. Moraes, H.B. Evans, D.P. O‘Brien and F.G. ClutsonTem sido mostrado por Smith (1950), Rogers (1952), McCulloh (1965 e 1966), Rasmussen (1975), L.Fehr (1983), Black (1986) e outros, que as leituras de gravidade em subsuperficie podem apresentar alem de uma alta precisăo, um grande volume de rocha investigado a partir do poço, gerando como resultado final seguras, medidas de densidade para determinados intervalos de profundidade, além de năo serem sensiveis a problemas como revestimento e desmoronamentos da parede do poço.
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Aquisiçăo Sísmica e Correçőes Estáticas: Relaçőes e Limitaçőes
By R.A. SantosOs levantamentos de sísmica de reflexăo realizados sobre camadas de intemperismo săo, comumente, caracterizados por registros que apresentam distorçőes dos atributos sísmicos dos sinais desejados, causadas, entre outros fatores, por baixas velocidades de propagaçăo das ondas elásticas. As correçőes dessas distorçőes săo obtidas a partir de deslocamentos nos tempos de chegada das reflexőes, que necessitam ser determinados com grande precisăo. Esses valores temporais săo dénominados Correçőes Estáticas e representam uma das etapas mais importantes do processamento de dados sísmicos.
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Borehole Geophysics: New Directions
By P. CarrionOne of the major chalenges in borehole geophysics is reservoir analysis. What information about reservoirs one can get measuring seismic arrivals in boreholes? When we talk about reservoirs, we are trying to obtain knowledge about porosity, saturation, viscosity, anisotropy and permeability. These parameters mainly dictate the reservoir characteristics which are important for oil production. In this presentation, we will discuss a possibility for direct mapping of the Vp/Vs ratio which is an important indicator for the petroleum reservoir. In reservoirs with relatively high porosity in sands, this ratio can be substantially higher than in the surrounding host rocks. The proposed technique is a two step procedure. First, we separate compressional and shear arrivals and then compute compressional and shear wave velocities separately using tomographic inversion of first arrivals.
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Design of Land 3D Surveys for Stratigraphic Purposes
Authors J. Meunier and R. GarottaIt is common practice to record 2D lines with a source and receiver iinterval twice the CDP interval. This presents the double advantage of higher energy and of optimum multiple and radial noise attenuation. In 3D seismic, practival or theoretical schemes are used to define the stacking fold, which as it is much less than a 2D fold, is obtained by low density destribution. Structural requirements, i.e. the anticipated depth and dips of the target, are taken into accoutn in approximately the same way as for 2D seismic in order to define the bin size, shooting distances and migration aperture.
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Steep-Dip Time Migration in Lateral Velocity Media
More LessPoststack 20 seismic depth migration methods have been a common practice for imaging in structurally complex regions. However, a good knowledge of interval velocities is required. An alternative approach to improve the accuracy of the imaging in an inhomogeneous earth is to use a time migration suitable for lateral varying velocity. This paper describes a numerical algorithm to perform a linear transformation of the scalar wave equation in media with varying velocity. We implement a nine-point star pattern finite difference scheme following Zhiming Li (1986; work which overcomes the dip limitations for the conventional paraxial operators and we show through synthetic and real examples the quallty of the imaljing without a proper knowledge of the velocity field
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Phase Screen Method for One-Way Wave Extrapolation
Authors P.A. Costa, R.-S. Wu and M.A.B. BotelhoThe method is based on the symmetric splitting of the wave operator in inhomogeneous media. In each step of calculation the propagation effect and scattering effect call be treated separately. The forward-scattering effect can be equivalented to passing through a phase-screen. At each point of the screen, the wavefront is distorted by the amount corresponding to the integral of slowness perturbations along the propagation path. The method offers a fast algorithm for 3-D wave propagation in inhomogeneous media. In order to check the validity and accuracy of the method, we compare the results by this method for sorne canonical cases, e.g. the acoustic wave scattering by a Gaussian heterogeneity, and by a circular cylinder, with those calculated by finite difference. The results show that the method can calculate the one-way wave propagation with good accuracy and the computation speed is considerably faster than the finite difference algorithm. Besides, the method requires much less storage space, and therefore make it a promissing tool for 3-D wave datum processing.
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Substituiçăo de Camadas com as Integrais de Kirchhoff e Rayleigh
Authors W.P. Gouveia Jr. and A.L.R. RosaO procedimiento de substituiçăo de camadas tem como objetivo retirar os efeitos associados ŕ variaçăo lateral de velocidade junto ŕ superfície, nas reflexőes provenientes das camadas mais profundas. É um procedimento importante, principalmente no processamento marítimo, onde a topografia do fundo do marapresenta-se irregular em muitas situaçőes. Os dados resultantes podem ser submetidos, com melhor desempenho, ŕs técnicas convencionais de empilhamento e de migraçăo em tempo.
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Exact Analytical Solution for the Seismic Tunneling Problem
By K. SchielAn exact analytical solution for the tunneling problem is presented. The canonical configuration consists of two acoustic half-spaces in contact including a source. Classical solutions are based on numerical schemes or on other well known methods like the Cagniard-de Hoop method, the WKBJ method, or the steepest descent method. In common with the Cagniard-de Hoop method and as opposed to the other methods mentioned above, the one presented here doesn't use approximations and is explicit in the space-time domain. Furthennore uniqueness of the solution is established which is not the case with the Cagniard-de Hoop method or any other classical solution known. The Green's functions and the seismograms calculated with the method presented here diverge significantly from that obtained with the Cagniard-de Hoop method.
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Reflection and Transmission of Elastic Plane Waves at the Interface between Anisotropic Media
Authors J.S. Protázio and M. SchoenbergAnisotropy affects the reflection and transmission of elastic waves significantly as soon an the energy propagates over a wide angular aperture. Plane wave reflection and transmission coefficients at an interface between anisotropic elastic media can be expressed explidtly in terms of 'iIupedance matrices", as long as the plane of the interface is a mirror plane of symmetry of both media. For the two dimensional case, plane strain 'quasi-compressional' (qP) and 'quasi-transverse' (qS) waves (associated displacements in the plane of propagation) are uncoupled from the anti-plane strain SH waves (associated displacements perpendicular to the plane of propagation) and the plane of propagation' also must be a mirror plane of symmetry of both media, implying that both media are at least orthorhombic. This case includes the important problem of reflectivity and transmissivity between isotropic and transversely isotropic media. The ultimate aim of amplitude vs. offset (AVO) analysis in this case is the estimation of the density ratio and the elastic constants in each medium on which the reflection and transmission coefficients depend. However, these constants are not equally well resolved. For all models that could conceivably represent the anisotropy in real rock masses, even the anisotropy of the reflecting medium can not be estimated, since each anisotropic medium has its 'quasi-equivalent' isotropic medium which gives reflection coefficients close to those of the anisotropic medium, ovr a wide range of incidence angles.
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Tempos de Trânsito de Ondas Quase Cisalhantes em Meios Heterogęneos Anisotrópicos: Precisăo do Metodo de Perturbaçăo
More LessAté 1977 (Bamford e Crampin, 1977), as observaçőes de meios anisotrópicos se restringiam praticamente a alguns km no topo do manto superior, tanto sob oceanos quanto sob continentes. A partir de 1984, observaçőes de separaçăo das ondas cisalhantes na crosta, fenômeno característico da propagaçăo de ondas elásticas em meios anisotrópicos, se multiplicaram e hoje se reconhece esta característica em quase toda onda cisalhante se propagando nos 10-20 km superiores da crosta (Crampin, 1987). Esta distribuiçăo quase geral de meios com comportamento anisotrópico na Terra justifica o interesse crescente por seu estudo.
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Ray Tracing in Layered Anisotropic Media
Authors J. Costa, M. Schoenberg and D. MillerTwo dimensional ray tracing in layered elastic media is shown to be a simple procedure, whether the Iayers are isotropic or anisotropic, when certain criteria are met, those of 'mild anisotropy'. This implies not that the 'anisotropy parameters' are met, but that the medium behaves so that there is no anomalous polarization, there is no triplication, and any quasi-shear wave is slower than any quasicompressional wave. The two-point scheme uses the fact that all rays from source to receiver, including converted and reflected waves, have a common value of horizontal slowness (Snell's Law). Possible values of horizontal slowness have bounds depending on the specified layers through which the ray passes, and the specified ray type in each layer. Whenever a ray originates from the source with a horizontal slowness within these limits, it will reach the receiver depth, although in general at a point that is horizontally offset from the actual receiver position. When the conditions of mild anisotropy are met, range increases monotonically with horizontal slowness, yielding &. unique, easy to find, horizontal slowness, and its associated ray which arrives at the receiver depth at the correct range. The scheme is analysed in detail for the wave modes in the vertical plane of a transversely isotropic medium or in a plane of symmetry of an orthorhombic medium. In particular, the modes studied are those whose displacements lie in the plane of propagation, the so called qP and qS modes.
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Inversăo em Geofisica de Poço: Um Estudo sobre Ambiguidade
Authors A.B. Buoro and J.B.C. SilvaA interpretaçăo,de formaçőes geológicas complexas a partir de dados de perfilagem de poço pode ser feita utilizando-se toda a informaçăo disponível simultaneamente. Para isto é necessaria solucianar o sistema de equaçőes associado ao modele interpretativo proposto a priori, a que caracteriza um problema inverso em geofisica. Este problema năo tem soluçăo única e é instável uma vez que a quantidade de informaçăo contida nos dados geofisicos é via de regra insuficiente para determinar a complexidade das formaçőes geológicas, o que caracteriza portanto um problema ambíguo.
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True Amplitude Migration
Authors P. Hubral, M. Tygel and J. SchleicherA primary offset reflection of a point source from a smooth reflector within a laterally inhomogeneous velocity earth model is (within the framework of ray theory) defined by parameters pertaining to the reflected ray. The geometrical spreading factor - usually computed along the ray by dynamic ray tracing in a forward modeling approach - can be recovered from traveltime measurements at the surface. As a consequence, offset reflections can be time- and depthmigrated such that the geometrical spreading factor along the offset ray is removed. This leads to a. so-called "true amplitude migration. In this work, true-amplitude timemigrated reflections are obtained by nothing more than a simple Kirchhoff-type weighted diffraction stack, followed, essentially, by a time derivative of the diffraction-stack traces. For small transmission losses of primary offset reflections through intermediate layer boundaries, the true-amplitude time· migrated reflection provides a direct measure of the angle-dependent reflection coefficient at the reflecting lower end of the primary reflected ray. The time-migrated field-can easily be transformed into a depth migrated field with the help of image rays. The theory-described here is similar to the inversion approach derived by Bleistein (1987) based upon fundamental ideas expressed by Beylkin (1985). The similarities and differences will be elaborated.
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3-D Migration to Zero Offset for a Constant Velocity Gradient
Authors W.S. French and W.T. PerkinsDMO processes are designed to move energy to where it would have been recorded by a zero-offset experiment conducted on the datum, For a constant velocity model, the spatial correction is always on a line between the source and the receiver to a position updip from the trace midpoint. and the temporal correction is a shift towards zero time additional to NMO, there being no DMO effects for pure strike shooting. In the case of a non-constant velocity field refraction effects complicate the situation, and the appropriate operator for a given reflection title becomes an areal surface whose characteristics may be counterintuitive; for example, there can be marked "DMO" effects even for strike shooting, with spatial and temporal corrections occurring in a direction opposite to the corrections for dip shooting. In this paper+H182 we derive the MZO (Migration to Zero Offset) operator for a velocity which increases linearly with depth, and we exbibit the surprising and important effects of this operator when applied to data for which the source-receiver orientation is strike to the reflectors.
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Pre-Stack Partial Migration in the Double Radon Domain
Authors J.T. Fokkema and M. VissingaIn this paper we propose an alternative approach for the NMO, DMO, CMP stacking and post-stack migration route. Our approach is based on a high-frequency analysis of the data after a double Radon transform with respect to the true field coordinates. 'It will be shown that the spatial complexity of the data leads to a well-defined topological subdivision in this space. The migrated result is obtained by a single inverse Radon transform. applied in the different subdomains.
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Uma Técnica Mista para Substituiçăo de camada
Authors W.P. Gouveia Jr. and P.L.M. OsórioA presença de interfaces de topografias irregulares próximas ŕ superficie caracterixa uma das situaçőes em que processamentos adicionais săo necessários para se obter urna imagem confiável do subsolo de interesse exploratório. Tais topografias, que ocorrem com alguma frequęncia no levantamento marítimo, onde o fundo do mar apresenta esta característica irregular, impőem um forte contraste de velocidades a propagaçăo do campo de ondas sísmico. Consequentemente, as reflexőes registradas em superficie provenientes de camadas mais profundas perdem suas caracteristicas hiperbólicas, seja em grupos de tiro comum como tambem em grupos de ponto medio comum.
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