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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 - 100 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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Aplicaçăo de um Método de Inversăo năo Linear ŕ Eliminaçăo de Reflexőes Múltiplas
More LessUm novo método năo-linear de inversăo de dados sísmicos, ora em desenvolvimento, é abordado. O método aplica-se a meos multidimensionais e tem sido explorado como um procedimento de supressăo de reflexőes múltiplas. Neste trabalho mostramos, de forma sucinta, o seu funcionamento e o aplicamos a um dado analítico que representa um meio unidimensional constituido por uma superfície livre e um único refletor, com ondas planas propagando-se em incidęncia normal.
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Ray-Born Synthetic Seismograms in Complex Structures Containing Scatterers
Authors A.C.D. Miranda and V. ČervenýThe method based on a hybrid combination of the ray theory with the Born approximation can be used to compute synthetic seismograms in complex laterally varying layered structures containing small sca.tterers. The scatterers can be combined to form objects of a complex shape. The wave field in the background laterally varying layered strudure is computed by the ray metbod and the scattered wave field by the Born approximation. A computer program package designed for such hybrid ray-Born computations in 2-D models is briefly described and numerical applications are presented. The ray-Born numerical modelling of seismic wave fields considerably extends the possibilities of the ray modelling.
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Método de Fourier com Aproximaçăo de Alta Ordem no Tempo e Transformada Rápida de Hartley no Espaço
By R. OliveiraA principal característica do M, é o calculo das derivadas espaciais através do usa da propriedade da derivada da Transformada de Fourier, alcançando assim, alta precisăo. Já a derivada temporal é calculada através da aproximaçăo par operador de diferenças finitas (DF) de segúnda ordem (Kosloff, et al. 1982, 1984, Oliveira, 1988). Este trabalho, propőem usar a propriedade da derivada da Transformada de Hartley, para calculo das derivadas espaciais (Saatcilar, 1990) e operadores de DF de alta ordem, para a derivada temporal (Dablain, 1986).
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Correction of Wavelet's Shape in Depth Migration
By S.V. GoldinWhile propagation through complicated media seismic signal passes such singularities as points of focusing and caustics having changed its shape with accordance to Hilbert transformation. Is this distortion compensated at reverse wave field continuation? If it is not, does one have possibility to apply a filter-corrector under the condition that geometry of the reflector is unknown? The positive answer for the last question is given in the paper. First, it is shown that reversed wave field continuation automaticaly compensates influence of the singularities that appear due to reflector's geometry.This fact is independent on the kind of pattern used for migration. Second, knowing the velocity model of overburden it is easily to construct a filter-correctcr which compensate influence of other singularities. The result is derived from analysis of the migration integral and Kirchhoff's approximation of reflection wave with help of the method o:f discontinuities recently developed by thla author.
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Is Geotomography Doomed?
Authors P. Carrion, G. Boehm, A. Vesnaver and I. PettenatiGeotomography has emerged from medical tomography but unlike medical tomography it largely does not work well. Why it happens? The answer is trivial: seismic experiments are different from medical and do not suit tomographic procedure. What can be done? To reject tomography as a tool for velocity reconstruction or to adopt it with modifications? Geotomography is probably a unique tool for velocity reconstruction. Conventional velocity analysis is too simplistic and is limited to layered media which do not bear much interest. In this presentation we introduce a possibility to significantly improve tomography and to make use of it in real situations related to typical seismic experiments. We achieve it with the help of the so-called compensation operator which reduces the dependence of results upon angular coverage and thus adjusts it to real world.
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Tomographic Inversion of Travel Time Data for Estimating VelocityStructure and Discontinuities
Authors J.A. Madrid and J.C.V. TraslosherosWe present a method to invert travel time data for determining the velocity structure and geometry of discontinuities in 2-D laterally varying media. The algorithm is based in the construction of a matrix of partial derivatives of travel times where the parameters may be either the velocity or the depth, or both, and SVD. The partial derivatives are computed using previous formulae for derivatives with respect to the velocity developed by Madrid (1986), and a ray tracing technique based on the partition of a medium in triangular regions wherein the gradient of velocity is constant. This kind of parameterization allows us to implement specific constrictions on the variations of depth parameters that are adequate in the search of discontinuities. The method is tested with synthetic data in the presence of noise. A simple real example is included.
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Técnicas Estatísticas para Prever Cobre Usando Perfilagem Geofisica de Poços
Authors D.A. Santos and O.P. VermaMétodos geofisicos podem ser utilizados para detectar minerais metáIicos devido as suas diferentes propriedades físicas. Isto ocorre diretamente, quando estes minerais possuem quantidade suficiente para afetar as propriedades eIetricas. Elemento metálico como o cobre, geralmente se encontra em pequena quantidade e năo é suficiente para ser dectado diretamente por metodos geofísicos. No entanto, ele esta associado a algum tipo de feiçăo geologica ou elemento que é dectável facilmente. Desta forma, técnicas geofisicas tem sido utilizadas para delinear alteraçőes associadas e/ou feiçőes favoráveis a mineralizaçăo (Urbank and Bailey, 1988). Este trabalho baseia-se em estabelecer uma correlaçăo entre a mineralizaçăo de cobre através de seus teores geoquímicos com parâmetros em perfilagem de poços. Para isto utilizou-se os metodos elétricos: polarizaçăo induzida (IP), resistividade aparente (Pa), condutividade apaxente (Oa), logarítmo da. Pa. (Log(pa)), potencial espontâneo (SP), resistęncia (Re); e raias gama (i). As possíveis correlaçăo săo feitas quantitativamente por análise R modal e qualitativamente através de exame visual. Fizemos ainda, a regressăo múltipla destes dados e usamos os coeficientes obtidos para prever furos de uma mesma área local. Neste trabalho foram utilizados dados obtidos em poços localizados na área de Saloba (furos 68 e 69) na regiăo de Carajás-PA.
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Tomografia Sísmica de Tempos de Percurso com e sem Vínculo
Authors D.J. Carneiro and P. CarrionMétodos tomográficos sísmicos podem ser divididos em duas categorias, que săo: tomografia sísmica de difraçăo (Devaney, 1984; Slaney Et al., 1984; Wu and Toksoz, 1987), e a tomografia sísmica de tempo de percurso. Subclassifica-se a tomografia sísmica de tempo de percurso em: tomografia sísmica. de tempo de percurso sem vínculos e tomografia sísmica de tempo de percurso com vinculos.
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3-D Inversion of Seismic Reflection Data Using Dynamic Ray Tracing
Authors M.A. Castro and V. ČervenýPossible applications of the dynamic ray tracing in the 3-D Born and Kirchhoff' inversion of seismic reflection data are discussed. It is snown that the most important quantities in the inversion integrals, such as the Beylkin determinant and the amplitudes of the ray theory Green functions, can be determined using the dynamic ray tracing procedure. The dynamic ray tracing can be simply performed along known rays in a very general, laterally varying layered background medium. As the cost of the dynamic ray tracing is merely a small percentage of the cost of the standart two-point ray tracing, the authors believe that the inversion algorithms which use dynamic ray tracing, will be useful in the inversion. They will find valuable applications particularly in complex 3-D structures.
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Reflection Tomography: Practical Aspects and Test Cases
By M. Silva A.Seismic tomography has proven to be a powerful tool in velocity determination. Originally, the method was applied to transmission data, and more recently has been adapted to reflection data by Bishop et al. (1985). In newly explored areas, where reflection data is the only data available, and where the geology and structure are complex, reflection tomography is the best velocity determination method since other conventional velocity analysis methods fail. In this paper, we show the basic principles and practical aspects of this method as well as some test cases that prove its efficiency.
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Multi-Frequency Backscattering Tomography (MFBT) for Boundary Scattering and for Volume Scattering
Authors R.S. Wu and F.V. AraújoThe Multi-Frequency Backscattering Tomography (MFBT) for boundary scattering based on the Physical Optics approximation is formulated in this paper. It is shown that the reconstruction formulas are the same as the MFBT for volume scattering (Wu and Araujo, 1991) except for a factor of ik/2. The reconstruction algorithm using direct FT makes the method extremely fast compared with other multifrequency migration or inversion methods, even much faster than the single-frequency diffraction tomography. Numerical tests show that for large scatterers or interfaces the MFBT for boundary scatterirlg give better results than that for volume scattering.
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Born Seismic Migration-Inversion
More LessSeismic migration and inversion describe a class of closely related processes sharing common objectives and underlying physical principles. Born inversion represents a feasible approach in reflection seismic data processing, besides it is closely related to classical migration concepts. Born approximation leads to a linear integral equation relating data and scallering potential, but neglect multiple scattering. Applicalion of high-frequency approximation leads to simple relations between Born inversion, (F-K) frequency wavenumber migralion and Kirchhoff migration. We assumeme a model. of the subsurface characterized by weak fluctuation in velocity, adopting a comanl reference velocity and negligible density variations. We deal with common·midpoint stacked data, that can be considered as an approximation of a zero-offset data. In this paper we attempt to review, theoretically, the relationship between wave-equation migration and wave equation inversion, and present some numerical examples of Born seismic migration-inversion.
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A Influęncia de Zonas de Baixa Velocidade na Inversăo Tomográfica Robusta
More LessA tomografia aplicada na sísmica permite que se obtenha a distribuiçăo da velocidade de um meio a partir do tempo de viagem da onda da fonte localizada num poço até receptores dispostos na superfície e/ou outro poço. A relaçăo entre o tempo medido e a velocidade que se quer determinar é estabelecida pela equaçăo de Fermát. Nesse trabalho utilizamos a tomografia de transmissăo, a qual faz uso apenas da onda direta.
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Seismic Waveform Inversion Using Global Optimization Methods
Authors P.L. Stoffa and M.K. SenA nonlinear optimization method based on the principles of natural selection is used to invert plane wave decomposed seismic data. We develop a genetic algorithm (GA) that uses an ensemble of subsurface models to create synthetic plane wave seismograms which are compared to observed seismic data. A normalized cross correlation function (fitness function) is used to compare the model seismograms to the data. The GA then selects the models with bigh fitness values to generate a new generation of trial models through the process of reproduction. We show that our GA works. We also show that its rate of convergence to a population with bigh fitness values can be significantly increased by defining the fitness function as a probability and using a control parameter analogous to the temperature employed in simulated annealing (SA) that decreases with each generation of modell. Further, we show that by repeating the process, we can estimate the mean model, the posterior probability density in model space and the posterior model covariance matrix.
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A Focus on the Elements of Successful Onshore 3D Imaging
Authors W. Ritchie and K. DingwallSuccessful imaging of onshore three-dimensional (3D) seismic data requires the interaction of innovative field design, cost-effective acquisition and. importantly. an approach to data processing which truly comprehends the 3D nature of the data. Much attention was focussed, in the late 1980's, on the key aspects of geometrically imaging 3D seismic: 3D migration, both partial pre-stack and post-stack, and the associated definition of the 3D velocity field. While these elements are essential components of an imaged solution especially for onshore 3D seismic, they are only part of the total equation. Without the complementary support of 3D signal enhancement tools for 3D noise elimination and 3D surface consistent processes, the later geometric imaging processes would fail to produce the desired result. This paper discusses both aspects of a total 3D processing solution with examples to highlight tools for 3D noise elimination, 3D surface consistent coupling compensation as well as 3D velocity and migration and describes a comprehensive sequence for optimum 3D onshore imaged information.
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Wave Equation Multiple Supression Using Constrained Cross-Equalization
Authors D.J. Monk and R.M. LansleyA method for improving the attenuation of water layer multiple energy is suggested. The improvement is achieved using wave equation extrapolation to generate an initial model of the multiple energy , and then constraining the way the time variant wavelet is modified to fit the observed multiple energy. Reconciling the initial multiple model wavelet with the input data is a critical part of this process, and several techniques have been suggested previously by other authors. The approach used here is to fit time, amplitude and phase of the wavelets, by adapting the model trace using a weighted sum of four traces which can each be derived from the initial multiple model trace. Results on real data suggest that attenuation of primary energy is minimized using this technique, without diminishing the level of multiple attenuation.
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Utilizaçăo da Matriz de Transiçăo no Estudo de Espalhamentos Acústicos
By A.D. MattosO fenômeno do espalhamento de ondas em meios acústicos e elásticos encontra importantes aplicaçőes em uma vasta gama de atividades, entre elas a exploraçăo geofísica. Por esse motivo săo elaborados rotineiramente programas de modelamento que lançam măo dos mais diversos métodos.
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Interpretaçăo de Perfis em Carbonatos Fraturados da Bacia do Paraná - Maranhăo
Authors A.A.S. Laranjeira and H.B. EvansEste trabalho é um estudo de interpretaçăo de perfis em carbonatos fraturados de mineralogia complexa. Os poços X e Y, perfurados na Plataforma. do Pará-Maranhăo, registram um pacote de rochas carbonáticas de mais, de 4000 m de espessura, que apresentam fraturas subverticais descontinuas, parcialmente tamponadas por calcita. A litologia é complexa, tanto do ponto de vista da composiçăo mineralógica, tamanho e forma dos grăos, quanto da geometria dos poços. A presença de fraturas diminue o expoente porosidade da equçăo de Archie para até 1,3, faixa considerada. inerente a. rochas fraturadas. A identificaçăo dos tipos litológicos e feita baseada em Evans(1971), Clavier e Rust (1976) e Schlumberger (1989). A saturaçăo de óleo é calculada com a fórmula de Archie aplicada com expoente de porosidade variável, calculado a partir do modele de Rasmus(1983), e a partir do método estatístico de Porter et al (1969). Um estudo anterior de porosidade e saturac;a.o de fluidos nos poços da Plataforma do Pará e encontrado em Beer (1986).
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Um Algoritmo Eficiente para Obtençăo de Filtros de Forma e L-Preditivos
By M.J. PorsaniPara sinais transientes, a exemplo de um pulso sísmico, a premissa usual de que os dados possuem valores de amplitude nula ao início ao final do pulso, é de fato válida. Neste caso a matriz das equaçőes normais, é a matriz banda-estruturada Toeplitz esimétrica, (Robinson, 1967).
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Radiation Pattern of Seismic Sources Via Disc-Source Analysis
Authors P. Carrion and E.E.S. SampaioSeismic exploration industry is largely based on the concept of the explosive point source, whose radiation pattern is spherically isotropic in acoustic homogeneous media.. In elastic media. the radiation pattern of a point source is not uniform over the phase surface and depends on the load direction and on the type of the generated waves. The situation drastically changes when one considers the drill-bit radiation source. The use of drill-bit as a source of energy was advocated a few decades ago, but it 'rang hollow until recently. Nowadays it emerges as one of the most attractive technologies: the method does not require any borehole recording, and therefore no risk to the borehole is involved. What is striking, however, is that the drill-bit source is poorly understood, probably due to the high complexity of its mathematical treatment. This makes difficult an accurate interpretation of continuously reeorded data. In the current study we develop a theory for disk-type sources with a combined load that generates elastodynamic waves, that tackles the drillbit source radiation pattern problem. We derived an explicit expression for the drill-bit source and solved the inhomogeneous wave equations for the Lame's potentials using Fourier-Hankel transform. We show that the radiation pattern of the drill-bit source significantly differs from the point source radiation pattern. This is in agreement with the observed data generated in the drill-bit experiments, demonstrating the viability of disk-type sources,
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Effects of Surface Roughness or Diffusivity on Migration, and Lateral and Vertical Resolution
By R.W. KnappWhen the wavelength of the roughness of a reflector surface is much Iess than the wavelength of the lateral wavelet it confounds the process of migration and produces a vertical effect that appears as a velocity gradient when the data are inverted. Lateral effects and vertical effects become indistinguishable and the problem is irreparable by any processing of the data. The problem is: more acute on high-resolution data than lower-resolution data because surface roughness is a small-scale feature. For it to affect the data, wavelengths must be dimensionally similar to the scale of the roughness. The real data example shown, for instance, had a peak frequency of about 500 Hz and a dominant frequency of about 350 Hz. That is not to say that the effect might not be seen ·on low-frequency data, just that opportunities increase with higher frequencies. An interpreter aware of the effect of surface roughness might know to consider both possible interpretations, but it requires independent information, such as a onedimensional vertical velocity log or other geological knowledge to separate the two. The distinction between the two is geologically significant. Surface roughness and velocity gradients have different geological implications, so distinguishing the two affects the geological interpretation of the region in question.
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Scale Space, Fractals and a Touch of Chaos
By T.J. UlrychThis paper deals with the fundamental issue of scale. The thesis which is presented is that scale is a concept which unifies some of the most current and interesting ideas in geology and geophysics. The introduction of this ooncept in a. formal wa.y into the literature is due to Witkin (1983) who coined the name scale space and applied it in a fascinating manner to the representation and decomposition of signals. Leaney and Ulrych (1988) extended this approach to event tracking and matching of well data. Scale, of course, is what fractals are all about and the paper goes on to deal with some recent results which describe the fractal nature of the reflectivity series and consequently of the underlying sedimentation process. Since fractal structure is a new geometric property which is exhibited by chaotic motions, the paper concludes with a brief look at a canonical chaotic .example and examines the possibility of wavelet estimation using a particular measure in phase space.
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Revisitando a Correçăo de NMO
By O.O. DuarteComo se sabe, a correçăo de "nmo" tem por objetivo eliminar a influęncia da distância fonte-receptor nos dados de reflexăo sísmica, simulando o registro que sería obtido com o receptor muito próximo da fonte. Um ponto fundamental para a análise desta correçăo é que o afastamento fonte-receptor causa, além da compressăo da funçăo refletividade, um aumento da amplitude variável com o tempo. Assim, por uma questăo de simetria, a correçăo de "nmo" deve ser acompanhada por uma reduçăo equivalente das amplitudes.
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Uma Nova Opçăo para a Eliminaçăo de Fatores que Degradam a Resoluçăo Sísmica
Authors A.S. Oliveira and R.J. CamposDe acordo com o modelo convolucional, o sinal sísmico registrado é o resultado da convoluçăo entre varíos fatores, que podem ser classificados segundo a dependęncia ou năo do offset e do tempo de transito. Săo fatores dependentes, por exemplo, as efeitos dos arranjos de fontes e receptores, a refletividade, a absorçăo seletiva de altas frequęncias, etc. Năo săo caracterizados por tal dependencia a assinatura intrínsica dos elementos da fonte, a resposta impulsiva dos receptores e do instrumento. Incorporando as reflexőes na superficie do mar (fantasmas) a uma estimativa dos efeitos dos arranjos de fontes e receptores, e possivel obter, por deconvoluçăo, uma assinatura que contenha as contribuhőes da fonte, dos recepetores, do instrumento, do efeito bolha e etc. Isto e realizado sobre a onda direta que, em aguas profundas e facilmente identificável.
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Influence of a Near-Surface Structure on Seismic Wave Fields and Multiple Suppression
Authors F.C.M. Andrade and V. ČervenýThe hybrid ray-reflectivity method is applied to the numerical modelliog of seismic wave fields in laterally varying layered models containing a thin near-surface low-velocity layer. The computatioos within the laterally varying layered model are performed by the ray method, but the thin near surface layer is attacked locally by the matrix methods. The thin layer need not be homogeneous, it may include arbitrary inner layeriog and it may lightly vary laterally. All multiples within the layer are automatically taken into account. Numerical examples of hybrid ray-reflectivity seismograms for two models of the thin near-surface layer are presented. An inverse algorithm to remove the effects of a thin near-surface layer from seismograms recorded at the Earth's surface is proposed.
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3-D Static Corrections
Authors W.N. Amorim and W. EckhardtThe calculation of static corrections using the arrival times of refracted waves present in reflection seismograms by means of the method of Seismic Refraction Tomography (SRT) for 2D acquisition geometry has succesfully been applied to straight and crooked seismic lines. The redundancy as obtained by present day CDP shooting is used to estimate the variations in thickness and velocity of the weathered layer, which cause static shifts in the observed reflection times and hence distortions in the final stack. This paper deals with the extension of the method to any acquisition geometry, in particular 3D. After a short review of the basic theory we will highlight some aspects for 3D geometry. Then we discuss some implementation considerations after which real data examples will be shown. We conclude with observing that the method results in an improvement of the quality of the stacked section, especially for the 3D case where the conventionally obtained static corrections by means of refraction seismics do not account for the spatial variations in thickness and velocity of the weathered layer. This implies a pleasant 'fringe' benefit of the method, vz. that the shallow refraction survey may be omitted.
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Atenuaçăo de Alinhamentos Gerados por Filtros de Mergulho
Authors A.B. Abrăo and H. MalebrancheOs filtros de mergulho (ou velocidade) que săo aplicados nas fases pré ou pós empilhamento, objetivam apenas a atenuaçăo dos ruídos coerentes que mascaram os dados sísmicos. Entretanto, alteram o espectro dos ruídos incoerentes induzindo coeréncia local e modificando o carácter dos dados. A consequente aparęncia mixada dos refletores e a presença de alinhamentos onde antes só havia ruído aleatório, diminui a qualidade dos dados e dificulta a interpretaçăo. Essa é uma razăo pela qual esses filtros săo apenas parcialmente aplicados.
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Filtro de Feixes Gaussianos
By R.C. PestanaA transformaçăo de sismogramas de fonte pontual em sismogramas de feixes gaussianos pode ser de grande valor no estudo de meios que possuem a grande vantagem de se propagarem em uma única direçăo e permitem, ainda, uma iluminaçăo mais localizada dos objetos a serem investigados. Já as ondas planas, as quais se propagam também em uma única direçăo e necessitam de um suporte espacial infinito, fornecem uma resposta mais global.
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Geophysical Monitoring Steamflood Advance
By J.M. SoutoA new analytical technique was developed to monitor the changes in oil water saturation during thermal recovery of tar sands. During fracture assisted steamflood recovery, the reservoir is heated and depleted of most of its oil by the injected fluids. Changes in temperature and oil water saturation produce a significant change in resistivity at both the microscopic scale (grain size, pore flushing), and at the macroscopic scale (reservoir size. production pattern). This geophysicial technique uses geothermal technology to detect the steamflood advance from the surface by using a large electromagnetic signal generator and a permanent network of receiving stations to detect and measure the curvature of the secondary electric: field induced in the alteration zone produced by the the steamflood injection. The analytical technique allows interpretation of the observed anomaly in terms of thickness and lateral extent of the invaded zone. In addition, the technique was designed so that the reciever grid network is reoccupied later, producing more resistivity "snapshots", allowing measurements of the changes in distribution of injected fluids, temperature and oil water saturation during various phases of steamflood recovery of heavy oil.
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Assinatura da Fonte e seu Espectro Angular por Inversăo de Sismogramas de Ondas Planas
By S.S. SantosEm um experimento sismica, maritima ou terrestre, a assinatura da fonte é pouco conhecida ou desconhecida o que dificulta a soluçăo de problemas geofisicos onde o conhecimento da assinatura da fonte e necessario. Este trabalho sugere um novo metodo para estimativa de assinatura da fonte baseado na combinaçăo CD/ERT (continuaçăo descendente/extrapolaçăo reversa no tempo) onde fonte e receptores săo colocados na superficie a metodo e iterativo e consiste em computardor a cada passo, os valores iniciais em t=T (onde T é metade do comprimento do sismograma) e a assinatura da fonte. Uma propriedade importante do metodo CD/ERT e que năo requer a hipótese de fase mínima. O número de iteraçőes depende da precisăo desejada e o algoritmo é ilustrado para vanas assinaturas com diferentes amplitudes e fases. Este método também é usado para a reconstruçăo do padrăo de radiaçăo da fonte e pode ser aplicado a dados sísmicos reais desde que o modelo de velocidade seja previamente conhecido.
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Amplitude Anomalies,and AVO
By W. EckhardtIn the last decade Petrobrás drilIed a great number of wells based on the presence of amplitude anomalies mostly bright spots in seismic data. Not all of these wells proved to be producers. This study aims to investigate whether the application of amplitude versus offset (AVO) or angle analysis techniques would have increased the success rate of drilling. A second aim of this study is to point out for what type of data AVO analysis is feasible and what suite of preprocessing is needed before the data are submitted to AVO analysis.
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Influęncia do Binômio Fm. Barreiras/Carbonatos na Qualidade Sismica na Regiăo de Japaratuba (SE)
By F.A.F. GomesA área do Baixo de Japaratuba e adjacęncias (fig. 1) ostenta má qualidade sísmica e como reflexo apresenta uma baixa densidade de linhas sísmicas para o atual estágio exploratório da Bacia Sergipe-Alagoas. Na área afloram clásticos da Fm. Barreiras sobrepostos a carbonatos das Fms. Riachuelo e Cotinguiba.
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Wavelet Estimation and Deconvolution by the Vertical Double-Streamer Experiment
Authors H. Huck, M. Tygel and A. MatthiesIn this paper, we give a deterministic estimation scheme for an arbitrary (i.e. mixed delay) source wavelet and the reflectivity of a stack of horizontal layers. From a marine vertical double-streamer seismogram the wavelet estimation and the deconvolution can be performed without any a priori information on the souree pulse or the reflection coefficients of the layers. For this purpose, the recordings at two depth levels in the uppermost (water) layer (see Fig.1) are decomposed in their plane wave contributions. The decomposition lead's to the consecutive recovery of the medium reflectivity and the source wavelet in the slowness-frequency (p -w)-domain. The proposed scheme is based completely on the point source response of a layered acoustic medium (bounded by a free surface above and a homogenous half-space below) given by the wave equation (see Ziolkowski et al. (1987), Tygel and Hubral (1987), Tygel et al. (1991)).
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The Accuracy of Linearized Elastic Parameters Estimation
Authors B. Ursin and E. TjĺlandElastic parameters can be found from amplitude versus offilet (AVO) curves. A fourth-order polynomial can be fitted to the AVO-curve and estimated by a least squares technique. The polynomial coefficients are linearly related to a set of elastic constants. In this paper the statistical properties of the estimated polynomial coefficients and elastic model parameters are investigated. An eigenanalysis is also performed of the normalized covariance matrices.
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Where are the Fault Plane Reflections ?
Authors A. González, W. Lynn, W.F. Robinson IV and S. MackayTwo imaging methodologies that we expect to give comparable results are 1) DMO followed by CMP stack and poststack time migration and 2) prestack f-k time migration. For the most part, these two approaches do indeed give similar images; yet in one important area they are nearly always different. Specifically, the approach of DMO-stackmigration routinely images reflections from fault planes; whereas the approach of prestack f-k migration routinely does not. This observation is not limited to anyone geologic basin, but is seen worldwide. This paper investigates several possible reasons that could explain the differences in fault-plane imaging: 3-D effects, vertical velocity variation, acquisition geometry issues, and transverse isotropy. We find that only transverse isotropy can single-handed explain the different responses of the imaging approaches. We further show that the sensitivity of prestack f-k migration to transverse isotropy can be used to estimate the amount, in some average sense, of transverse isotropy. This has importance not only for selection of imaging parameters, but also for interpretation and time-to-depth conversion. In this paper we also show that using an aneliptic dispersion relation in prestack f-k migration compensates for the effects of transverse isotropy, thus reducing the dip dependency of migration velocities.
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Interactive Imaging and Interval Velocity Analysis in Complex Geological Areas
More LessWe present an interactive procedure forthe estimation of interval velocity models and the corresponding depth images. This procedure utilizes internal velocity analysis in conjunction with both prestack and poststack migrations and seems to yield reliable results. A major advantage of the procedure is that geological interpretation rnay be used throughout the process.
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Seismic Imaging of Salt Structures in the Gulf of Mexico
Authors D.W. Ratcliff, S.H. Gray and N.D. WhitmoreMajor hydrocarbon accumulations are often associated with salt structures in the Gulf of Mexico. Correctly imaging the salt structures has been and will continue to be an important exploration tool. Defining the salt boundaries and the surrounding sediments enables the geophysicist to explore more effectively for those hydrocarbon accumulations. For example, imaging steeply dipping salt face reflections can assist in optirrizing salt flank well locations. Properly migrating salt overhang reflections that have travelled through salt can aid in determining updip limits of possible reservoir sands. Imaging below salt can help identify structural features that trap significant hydrocarbon resources. Salt imaging technology can also be used to reduce some of the risks associated with salt flank, salt overhang and subsalt drilling.
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The Evolution of Stacking Reflection Data in Areas of Complex Geology
By W.S. FrenchSignificant changes occur every few years in the way we process seismic reflection data from areas of complex geology.
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A Case Study of Pre-Salt Imaging
Authors M.A. Brzostowski and W. RitchieConventional imaging techniques operating in the time domain are recognized to poorly resolve complex geologic environments. Complex subsurface geology with rapid spatial changes in interval velocity requires pre-stack imaging techniques operating in depth. However, these techniques rely on good definition of the velocity field. A common approach to defining this field involves multiple iterations of pre-stack depth migration through successively deeper velocity layers - an expensive and time-consuming process. In this paper, we describe a technique to cost and time effectively develop the required velocity model using travel-time inversion, of followed by replacement of the complex velocity field with a simplified field thereby removing nonhyperbolic moveout effects and allowing implementation of the most cost and time effective imaging solution. The technique is applied to data from the Pre-Caspian Basin of the Soviet Union where the ability to image below significant accumulations of Permian salt and overcome the false time pull-ups due to the fast saIt velocity is essential to accurate mapping of this basin. Following development of the velocity model using travel-time inversion, its use in conjunction with both pre and post-stack time and depth migration techniques is demonstrated in a set of seismic data comparisons. The conclusion is that the concept of Replacement Dynamics provides an attractive, cost-effective alternative to other computationally intensive techniques in unraveling many areas of complex subsurface geology.
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An Asperity-Deformation Model for Effectivity Pressure
Authors A.F. Gangi and R.L. CarlsonThe variations of the mechanical and transport properties of cracked and/or porous rocks under isotropic stress depend upon both the confining pressure (Pc) and the pore-fluid pressure (Pp). To a first approximation these rock properties vary with differential pressure, Pd = Pc - Pp; at least for low differential pressures. However, at higher differential pressures. the properties vary in a more complicated, way upon the two pressures. The concept of effective pressure, Pe, is used to denote this variation; that is, the rock properties are constant for constant effective pressure even though the confining and pore pressures change.
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Macro Model Optimization and Depth-Migration
By R. MarschallAny procedure to go from time to depth requires (among other features) a macro-model, i.e. a velocity model. This macro-model therefore has to be specified if one wants to apply any type of depth-migration, pre-or poststack. However, the same macro-model specification is needed if one wants to depth-convert an interpretation of e.g. a 2-D time section. The ever-lasting problem here was given by the question: how good is the final resulting depth map away from the fixpoints, i.e. wells, especially when the depthing is done by applying e.g. APPARENT VELOCITIES. In this work we describe the CONCEPT OF VELOCITY-CHECKiNG BY WAVEFRONTS which is applicable to both, i.e. conventional depthing problems as well as to depth-migration related problems, simply by using the given redundancy of normal seismic data.
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Wave Theoretic Multiple Suppression and Source Signature Estimation
By A.B. WegleinMultiple suppression has a long history in exploration seismology. Although our current methods (NMO-stack, p-r and f-k filtering and modeling} are often effective there are also many cases where muttiples remain a problem. The goal of a recent wave theoretic mutliple suppression method (Weglein and Stotl, 1991, and Carvalho, Waglein and Stolt, 1991) is the suppression of multiples that are not susceptible to our current mutliple removal methods. Two categories of multiples are targeted: (1) those that satisfy the assumptions of moveout separation methods (homogeneous overburden, horizontal reflectors, and knowledge of the velocity of a primary) but are too close to a primary in moveout, and {2} those that do not satisfy one of (or a combination of) the above listed assumptions. The method we are proposing Is a nonlinear migrationinversion procedure based on the T operator of scattering theory. Each successive term in the series solution requires the same migration operation.
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Seismic Imaging of Rift-Related Subsalt Structures Using Prestack Depth Migration
Authors S. MacKay, D. Tufekcic and A. GonzálezThe North Sea and the Gulf of Suez contain classic examples of rift-type graben structures. Typical of such geologic regimes, substantial thicknesses of evaporites were accumulated during the inijiation of rifling. Evaporites, especially salt, provided ideal seals for the faulted pregraben and graben-fill sediments. Subsequent burial then yielded the mechanism for halokinesis and the resultant traps associated wijh salt diapirislm. Anhough contributing to many of the conditions necessary for hydrocarbon trapping, salt presents a severe impediment to seismic imaging. Irregular masses of salt act as distorting lenses to refiected seismic energy. To form accurate images of the subsurface, prestack depth migration is needed.
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Finite-Difference Reverse Time Migration of Multi-ConfigurationMarine Seismic Data
Authors M.A.B. Botelho and P.L. StoffaPre-stack reverse time migration requires that each subsurfacee grid point have a unique imaging time. This time is used to stop the reverse time propagation. Here we compute the required imaging times by using a finite difference algorithm to solve the forward problem. These times, which correspond to the arrival time of the direct wave, are usually found by ray tracing. The procedure we present consists of forward finite difference modelling and detection of the energy of the direct arrival at each subsurface grid point.
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Climate Change in the Northern Hemisphere: The Record from Borehole Temperatures
By H.N. PollackRock temperatures at shallow depths within the Earth are an archive of temperature changes that have occurred at the surface of the Earth in the recent past. Thus subsurface temperatures comprise a valuable complement to surface meteorological data in understanding the Earth's surface temperature history, particularly for times prior to the establishment of a worldwide network of meteorological stations. The subsurface observations are relevant to an assessment of the role of atmospheric greenhouse gases in the global warming of the 20th century.
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Reconstruçăo de Mudanças CIimáticas a Partir de Perfis Geotérmicos: Avanços Teóricos
By F.B. RibeiroA identificaçăo e remoçăo de perturbaçőes climáticas no cálculo de densidade de fluxo térmico e a reconstituiçăo das variçőes climáticas recentes săo problemas clássicos da geotermia. O reconhecimento de que tais perturbaçőes ocorrem de forma mais generalizada do que se imaginava a pouco mais de uma década e também a crescente preocupaçăo com o meio ambiente tęm, recentemente, aumentado o interęsse pelo problema. A primeira parte deste trabalho apresenta uma revisăo dos métodos de simulaçăo de perturbaçőes climáticas sobre perfis. Na segunda parte, discute-se a aplicaçăo dos métodos de inversăo na reconstituiçăo das variaçőes climáticas a partir de perfis geotérmicos.
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Variaçăo Temporal de Perfis Geotérmicos Rasos Associada a Alteraçőes MicrocIimáticas na Amazônia
Authors J.R.S. Souza, R.L.C. Araújo and M. MakinoA energia solar incidente na superficie terrestre é fundamentalmente refletida, reirradiada (transporte de calor sensivel) ou convertida em calor latente de vaporizaçăo, fotossintese, bem como em outros processos biológicos. Além disso, uma parte apreciavel dessa eneria é transmitida e armazenada abaixo da superficia afetando o regime térmico do solo (Sellers, 1974; Souza et al, 1989).
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Evidęncias Geotérmicas sobre Variaçőes Climáticas Recentes no Hemisfério SuI
By V.M. HamzaAs flutuaçőes climáticas exercem influęncias significativas sobre o balanço energético da superfície da Terra, com consequęncias notáveis, tanto na geosfera como na biosfera. Os efeitos térmicos das alteraçőes energéticas, na superfície, penetram em profundidade numa forma muito semelhante ŕs penetraçőes das variaçőes diurnas e sazonais da radiaçăo solar.
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Tendęncias e Flutuaçőes Seculares da Pluviometria no Brasil
Authors T.M.B.S. Xavier and A.F.S. XavierA pluviometria é um parâmetro muito utilizado para detectar e avaliar mudanças climáticas. Neste trabalho săo estudadas tendęncias e flutuaçőes de séries longas de chuva, no Brasil, referentes a períodos úmidos do ano e anos hidrológicos. A metodologia envolve o uso de técnicas de séries temporais, para detectaçăo de tendęncias e a análise de quase-ciclos. Discutem-se os resultados preliminares obtidos, quanto a sua possível interpretaçăo em termos da influęncia de fatores físicos e antrópicos sobre o clima.
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Aspectos Climáticos da Variabilidade de Cotas Máximas e Minimas do Rio Negro: 1903 - 1985
Authors A.F.S. Xavier, O. Massambani and T.M.B.S. XavierA análise de vazőes ou de alturas de rios é importante, no contexto de estudos hidrometeorológicos, desde que esses dados representam uma integraçăo da chuva e do escoamento superficial nas respectivas bacias coletoras, bem como, de outros processos de alimentaçăo. Por outro lado, o vies inerente a observaçőes de tipo puntual fica praticamente eliminado.
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