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5th International Congress of the Brazilian Geophysical Society
- Conference date: 28 Sep 1997 - 03 Oct 1997
- Location: São Paulo, Brazil
- Published: 07 November 1997
141 - 160 of 416 results
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Digital 5' x5' Gravity Maps of the Săo Francisco Craton, the Marginal Fold/Thrust Belts and Contiguous Continental Margin/Oceanic Basins
Authors Eder Cassola Molina, Naomi Ussami and Yára Regina MarangoniDigital 5'x 5' Free-air, Bouguer and residual anomaly maps are presented based on over 45,000 gravity stations collected by different institutions and stored at IAG-USP gravity data bank. The area defined by 54-32°W and 8-24°S covers the Săo Francisco craton and its marginal fold/thrust belts, the conterminous eastern continental margin and oceanic basins. The gridded map was obtained using the least-squares collocation technique. The nominal resolution of the present maps is 5' in areas with good data coverage. In the oceanic region, conventional marine gravity data were processed with satellite altimetric data obtained by GEOSAT Exact Repeat Mission. The gridded anomaly maps, together with the stations istribution are made available in digital form. An improvement in data coverage is expected as new data are collected and contributed to the data-bank, so that continuous update of the present model will be possible. For obtaining these data and more information, connect http://www.iag.usp.br/geofisica/sfcraton.html.
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Modelagem Gravimétrica Direta e Inversa Aplicada no Cálculo da Espessura Sedimentar da Sub-Bacia de Cedro e do Testemunho Sedimentar de Fátima, Nordeste do Brasil
A Sub-bacia de Cedro e o Testemunho Sedimentar de Fátima fazem parte do conjunto de pequenas bacias interiores do Nordeste do Brasil (Cordani et al. 1984) (Fig. 1), que implantou-se regional mente no contexto de rochas pré-cambrianas pertencentes a Província Borborema. O embasamento é composto por xistos, gnaisses e granitos das faixas Piancó-Alto Brígida e Pajeú-Paraíba, entre os lineamentos de Patos e Pernambuco, no Domínio da Zona Transversal. Litoestratigraficamente as áreas sedimentares possuem uma coluna estratigráfica simples constituída no máximo por tręs formaçőes exceto as grandes bacias Araripe, Potiguar, Recôncavo-Tucano-Jatobá, nas quais está preservada a coluna estratigráfica completa.
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Modelagem Gravimétrica "2 D" através do Complexo Granítico Quixadá-Quixeramobim, Regiăo de Algodőes e Uruquę, Ceará
Authors R. Mariano, G. Castelo Branco and Mauro Lisboa SouzaA regiăo em estudo localiza-se na porçăo central do Estado do Ceará sendo caracterizada por uma unidade geotectônica do Proterozóico Superior (Complexo Granítico) encaixada em rochas do Proterozóico Inferior (Complexo Gnáissico-Migmatítico) e Proterozóico Médio (supracrustais do Grupo Ceará). A regiăo e de geologia extremamente interessante sendo caracterizada por seis Fácies graníticos (Muxuré Velho II, Uruquę, Água Doce, Muxuré Velho I, Serra Branca, Muxuré Novo) e por estar delimitada por duas zonas de cizalhamento dúcteis em íntima relaçăo com o mecanismo de posicionamento das rochas graníticas (Torquato et al., 1989). A regiăo do Complexo Granítico envolve uma área de aproximadamente 1.800 Km2.
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Inversăo Gravimétrica 3D para Superfície entre Dois Meios Homogęneos, com Aplicaçăo ŕ Depressăo Circular de Colônia (Săo Paulo/SP)
Authors Gino Francisco dos Passos and Wladimir ShukowskyEm gravimetria, a determinaçăo da distribuiçăo de densidade de massa em subsuperfície correspondente a uma dada anomalia observada é um problema inverso contínuo mal posto, uma vez que existem infinitas distribuiçőes de densidade de massa correspondentes ao efeito observável em superfície.
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Gravity Signature of the Morungaba Granitoids
Authors Wladimir Shukowsky, Silvio R.F. Vlach and Kellerman A. NovaesThe analysis of a gravity survey of the Morungaba granitoids (eastern Săo Paulo State) is presented here as a performance test of the use of a fast and inexpensive gravity surveying technique in the assessment of structural characteristics of granitoids. It is shown that by the use of adequate data processing applied to relatively low precision gravity data acquired over a rugged terrain it is possible to identify anomalies with a few milliGal amplitude, which are suitable for structural interpretation.
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Compartimentaçăo Tectônica do Sudeste do Brasil e suas Relaçőes com as Zonas de Fraturas Oceânicas
Authors E.C. Alves, M. Sperle, S.L.M. Mello and S.E. SichelA compreensăo da interrelaçăo das estruturas geológicas do oceano profundo com as províncias geológicas localizadas na borda continental emersa do sudeste do Brasil e o significado tectôni co de sua topografia, tęm que estar integrado a geologia da margem continental e da bacia oceânica adjacente. O estudo das zonas de fraturas oceanicas (ZFO'S) desde a cordilheira mesoceânica até a margem continental tęm revelado que estas estruturas desempenham um importante papel no estudo do condicionamento tectônico da borda continental emersa e da margem continental adjacente. Neste contexto será feita uma análise dos principais estudos realizados na bacia oceânica do Sudeste do Brasil, onde procurar-se-á mostrar possíveis relaçőes entre estruturas do bordo continental e as ZFO'S na compartimentaçăo tectônica do sudeste brasileiro.
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Mapas Gravimetricos de Goias e Distrito Federal
Authors Yára R. Marangoni, Miguel D. Gago and Maria Cristina LobiancoOs anos 90 estăo sendo marcados pela comunicaçăo eletrônica, Revistas, jomais, artigos, mensagens, arte, tudo está na rede de computadores e tudo pode ser visto por milhóes de pessoas. Assim, as Geocięncias năo podem mais se limitar a produzir mapas coloridos em papel a urn custo alto e produçăo limitada, devendo caminhar rapidamente para a cornunicaçăo digital. Boa parte dos mapas ja săo confeccionados eletronicamente, mas a sua veiculaçăo ainda é fortemente dependente do papel. O que se pretende neste projeto é produzir mapas de anomalias gravimétricas do estado de Goias e do Distrito Federal, a serem distribuídos tanto por via disquete, como atraves da Internet.
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Métodos Potenciais na Bacia do Paraná: Alguns Aspectos a Considerar na sua Interpretaçăo.
More LessCom os trabalhos de prospecçăo de petróleo desenvolvidos pela Petrobras na última década na Bacia do Paraná, novos dados de métodos potenciais foram incorporados ao acervo da Bacia, destacando-se os levantamentos aeromagnetométricos (fig.OI), gravimétricos e perfis de magnetometria e gravimetria ao longo de linhas sísmicas.
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Interpretaçăo Preliminar de Dados Gravimétricos da Bacia de Săo Paulo (Brasil) e Regiăo Circunvizinha
More LessNesse trabalho săo apresentados os resultados preliminares da análise de dados gravimétricos no quadrilátero situado entre as longitudes de 47°09'W e 45°58'W e as latitudes de 23°08'S e 23°57'S, abrangendo uma área total de cerca de 11 mil Km2 na qual se situa a Bacia de Sao Paulo e regiăo circunvizinha.
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Gravity Imaging the Continental Margin of Brazil
Authors Luiz F.S. Braga, J.A. Bach de Oliveira, J.D. Fairhead and D.C. MantonPetrobras has systematically investigated the continental margin of Brazil using gravity surveys since the early 80's. These surveys include sea bottom gravity measurements, detailed 2D surveys and more regional surveys extending into deep water. This effort resulted in a near total coverage of the margin.
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Interpretacion Preliminar del Efecto Gravimetrico de la Cuenca Ischigualasto - Villa Union (San Juan y la Rioja - Argentina
Authors Francisco Ruiz and Antonio IntrocasoEn la profunda cuenca sedimentaria Ischigualasto - Villa Unión, se efectuó un relevamiento gravimetrico areal que cubre toda la comarca, a partir del cual se dispone de una base de datos adecuada para su estudio integral. Con el propósito de evaluar la respuesta gravimétrica cortical de la fosa, se aplicaron tecnicas de separación de anomalías sobre la carta de gravedad corregida por el efecto del relleno sedimentario. Se emplearon tres rnetodos de filtrado: a) Prolongación Ascendente de Campos Potenciales, b) Superficies de Tendencia, y c) Suavización Grafíca. Los resultados obtenidos en todos ellos son consistentes y muestran anomalfas residuales positivas en la region de la cuenca, con máximos del orden de 40 mGal y cuya longitud de onda es levemente superior a su ancho. La residual puede atribuirse a cuerpos intrusivos, hipótesis respaldada por evidencias de vulcanismo básico,
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Interpolation For Aeromagnetic Data Using Magnetic Variogram for Kriging
Authors Armando Z. Remacre and Marcelo A. BraghinBefore to start geophysics analysis, it is necessary evaluate the accuracy of the surface created from an aerogeophysical survey. There are many interpolators available for this task. One of the most used is spline or minimum curvature. However, not always is the best choice, even if some parameters would be changed. This paper presented an application of kriging a surface using a magnetic variogram model for aeromagnetic dataset.
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Contributions to Understanding Tectonic Processes from Electromagnetic Studies in Canada as part of Project LITHOPROBE
More LessProject LITHOPROBE is Canada's national, collaborative, multidsciplinary earth science program directed towards obtaining an enhanced understanding of how the North American Continent evolved. Research is focussed along ten transects that span the country geographically from Newfoundland (east coast) to Vencouver Island (west coast), and span geological time from the Hadean (>4 Ga) to the present. Along every transect is conducted seismic reflection, seismic refraction and electromagnetic surveys, as well as geological, geochronological and geochemical research activities.
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Can Magnetotellurics Map Crustal Strain Levels? Examples from North America and New Zealand
More LessMagnetotelluric (MT) results from three diverse tectonic settings exhibit strong correlations between increased electrical conductivity and expected depths of enhanced crustal strain. The environments are a young caldera complex and the Basin and Range (BR) Province, both in North America, and the transpressional Southern Alps of New Zealand. Conductivity patterns at >10 Ian beneath the Valles caldera are strongly congruent with isotherms projected from geothermal drill holes up to 4.5 Ian deep.
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Deep Electromagnetic Probing of the Slave Craton
Authors Alan G. Jones and I.J. FergusonDuring August-October, 1996, sixty magnetotelluric sites were acquired in northwestern Canada, as part of the Lithoprobe SNORCLE transect, from the Slave craton to bounding Proterozoic terranes to the west. Preliminary analyses and interpretation of the MT data indicate features that are important for the Slave craton specifically, and possibly for the geological history of the early Earth in general.
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Electromagnetic Fields on the Seafloor: Magnetotelluric Sounding and Motional Induction
More LessNatural electromagnetic fields in the oceans are induced by both external, ionospheric and magnetospheric, electric current systems and by the dynamo interaction of ocean water currents with the earth's geomagnetic field. The effects of these two sources divide naturally at a period near the inertial value (about 1 day at mid-latitudes). At shorter periods, external sources dominate both the electric and magnetic fields and, except for the ocean tides, it is difficult to detect motionally-induced electromagnetic fields. In this period range, seafloor data may be utilized for magnetotelluric sounding of the electrical conductivity of the oceanic lithosphere and mantle. By contrast, at periods longer than a few days where the horizontal length scale of the water velocity field is large compared to the water depth, motional induction is predominant in the horizontal electric field and dwarfs all external contributions at periods longer than about a week, depending on location. The motional horizontal electric field is proportional to the vertically-integrated, seawater conductivity weighted horizontal water velocity, which is very nearly the depth-averaged (barotropic) velocity in most places. Because of their inherent spatial averaging properties, electric field measurements are finding increasing use in physical oceanography where the barotropic velocity is of interest. Motional magnetic fields are much weaker than the horizontal electric components, and have not been detected away from intense boundary currents at long periods. This means that outside of energetic current regimes, the geomagnetic depth sounding method is usable in the deep ocean to very long periods. In addition to the conventional (horizontal electric and triaxial magnetic field) components, it is feasible to measure the vertical electric field in the ocean.
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Volcanism and Tectonism at the Mid-Atlantic Ridge
Authors J.R. Cann, D.K. Blackman and D.K. SmithOceanic crust is constructed in a narrow zone no more than 20 km wide at the axes of mid-ocean ridges. The zone is segmented into elongate volcano-tectonic units tens of kilometers long within which magmatism and tectonism interact rapidly to build and deform the new lithosphere. At the slow-spreading Mid-Atlantic Ridge magmatic and tectonic processes interact in a particularly complex way, since magma chambers are small and short lived, and the lithosphere may be deformed by brittle dyking or faulting, or by ductile shear. On average about 10% of the extensional strain is accommodated by faulting and 90% by dyke intrusion, but locally, and especially near the ends of spreading segments, magmatic extension decrease, and tectonic extension may increase by factors of 2-5.
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Electrical Conductivity Distribution in the Parnaíba Basin and its Tectonic Significance
Authors B.R. Arora, N.B. Trivedi, A.L. Padilha, I. Vitorello, A. Rigoti, F.H. Chamalaun and Sérgio L. FontesA magnetometer array study was operated in the north-northeast Brazil as a collaborative exercise between the Instituto Nacional de Pesquisas Espaciais - INPE and the Flinders University of South Australia. The analysis and interpretation of the magneto-variational (MY) field data has promoted further the spirit of the international cooperation. This is the first exercise of its kind in the Brazilian shield, aimed at mapping the internal conductivity distribution on a regional scale. The present paper summarizes the primary features of the conductivity distribution deduced by subjecting the array data to various data processing techniques of Geomagnetic Deep Soundings - GDS (Gough and Ingham, 1983; Egbert and Booker, 1993). The tectonic significance of the mapped conductive zones in the geological evolution of the Parnaiba basin is emphasized and possible sources of the enhanced conductivity summarized.
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Computer Modeling of the Strike Effects on MagnetoTelluric Data from 2 D Earth Under the Equatorial Electrojet
By Luiz RijoIt is well known that the Equatorial Electrojet (EEl) currents are a source of perturbations on I-D Magnetotelluric long period data (Hermance &Peltier, 1970; Peltier & Hermance, 1971; Rijo, 1991; Rijo, 1993). For the simple case of 2-D geological structures parallel to the E-W direction of the equatorial electrojet, this phenomenon is well documented in the literature (Mota & Rijo, 1992). However, the strike of 2-D geological structures are not always restricted to E-W direction, thus it is appropriate to investigate the general situation of structures having arbitrary orientations with respect to the EEl direction.
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Integration of 3-D Hydrogeophysical and Hydrogeochemical Data from Picos Area in Piaui State, Northeast Brazil: Progress Report
Authors Max A. Meju, Sergio L. Fontes, Emin U. Ulugergerli and Antonio R.S. FilhoThe groundwater resources of the semi-arid eastern margin of the basin are relatively poorly understood and a major international research program has been set up to shed some light on some of the current problematical observations using an integrated geophysical, hydrogeological and hydrogeochemical approach. There are important confined aquifers in the area of study (which is centred on Picos City) but a significant proportion of the groundwater resources is saline in nature. The boreholes tapping the regional aquifers at some locations provide inorganic hydrogeochemical and fluid resistivity data allowing partial mapping of saline plumes (of possibly hydrothermal nature) which pose a serious threat to major water supply schemes in the area. Joint transient electromagnetic and magnetotelluric 3-D surveys were conducted in the area in 1996 to provide constraints on the borehole data and to facilitate a better understanding of the hydrogeochemical anomalies in the area. Preliminary results from the geophysical surveys are discussed and compared with pre-existing hydrogeochemical data in this paper.
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