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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
201 - 300 of 416 results
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Slip Surfaces at Inner Corner Highs on the Mid-Atlantic Ridge
Authors B. Janssen, D. Blackman and Joe CannWe present recently collected high-resolution data from the Atlantis Fracture Zone which is situated on the Mid-Atlantic ridge at 30° north. The Sirnrad multibeam bathymetry has ground resolution of 50-100m and the deep-towed side-looking sonar device (TOBI) has a resolution of 3-6m. The combination of imagery proved a powerful tool for interpreting the Ridge Transform Intersections (RTI).
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Some values of Effective Elastic Thickness determined for the South American Plate
Authors M.S.M. Mantovani, S.R.C. de Freitas and W. ShukowskyIn the last decades, the effective elastic thickness (Te) has been considered an important parameter for the study of the lithosphere rheology.
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Flexural Rigidity at Parnaíba Basin - Brazil
Authors R.M. Vidotti, C.J. Ebinger and J.D. FairheadThe intracratonic Parnaibá basin, located in Northeast Brazil, covers an area of approximately 600,000 km2. It is surrounded by Brazilian cycle (700-500 Ma) orogenic belts and its sedimentary cover ranges in age from Silurian to Cretaceous. These sequences have been intruded by volcanic rocks of Early Jurassic and Early Cretaceous ages. The maximum sedimentary thickness is around 3500 metres, based on surface geology, geochemical surveys, exploratory wells and seismic sections (Góes et al, 1993).
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Subsidence Modeling of the Parnaibá Basin (NE Brazil)
More LessThe tectonic subsidence of the Parnaiba Basin was estimated through systematic backstripping using 22 boreholes which reached the metamorphic or sedimentary basement. A non-uniform lithospheric tretching model was used as a first approximation for the modeling of the tectonic regime. This procedure was suggested by the presence of several grabenlike structures, as confirmed by a few shallow seismic sections, exploratory boreholes and gravity maps.
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Regional Gravity and Aeromagnetic Investigation of the Takutu Basin, Roraima - Brazil
Authors J.V. Miranda, C.J. Ebinger and J.D. FairheadA regional gravity study has been undertaken over the Takutu Basin, in Roraima-Brazil, sponsored by the ABGP (Anglo Brazilian Gravity Project) with logistical assistance provided by the University of Leeds, Earth Sciences Department, the ON (Observatório Nacional do Rio de Janeiro-Brazil) and the CPRM (Companhia de Pesquisa de Recursos Minerais-Brazil). The gravity survey carried out in this region helps to delineate tectonic provinces and to define the boundaries of the Mesozoic Takutu rift basin. The only existing gravity data available, prior to this work, was part of a country-wide data acquisition project from the city of Manaus along the road to the Venezuelan border, forming a long N-S profile. A new set of 291 gravity measurements, with 3km spacing was established using a Lacoste & Romberg gravity meter.
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Caracterizaçăo Geológica e Geofísica de uma Estrutura na Borda Leste do Graben de Umbuzeiro, Bacia Potiguar/RN, e Implicaçőes Neotectônicas
More LessRecentemente, vários estudos de cunho neotectônico estăo sendo realizados no Estado do Rio Grande do Norte, com o objetivo de melhor compreender a deformaçăo atual da crosta, caracterizando as estruturas produzidas e os regimes tectônicos envolvidos. As ferramentas utilizadas nesses estudos săo variadas, destacando-se as análises de mecanismos focais, de rede hidrográfica, de geomorfologia, de fotosensores e levantamentos geológico-estruturais.
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Surface-wave Tomographic Inversion of the South American Lithosphere (The SISAL Project)
Authors J.A. Rial, O.Y. Vdovin, M.H. Ritzwoller and A.L. LevshinWe are studying the dispersion characteristics (measurements of group velocity) of broad band (15-2oosec) Rayleigh and Love waves propagating across the South American continent and adjacent oceans. The ultimate purpose is to invert for a high resolution, tectonic scale (lateral:5ookm) crustal and lithospheric model covering the South American, Cocos, Caribbean, Nazca and Scotia plates to depths of approximately 200 km. To date, data from more than 400 events have been analyzed yielding in excess of 4100 paths across the continent and neighboring regions. These measured group velocities are summarized in the form of tomographic maps for group velocities of Love and Rayleigh waves with periods ranging from 20 to 150 sec. We report the results of a resolution analysis (checker-board test) for a variety of periods for both Rayleigh and Love waves. The estimated group velocity maps are compared with those predicted from a recent model in which the crust is CRUST-5.1 (Mooney et. al., 1996) and the mantle is S16B30 of Masters et.al., (1996).
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Topographic and Structural Expressions of the Ongoing Compression in Midplate South America
New borehole breakouts and focal mechanisms confirm previous results obtained for the World Stress Map and greatly improve our knowledge of the midplate stress field: present-day horizontal compression prevails throughout midplate South America (Assumpcao, 1993; Lima et al, in press). Preliminary geodetic space-based results obtained from the French system DORIS have shown that, for the 3- year interval of observations, the ten available base lines are all shortening. The absolute velocities of the three stations located on the western edge of the plate are oriented to the east, suggesting that most of the shortening is absorbed by the Andes. Nevertheless, two other base lines that do not cross the Andean edge (between Kourou, French Guyana, and Cachoeiro, Southeastern Brazil; between Rio Grande, Southernmost Chile and Kourou) indicate that shortening is also occurring in a midplate setting.
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First Determinations of the Crustal and Upper Mantle Structure in the Amazon from Seismological Data
Authors Joăo Willy Corręa Rosa and Frank ScherbaumWe have installed a temporary seismological array of three broadband stations in a north-south trending line from the Pitinga (PTGA) IRIS station to the city of Manaus. This is a summary of the first stage of a long-term project for the study of the geologic structure of the Amazon Basin, between the Volkswagen Stiftung, the Universitiit Munchen, GFZ-Potsdam and the University of Brasilia. Since the Pitinga (PTGA) IRIS station is located on the shield right at the border of the Amazon Sedimentary Basin, and Manaus is located on the center of the basin, the purpose of this study is to show a first picture of the variations observed in the crustal and upper mantle structure of the Amazon Basin.
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Crustal Thicknesses in SE Brazilian Shield with Receiver Function: Isostatic Compensation by Density Variations in the Lithospheric Mantle.
Authors Marcelo Assumpçăo, David James and Arthur SnokeThe Brazilian Lithosphere Seismic Project (BLSP: University of Sao Paulo / Camegie joint project, 1992-1996) operated temporary broad-band stations in 19 different sites in the southeastern Brazilian shield. Crustal thicknesses were estimated for 16 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The Ps, Moho converted phase, was clearly identified in most stations. Crustal thicknesses were estimated using an average crustal P-wave velocity of 6.45 km/s. VpNs ratios were used as 1.70 for the stations in the Sao Francisco craton and adjacent fold belt (determined from small, local earthquakes), and 1.73 for the other stations. Crustal thicknesses ranged from 37 to 47 km. Although there is a clear normal correlation between topography and Bouguer anomaly in the study area, Moho depths show the opposite pattern from the expected: areas of low topography and less negative Bouguer anomaly, such as the Parana basin, have thicker crust (40 to 47 km) compared with the high elevation areas of the craton and fold belt (38 to 43 km). This may indicate that lower density (~p about -0.04 g/cm3) of the lithospheric mantle under the Archean block of the Sao Francisco craton is required for the regional isostatic compensation. In fact, surface wave dispersion seems to require slightly lower velocities in the upper mantle beneath the craton as compared with the Parana basin. Alternatively, if the density contrasts between Archean and Proterozoic lithospheres is smaller than the values above, then the crust beneath the Parana basin may be denser compared with the craton and fold belt. This would be consistent with magmatic underplating in the lower crust beneath the Parana basin. However, reasonable densities anomalies « 0.06 g/cm3) in the Parana crust would still require low density for the Archean lithosphere
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Some Questions Regarding Methods to Image the Crust at the Ocean Continent Boundary
By M. TalwaniThe need to integrate geophysical data - gravity, magnetic, and seismic - to image the crust at the ocean continent boundary is obvious. In this paper we shall briefly discuss the seismic method
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Seismic Bright Spots, Magmatism, Rheology and Moho Tectonics: New Results from Crustal Seismic Profiling
Authors Larry D. Brown, Andrew Ross and Camelia DiaconescuRecent seismic reflection profiling in Tibet (Project INDEPTH) has discovered some the latest examples of anomalously strong seismic reflections, or "bright spots" in the deep crust. These reflections, characterized by negative reflection polarities, strong shear wave reflectivity, low bulk shear wave velocities and high local conductivities, are interpreted to be fluid, probably magma, trapped in the basement.
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Laminated Lower Crust and Other Reflectivity Patterns
More LessSeismic reflection profiles provide the highest resolution on the in situ structure of the lower continental crust. Reflectivity patterns correlate with distinct geologic settings. There is a tendency of the rigid upper crust to store the memory of old collisional belts, showing ancient thrust-and shear zones.
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Some results of Investigating the Crustal and Upper Mantle Structure in Central Europe by Means of Seismic Surface Waves
More LessThe Earth's crust in central Europe has been investigated in detail along several intemational profiles of deep seismic soundings. Investigations of surface waves allowed to extend the structural investigations into the upper mantle. Pronounced lateral inhomogeneities in the upper mantle were confirmed by the surfacewave method. Moreover, short-period surface waves, genetated by explosions, were used in studying the superficial structure of several sedimentary basins.
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The Observation and Explanation of Asymmetrically Reflected PP and SS Precursors
Authors Martin Schimmel and Hanneke PaulssenSince more than 30 years symmetrically and asymmetrically reflected phases are observed in Pwave coda of seismic recordings for distances ranging between 80 deg and 120 deg. These phases appear before the PP-phase and are therefore referred to as PP precursors. The symmetrically reflected precursors are known as PdP phases and have their reflection point at depth d midway between source and receiver.
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Microtemperature Signals of the Earth's Crust
More LessTemperature variations in the uppermost crustal layers result from several external as well as internal effects. These variations superpose the basic and rather stable energy flow from the earth's interior. Variations of the surface temperature, caused by diurnal, annual or long-term waves, jerks or other changes, dissipate into the subground to different extents and may be detected by measurements in the subground. Also environmental conditions, e.g. the cutting of tropical forest, cause a variation of the subsurface temperature as demonstrated with measurements.
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Petrology and Geochemistry of the South Atlantic Ridge from 0° to 42° S
More LessPetrological and geochemical variations along the South Atlantic Ridge show that eruption temperatures for the South Atlantic Ridge primitive basaltic glasses (MG#>64) decrease systematically along the ridge to the south from 0° to 42°S which appears to be related to the lower mantle temperature gradient. The lower mantle (below the 650 km discontinuity) beneath the South Atlantic shows a lower seismic P wave velocity with a gradual increase southward (around 42°S) (Dziewonski, 1984). This seismic velocity is related to temperature, or degree of melt, where higher seismic P wave velocities indicate colder material. The lower mantle temperature also seems to play an important role in controlling the degree and volume of melting along the South Atlantic Ridge.
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Large Igneous Province
More LessLarge igneous provinces (LIPs), voluminous emplacements of predominantly mafic extrusive and intrusive rock, originate via processes other than those responsible for the creation of crust in the plate tectonic cycle, i.e., 'normal' seafloor spreading and subduction. They are produced when ascending hot plumes of mantle material reach the upper crust, and are generally considered to be relatively or totally independent of the movement of the Earth's lithospheric plates. LIPs and plumes therefore manifest a mode of mantle circulation not accounted for in the existing plate tectonics paradigm.
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Effects of Small-scale Convection in Igneous Crust Production at Rifted Continental Margins
Authors R.R. Boutilier and C.E. KeenObservations of large igneous crustal thickness at rifted volcanic margins has led to suggestions that small scale convection in the upper mantle may be in part responsible for increasing the delivery of magma to the overlying lithoSphere. We have quantitatively assessed this concept by numerical modelling H271of the viscous flow caused by divergent plate motions. The model consists of a viscous, temperature and pressure dependent, non-linear fluid whose behaviour is found by solving the Navier Stokes equations for incompressible flow in two dimensions using a finite element formulation.
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Were There Moving 'Plumelets' in the South Brazilian Continental Lithosphere
By V.M. HamzaIt is proposed that melt fronts generated by Tristan da Cunha plume during Cretaceous times is the deep thermal process responsible for the observed features in the spatial distribution of heat flow anomalies and alkaline magmatism in southern Brazil. Model studies indicate that plume derived melts can move over large distances without losing much of their thermal energy. Changes in the flow rates of such plumelets along networks of deep lithospheric channels can account for the observed age pattern of alkaline intrusives. It is likely that melt fronts are currently located at or near areas of high heat flow and suitable geoelectrical sounding methods should be capable of detecting their presence.
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Tectonic Evolution of the Rift Basins in the Northeastern Brazilian Region
Authors W.U. Mohriak, M. Bassetto and I.S. VieiraThis work discusses the tectonic evolution of the Tucano and Sergipe rift basins onshore and offshore the northeastern Brazilian region, based on integration of deep seismic reflection profiling, potential methods, geological data, and tectonic analysis. We address the following themes: a) the geologic evolution of the rifts; b) the potential methods response to the deep structure of the rifts; c) the seismic expression of major tectonic features; and e) a possible geodynamic model for the evolution of the rift system.
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Marine Geology and Geophysics of the Eastern Mediterranean
Authors J. Mascle and E. ChaumillonThe Eastern Mediterranean sea appears to be a relict oceanic space between two major continental plates Europe to the north and Africa to the south. Today this marine space includes two relatively deep basins: the oceanic crust floored Ionian sea to the west, believed to be of Mesozoic age and the Levantine basin where some remnants of a triassic oceanic crust have been indicated. The major morphostructure of the area are however the so-called Mediterranean Ridge, the Nile deep sea fan and the Cyprus outer arc.
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SAGE (Summer of Applied Geophysical Experience) Sets New Trends in Geophysics Education
More LessSAGE (Summer of Applied Geophysical Experience) focuses modern geophysical methods on major geologic problems using seismic reflection and refraction, gravity, magnetics, electrical resistivity, magnetotellurics (MT), and other electromagnetics. The four week field format includes approximately equal portions of hands-on field work, lectures/discussion, and computerprocessing/interpretation. The field area is in and adjacent to the Rio Grande rift in the State of New Mexico, USA. Major explosion seismic and vibroseis efforts, gravity, and MT have been successfully directed at understanding rift processes. Recent studies have applied shallow geophysics to local groundwater investigations and environmental problems at the Los Alamos National Laboratory (LANL).
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Implicaçős Tectônicas da Curva de Deriva Polar Aparente Mesozóica para a América do Sui
More LessEstudos paleomagnéticos provaram ser uma ferramenta poderosa na quantificaçăo dos deslocamentos das placas litosféricas, bem como em problemas de deformaçőes regionais. Rotaçőes crustais em relaçăo a qualquer eixo podem ser detectadas pelo paleomagnetismo, destacando-se as rotaçőes in situ, ou segundo eixos verticais, que de outra forma năo seriam facilmente identificáveis. Entretanto, para que isso seja possfvel, é necessário que os dados de regiőes deformadas sejam comparados a dados de áreas cratônicas. Estes dados levam ao cálculo de pol os paleomagnéticos de referęncia. A sequęncia de polos de referęncia de diferentes idades, vern a ser a curva de deriva polar aparente (CDPA) para urn bloco continental e deve refletir somente os deslocamentos da placa litosférica através do tempo.
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The Neoproterozoic Rodinia, its Break-up and the Formation of Gondwana: Paleomagnetic Evidence.
Authors M.S. D‘Agrella-Filho, R. Siqueira, R. Trindade, C.F. Ponte-Neto and I.G. PaccaHoffman (1991) elaborated a model proposing the Meso/Neo Proterozoic Rodinia supercontinent (Fig. 1), based on geological and (partly) on paleomagnetic grounds. He suggested that Baltica, Amazonia, Congo and Kalahari were the most probable units to border eastern and southern Laurentia, since they have similar Grenvillian orogenic belts (1.2-1.0 Ga). In his model, Hoffman (1991) suggests that the (present) northeastern Kalahari craton (including the Lurian belt) was flanked by East Antarctica, and that this craton belonged to the East Gondwana during the amalgamation of the Gondwana supercontinent in the Neoproterozoic.
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Nova Metodologia para a Determinaçăo da Suscetibilidade Magnética e da Magnetizaçăo Remanenre de Rochas
More LessUsualmente a determinaçăo da suscetibilidade magnética de rochas e efetuada em ponte de susceptibilidade utilizando-se corpos de prova cilíndricos de 1"x 2" e para a medida deste parâmetro físico em direçăo ortogonal é necessário a confecçăo de outro corpo de prova. A precisăo deste tipo de determinaçăo é, na prática, usualmente de 5 a 10% com sensibilidade de l x l O" u.c.g.s. No caso de nao de dispor de amostras de tamanho suficiente para a elaboraçăo de tais corpos de prova cilíndricos, pode se utilizar de fragmentos de rocha, mas neste caso a precisăo cae. Por este motivo, e para poder medir as propriedades magnéticas em pequenas amostras, o autor propôs uma nova metodologia para determinaçăo deste parâmetro em gemas, e rninerais (Haralyi 1993, 1994) cuja abrangencia e agora estendida para rochas é ensejando determinaçăo com maior precisăo e repetibilidade.
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Estudio Paleomagnetico de La Formacion Tipilihuque (Alumine, Argentina): Un Unico Evento Volcanico?
Authors G.H. Ré, O. Ruffo and R. TomezzoliEn la región comprendida entre Zapala y Junin de los Andes (prov. de Neuquén), se observan importantes afloramientos de secuencias basálticas de edad cenozoica; estas secuencias fueron ordenadas estratigráficamente como Basaltos del cero al siete; los cuales abarcan desde el Oligoceno (Basalto 0) al Holoceno (Basalto VII); y se los puede agrupar en dos grandes períodos magmáticos: uno pre- Mioceno y otro al Plio-Holoceno.
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Correlaçăo Geotectônica de Dados Aeromagnéticos e Gravimétricos no Norte do Cráton do Săo Francisco Regiăo Nordeste
More LessNa literatura especializada já é fato consagrado a utilizaçăo de dados aeromagnéticos e gravimétricos para interpretaçőes tectônicas. Os dados geofísicos fornecem um visăo em profundidade que corresponde a uma forma simplificada da terceira dimensăo dos dados geológicos, tornando-se imprescindível em áreas de escudos e faixas dobradas pré-cambrianas pouco conhecidas ou parcialmente encobertas por sedimentos fanerozóicos. O objetivo desta comunicaçăo é definir o arcabouço geotectônico de uma área complexa, caracterizada pela contigüidade de tręs províncias geológicas importantes, o Cráton do Săo Francisco (CSF), a Província Borborema (PB) e a Bacia do Parnaíba (BP), (Fig. 1).
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Estruturaçăo das Associaçőes Petrotectônicas do Escudo Sul-Rio-Grandense: Caracterizaçăo Geofísica Através da Magnetometria, Gravimetria e Gamaespectrometria
Authors A.F. Uberti Costa and L.A. D‘Ávila FernandesImportantes definiçőes referentes ŕ compartimentaçăo das associaçőes petrotectônicas encontradas no Escudo Sul-Riograndense foram obtidas através da análise de dados magnetométricos, gravimétricos e radiométricos. Essas informaçőes, que permitem também a observaçăo em profundidade de feiçőes geológico-estruturais significativas, e que pela primeira vez săo examinadas em conjunto, cobrindo toda a extensăo do Escudo, vern contribuir consideravelmente para urn melhor entendimento dos processos de estruturaçăo da crosta continental dessa regiăo.
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Estrutura Crustal do SuI e Sudeste do Brasil Inferida a Partir de Dados Aeromagnéticos e Geotérmicos
Authors P.L.F. Rocha, L.F.S. Braga and V.M. HamzaA utilizaçăo de dados aeromagnetométricos é de fundamental importância na exploraçăo de recursos minerais e na exploraçăo de petróleo, sendo empregado para definiçăo de lineamentos e faixas crustais em escala continental até a caracterizaçăo de detalhes estruturais dentro dos sedimentos.
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Correlaçăo entre Condutividade e Dilataçăo Térmica em Rochas Metamórficas da Regiăo de Săo Sebastiăo, Estado de Săo Paulo
Authors R.P. Pacheco and V.M. HarnzaAs propriedades térmicas dos solidos, tais como a condutividade e dilataçăo, săo geralmente dependentes de diversos fatores intrínsecos e ambientais. No caso de materiais geológicos, os fatores irnportantes săo: composiçăo mineralógica, estrutura cristalina, porosidade, pressăo e temperatura. Conhecimentos das influęncias destes fatores e das inter-relaçőes entre as propriedades termofísicas săo considerados básicos nos estudos do regime térmico do interior da Terra.
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Infravermelho Termal ( 8 - 12 urn ) na Caracterizacao de Rochas Vulcânicas
Authors A.P. Pachęco and Í. VitorelloEmissividades e inércias térmicas de amostras de rochas vulcânicas da Bacia do Paraná foram determinadas em Iaboratório por métodos radiométricos no infravermelho termal (8 -12 µm). Relaçőes lineares da emissividade e inércia térmica com Si02 mostraram nítida diferenciaçăo entre os litotipos básico e ácido e tendęncia de separaçăo no litotipo básico de amostras do norte e suI da Bacia.
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Domínios Estruturais da Porçăo Oceânica da Margem Sul-Sudeste Brasileira
Authors M. Bassetto and F.F. AlkmimO estudo dos dados sísmicos e potenciais do Projeto LEPLAC (Plano de Levantamento da Plataforma Continental Brasileira), no âmbito da porçăo oceânica da Margem Continental Sul-Sudeste do Brasil, revelou significativa compartimentaçăo morfoestrutural da área (Souza et al. 1992; Bassetto 1997). Natureza das estruturas dominantes do embasamento, espessura crustal ígnea, feiçőes do embasamento acústico e relevo do fundo oceânico săo parâmetres que exibem variaçőes consideráveis e relativamente ordenadas ao longo do setor estudado.
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Caracterizaçăo Geotérmica no Campus da Universidade do Amazonas
Authors P. Noronha Serra, R.L.C. Araujo, J.S. Carvalho and W.S. Marques PalhaDurante os meses de janeiro a dezembro de 1996, com o objetivo de verificar as variaçőes diurnas e sazonais de temperatura, foram realizadas, diariamente, medidas de temperatura em dois furos, denorninados respectivamente de Furo 1 e Furo 2, nas profundidades de 0,0m, 0,5m e 1,0m, sendo um sem (F1) e o outro com cobertura vegetal (F2). Tais medidas foram efetuadas nos horários das 10h e 18h. As medidas a 0,0m foram realizadas com o auxílio de um termômetro, cujo sensor é um termopar, com precisăo da ordem de ± 0,05°C. Nas de 0,5 m e 1,0m foram utilizados termômetros de termistor, com precisăo da ordem de ± 0,005°C. O presente trabalho é de fundamental importância para dar continuidade aos estudos sobre a influęncia da incidęncia da radiaçăo solar na superfície terrestre e da posterior perturbaçăo térmica propagada para as camadas subsuperficiais (Araujo, 1987; Carvalho, 1981; Souza et al., 1990;), bem como as variaçőes de temperatura decorrentes desse processo.
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Velocidade de Transmissăo da Perturbaçăo Térmica Superficial
Authors R.L.C. Araujo, J.S. Carvalho, W.S.M. Palha and P.N. SerraNa atualidade é intensamente difundido que a atividade de desmatamento na Amazônia provoca diversos distúrbios ambientais, a níveis local, regional e global (Souza et aI., 1996; Souza et aI., 1989; Araujo et aI., 1995). No entanto, năo há, ainda, informaçőes quantitativas suficientes para se elaborar urn modelo, plenamente aceito, o qual descreva, detalhadamente, a influęncia da cobertura vegetal sobre o regime geotermal raso, ou seja, sobre a estrutura geotérmica nas camadas mais superficiais da Terra. Tal conhecimento é de suma importância, face ao fato de que flutuaçőes geotermais em tais camadas acarretam consequęncias diretas na biosfera terrestre, como também, ser tal zona de fundamental interesse na prospecçăo mineral. Os resultados constantes neste trabalho săo relevantes no contexto da geoterm a, pois permitem o melhor conhecimento sobre a estrutura geotérmica rasa.
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Estudo Acerca das Influęncias Sobre os Valores de Condutividade Térmica
More LessNa regiăo Amazônica, a estrutura geotermal rasa sofre mensuráveis perturbaçőes térmicas, devido principalmente a grande incidęncia da radiaçăo solar na superfície terrestre. Além deste, outros fatores podem regular o comportamento térmico de uma área, tais atores săo: condiçőes do solo, precipitaçăo pluviométrica, evapotranspiraçăo, etc. Devido aos efeitos deste fatores, săo poucos os trabalhos de geotermia que consideram a zona situada desde zero até cerca de quarenta metros de profundidade, sendo a grande maioria dos estudos realizados a maiores de profundidade, a partir de onde considera-se que os efeitos transitórios năo săo mensuraveis.
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Variaçőes do Fluxo Geotermal Raso em Áreas Com e Sem Cobertura Vegetal: Um Estudo de Caso na Regiăo Amazônica (Brasil)
No presente trabalho foi realizado urn estudo sobre o regime geotermal raso na área do Campus da Universidade do Amazonas, no município de Manaus, no período de setembro/95 a junho/96. Os dados necessários para a realizaçăo deste trabalho foram coletados em dois locais, sendo um com e outro sem cobertura vegetal, a fim de melhor verificar as influęncias da radiaçăo solar incidente nos mesmos. Consideramos este trabalho de suma importância no âmbito da geotermia rasa na Amazonia, pois seus resultados permitiram urn maior entendimento da estrutura geotermal rasa; a determinacao da faixa de variacao do fluxo termico raso, alem de tomar possivel efetuar previsoes geotermicas.
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Relationship between Regional and Local Heat Flow in a High Enthalpy Geothermal Field-los Azufres, Mich, Mexico
More LessIn the last five years growing interest in low enthalpy and medium size geothermal prospects in Mexico increased the institutional support for thermal methods research. The Comision Federal de Electricidad (CFE) decided to analyze temperature data taken during drilling stops in boreholes of well known geothermal zones, to analyze the order of magnitude and characteristics of thermal conditions at different locations, relative to the faults acting as heat and mass flow channels, with the goal to define interpretative criteria for the study of less known areas.
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AMAZONScope: A BB Digital Seismograph Network for Monitoring Seismicity of the Brazilian Amazon
More LessTwenty years ago very little was known about the seismic activity of the Amazon region. The low population density and the delay in the installation of seismograph stations in the region contributed to the lack of knowledge about possible earthquakes. This situation only changed in the late 70's when studies about the seismicity of the Brazilian Amazon region were initiat+ed+H308 with the progressive installation of a network of stations through co-operative agreement between the University of Brasilia and Eletronorte (Electric Plants of Northern Brazil), The number of earthquakes that have already been recorded and the known cases of reservoir induced seismicity (RIS) show that the region bears an important level of seismic activity which needs to be carefully evaluated. To improve the capability of-seismic detection in this region, which represents 60% of the national territory, a modem network of stations, named AMAZONS cope, was conceived. Its purpose is to perform a continuous seismic monitoring program. The Network installation, operation and data analysis and interpretation, will be done by the University of Brasilia (UnB) Seismological Observatory (SIS) in collaboration with Eletronorte and of the Amazonian Military Command (CMA).
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Recent Seismic Activity in the S-W Edge of the Mato Grosso-Goiás- Tocantins (MGT) Belt (Central Brazil)
More LessIn Central Brazil lies one physiographic province with outstanding geological, geophysical and seismotectonical features, it is known as Goias- Tocantins seismic belt (Assumpcao et al. 1986; Veloso 1988; Fernandez et al. 1991), but for reasons to be explained bellow, we advance the more appropriate denomination Mato Grosso-Golas- Tocantins seismic belt. This physiographic province (Fig. 1) is a Proterozoic orogen formed by an inversion of an oceanic basin, provided by the convergence between the Amazon and the San Francisco-Congo cratons during formation of the eastern part of the Gondwana super-continent (Fuck et al. 1993, 1994; Fuck 1994). The present dynamics of this province is represented, among other factors, by the conspicuous seismicity ranging the microearthquake to small earthquake levels (Fig. lA) , that present a NE to SW trend, extending from Tocantins state (in north) to Mato Grosso state (in south)[hence, we consider more suitable to call it Mato Grosso-Goias-Tocantins seismic belt (MGT belt)]. During the beginning of 1996 a salient seismic sequence originated some 10 to 15 km north of the small town of Araguaiana (Mato Grosso). This paper is intended to introduce a brief presentation of the main features and findings related to this earthquake sequence.
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Reservoir Induced Seismicity at Nova Ponte (MG): Revisited
Nova Ponte reservoir (situated on the Araguari and Quebra Anzois rivers, planned capacity 1.28 x 109 m3 , maximum water depth 132 m; formed behind an earth-rockfill type dam with height 142 m and length 1600 m) is one of the recent cases of reservoir induced seismicity (RIS) in Brazil (see also Veloso et al., 1994 and 1995 or Ferreira et al., 1995 for presentation of other case histories of RIS in Brazil). Pre-impounding seismic monitoring has started 8 years before filling, using a single l-component SP analog seismic station, without detecting any local seismic activity in a radius of 80 km around the dam. Shortly after beginning of impounding (14 October 1994) quasi-continuous induced activity was detected. The seismic activity was triggered as soon as the lake attained about 20% of the full planned volume.
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Correlaçăo entre a Sismicidade e a Estrutura da Bacia Potiguar, NE do Brasil
Authors J.M. Ferreira, J.A.M. Moreira, R.T. Oliveira, M.K. Takeya and M. AssumpçăoA borda da bacia potiguar é de há muito reconhecida como uma das mais importantes regiőes sísmicas do país (Berrocal et al., 1984). No entanto, até o momento, năo se tentou ainda correlacionar a atividade sísmica com a estrutura da bacia, 0 que será feito neste trabalho.
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As Assinaturas Sísmicas Crustais da Sub-Bacia do Jandiatuba (Bacia do Solimőes)
Authors R.J. Vega Sacasa and J.B. Sena CostaO presente trabalho é uma síntese das informaçőes geológicas e geofísicas da parte ocidental da Bacia do Solimőes, denominada de sub-bacia do Jandiatuba. A análise de informaçőes da subsuperfície săo representados por dados de sísmica profunda na forma de uma seçăo sísmica com registro de 20 segundos, localizada na parte central da sub-bacia do jandiatuba, assim como da compilaçăo da literatura intemacional relacionada com programas de levantamentos a grandes profundidades realizadas em diferentes partes do mundo como o COCORP, BIRPS, DEKORP e ECORS. Estes resultados foram relevantes para fins de comparaçăo e correlaçăo com o objetivo de contribuir no entendimento geotectônico da refletividade desse segmento crustal da amazonia.
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Determinaçăo da Espessura Crustal na Margem Ativa da Regiăo Antártica
Authors L.C. Torres, B.S. Gomes and L.A.P. GamboaO propósito de presente trabalho consiste na apresentaçăo de urn modelo capaz de contribuir para determinar a espessura crustal da regiăo compreendida pela fossa Shetland do Sul, arquipélago vulcânico Shetland do SuI e bacia Bransfield, na regiăo Antártica. Para alcançar tal propósito, foi utilizado urn perfil geofísico contínuo (sísmica multicanal, magnetometria e gravimetria) com 393km de extensăo, coletado pelo Brasil durante os anos de 1987 e 1988. O perfil gravimétrico observado, após reduçăo de latitude e Eötvös, gerou a gravidade ar livre, a qual foi comparada com o perfil gravimétrico produzido pelo modelo proposto. Através da análise dos resultados, pôde-se comprovar a complexidade da geologia regional, apresentando uma tectônica compressiva no limite convergente de placas e uma tectônica distensiva no interior da bacia Bransfield.
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Sistema de Correçăo de Tempo, Baseado em GPS, para Estaçőes Sismográficas Analógicas e Digitais
O Observatório Sismológico (SIS) da Universidade de Brasília (UnB), em parceria com a iniciativa privada, desenvolveu e implementou um projeto de sistema de aquisiçăo de tempo preciso para estaçőes sismográficas analógicas e digitais. O sistema funciona utilizando informaçőes de tempo e pulsos de IPPS ( um pulso por segundo), provenientes de um GPS (Global Positoning System). Fornece como aídas informaçőes de hora, minuto, segundo e sinais de tempo codificados. A preciçăo e de um milisegundo. A execuçăo deste projeto resolveu, de forma relativamente barata, um antigo problema relacionado com a falta de urn sistema de tempo, que fosse, barato, preciso e apropriado para estaçőes sismográficas analógicas (Barros et al, 1991) e arranjos sismográficos digitais (Barros et al, 1993 e Barros 1995).
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Inversăo 2-D para a Distribuiçăo Global da Velocidade de Grupo das Ondas Rayleigh
Authors A.N. de Castro Santa Rosa and J.W. Corręa RosaDesde 1960, tem-se observado grande desenvolvimento técnico dos instrumentos com PRESS (1956) e SANTO & SATO (1966). Os registros disponíveis das estaçőes digitais săo obtidos das redes como: GDSN, IDA, IRIS e GEOSCOPE. Com todo este avanço, a precisăo das medidas cresceu e, como conseqüęncia disto, a tomografia utilizando dados de período longo, pode ser usada para, em grandes scalas, observar a heterogeneidade lateral de modelos na Terra. Temos, como exemplos destes trabalhos: SOURIAU & SOURIAU (1983), onde foi considerada a inversăo de dados de velocidade de fase semelhante foi o trabalho de ROSA (1986). Neste trabalho observamos as anomalias de velocidade de grupo relacionadas as grandes estruturas tectônicas na crosta e no manto da Terra.
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Inversăo Tomográfica com ondas Rayleigh no sul do Cráton do Săo Francisco e Faixa de Dobramentos Brasilía-Uruaçu
Authors A. Marchioreto and M. AssumpçăoNeste trabalho, determinaram as velocidades de grupo de ondas de superficie Rayleigh Rg de sismos regionais. Uma tomografia 2-D foi efetuada para quantificar as heterogeneidades laterais apresentadas pelas curvas. Por fim, a estrutura sísmica rasa (velocidade da onda S) foi obtida invertendo a velocidade de grupo regionalizada. A Figura 1 mostra os epicentros dos sismos, as estaçőes e o contorno das principais feiçőes geológicas que săo: o sul do cráton do Săo Francisco (SF) juntamente com o sul do Grupo Bambuí (BB), o suI da Faixa de Dobramentos Brasília-Uruaçu (BU), a Nappe de Empurrăo Socorro-Guaxupé (GX) e a Bacia do Paraná (PB).
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Evidencias de Anomalias Geoquimicas na Composicao do Manto sob a Porçăo Central do Cinturăo Móvel Costeiro e Regiőes Adjacentes (SE-Brasil)
Authors I. Ludka, C.M. Wiedemann, B. Lammerer and F. SöllnerO estudo de diversos magmatitos básicos, de idade e contextos geológicos diferentes, evidenciou zaracterísticas geoquímicas marcantes, peculiares ao manto sob a porçăo central do Cinturăo Móvel Costeiro. A geoquímica das rochas básicas mostra enriquecimentos anormais nos elementos incompatíveis Ba, Sr e ETRL (elementos terras-raras leves), além de uma alta razăo inicial de Sr87/Sr86, comuns a todas as ocorręncias estudadas. O detalhamento da petrografia e do quimismo mineral foi ponto de partida para caracterizar as rochas básicas como ígneas bem preservadas, representantes de fusőes mantélicas.
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Contribuiçăo Estratigráfica para uma Nova Visăo Paleomagnética das Formaçőes Cabeça, Longá e Poti (Bacia do Parnaíba), nas Regiőes Centro-Oeste e Nordeste do Estado do Piauí
Authors B. Vieira de Alencar, M. da Silva Borges and A.M. GóesOs dados paleomagnéticos provenientes das formaçőes Poti e Longá, na regiăo entre Floriano e Nazaré do Piauí, e das formaçőes Cabeças e Longá da regiăo de Batalha/Barras, foram reavaliados sob o ponto de vista estratigráfico. A partir de dados paleomagnéticos foram definidos 4 conjuntos estatísticos: 1) topo da Formaçăo Cabeças, 2) Formaçăo Longá, 3) base da Formaçăo Poti e 4) topo da Formaçăo Poti. Segundo as reavaliaçőes feitas nestes dados, variaçőes geométricas importantes foram constatadas, sugerindo que: 1) uma possível influęncia glacio-tectônica na Formaçăo Cabeças pode ter provocado rotaçőes/translaçőes nas camadas sedimentares; 2) a concentraçăo de direçőes medias próxima da vertical suscita a influęncia de clima mais frio durante a deposiçăo da Formaçăo Longá, possivelmente com tendęncia peripolar; 3) as variaçőes das inclinaçőes de intermediárias a baixas da Formaçăo Poti levam a idéia de uma aproximaçăo do equador durante a sua deposiçăo, talvez em clima temperado.
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Paleomagnetism of Carbonatic Sequences from the Bambui Group-Sao Francisco Craton
Authors M.S. D‘Agrella-Filho, M. Ernesto, R.I.F. Trindade, R. Siqueira, J. Nobre-Lopes and K. KawashitaThe Neoproterozoic period corresponds to a very important phase in the evolution of the Earth, when the Rodinia supercontinent desagregated to form the Gondwana (e.g. Dalziel, 1997). Although speculative models based on geological grounds can be proposed for the evolution of these supercontinents, paleomagnetism can establish the paleogeography of the envolved cratonic units more precisely. The Sao Francisco Craton is covered by extensive carbonatic layers which were deposited during Neoproterozoic. With the purpose of establishing the paleogeography of the Congo/Sao Francisco Craton in the Rodinia and Gondwana, a paleomagnetic study of these sedimentary covers is being carried out. Results from the carbonatic sequences of the Salitre Formation (Una Group, Bahia State) was already published by D'Agrella-Filho (1995). In this paper we present preliminary paleomagnetic results from the Bambui Group (Minas Gerais State), which is possibly correlated with the Una Group.
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Correlaçăo entre Orientaçăo Cristalográfica Preferencial de Anfibólio e Anisotropia de Sucetibilidade
More LessPara a análise estrutural de uma área tem sido utilizadas várias técnicas que procuram caracterizar de maneira realista o caminho da deformaçăo, assim como, o estado de deformaçăo finita. Entre as vorias metodologias, o estudo de petrotrama tem sido bastante utilizado, entretanto, isso tem consumido um grande intervalo de tempo, seja obtendo os dados através de um goniômetro de textura ou usando uma platina universal. O propósito desse trabalho e apresentar uma correlaçăo entre análise de anisotropia de suscetibilidade magnética (ASM), orientaçăo cristalográfica preferencial e o elipsóide de deformaçăo finita em gnaisses migmatíticos e quartzitos, procurando deterrninar regras que possam ser aplicadas a outros tipos de rochas. O estudo foi desenvolvido nas rochas embasamento cristalino da regiăo de Granja (extremo noroeste do Ceará) que é formado por gnaisses diversos, migmatitos e granulitos, e nos quartzitos da seqüęncia supracrustal. Estruturalmente, a área é recortada por zonas de cisalhamento (ZC) transcorrente regionais com direçăo NE-SW, exemplificadas pelas ZC de Granja, ZC Jaguarapi e ZC Araças. As amostras foram coletadas obedecendo uma gradaçăo deformacional, em relaçăo progressiva com as zonas de cisalhamento transcorrente.
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EI Chapualitense Inferior en Neuquen y Mendoza (Argentina): Implicancias con Respecto a su Edad
Authors G.H. Ré, R. Tomezzoli and O. RuffoLa región comprendida entre los 33° y los 46° S, presenta importantes afloramientos de secuencias basálticas de edad cenozoica; y se conoce como Zona Volcanica Sur. Estos afloramientos se ubican al este del frente volcánico actual y presentan características de volcanismo de intra-arco y de retroarco. El volcanismo de intra-arco se desarrolla entre los bloques precordilleranos, los cuales se encuentran separados por estructuras tipo graben que sugieren la existencia de un periodo de extensión intrarco durante el Plio- Holoceno.
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Magnetic Mineral Investigations of Rocks of the Urals Superdeep Borehole Section: Ore Formation Aspects
The main aim of the Urals superdeep borehole (SD-4) drilling is the description of structure, nature and evaluation of the Earth crust and ore bearing complexes in the zones of mobile continent belts after the example of the Middle Urals. Besides, an evaluation prospects of ore bearing deep horizons and revealing of new unknown sources of ore raw materials in a famous ore province is necessary for the development of models of typical for this region deposits.
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Mantle Diapirs and Sedimentary Basins Formation and Evolution. Case Study of the Pre-Caspian Depression and Sedimentary Basins of Brazil
By V.B. SvalovaThe origin and evolution of sedimentary basin could be a clue for oil-gas deposits forecasting. For simulation of the basin evolution and pressure and heat flow density distribution the thermomechanical modelling was used. Data on vitrinite reflectance were used as a geot-hermometer for paleothermal reconstruction of the sedimentary basin. Thermomechanical modelling together with field investigations and wells data gives the possibility to research the geothermal field changing during the sedimentary basin evolution. All geological, geophysical and petrological data were used for sedimentary basins evolution modelling. Different sedimentological models were used for analysing of sedimentary cover formation and evolution. For simulation of sedimentary basins formation and evolution the hierarchical mechanical-mathematical multi-layer models of high-vis cous fluid were used.
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Hidratos de Gás Marinhos: A Mega Ocorręncia da Bacia de Pelotas/Brasil:
Authors A.R.E. Sad, D.P. Silveira and M.A.P. MachadoHidratos de gás, também conhecidos como clatratos ou crioidratos, săo compostos sólidos congelados, formados por água e gás natural. Sua estrutura cristalina comprime de tal forma o gás que, em 1 m3 de hidrato, é possível ocorrer até 164 m3 de gás. Săo formados sob condiçőes de alta pressăo, baixa temperatura (inferior a +4°C) e concentraçőes adequadas de moléculas de gás na água dos poros dos sedimentos. Na natureza, tais condicoes coexistem nos assoalhos marinhos com laminas d'água superiores a 300/500 m (dependendo da temperatura de fundo da água), bern como em regiőes polares, associadas com permafrost (Max e Lowrie, 1996).
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Continental Lithosphere Deep Structure Researches on the Base of Scientific Deep Drilling
Authors Yu.l. Kuznetsov and V.B. SvalovaInvestigation of deep structure and geodynamic regimes of the lithosphere on the base of deep drilling and geotransects are extreamely important for fundamental Earth sciences development and promotion of the applied problems decision, especially understanding of processes of oil, gas and ore generation, evaluation of geothermal energy, interpretation of petrological, geological, geothermal and geophysical data, burial of radioactive waste.
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Studies of Deformation Processes in the Central Andes: New Data Shed Light on Old Problems
By H.J. GoetzeFor several years crustal processes near the convergent plate boundary of South America have been studied by the "Collaborative Research Center 267, (SFB 267)" entitled "Deformation Processes in the Andes". The studies of the last 4 years focus on interdisciplinary geoscientific research in the Central Andes crossing the fields of geology, geophysics, geodesy, petrology and remote sensing. This combination of geophysical surveys employing active and passive seismology, potential field data, and magnetotellurics, together with GPS monitoring, geological field work, petrological and geochemical analysis and rock age dating, and images of remote sensing provides new insights into the structure and tectonic evolution of this huge mountain range. Field activities are concentrated on a segment, extending between 20 and 30 degrees of latitude south, from the Pacific Ocean to the eastern Andean foreland, covering parts of NW Argentina, S Bolivia and N Chile.
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Propagation Efficiency of Long-Period Lg Waves in the Brazilian Shield and the Andes
Authors J.A. Rial and M.H. RitzwollerWe investigate the propagation efficiency characteristics of long-period (3-12s) Lg waves in continental South America as recorded in the last few years on stations of the GTSN, IRIS/uSGS, IRISIIDA and GEOSCOPE networks. This note reports results of a first look at the available Lg data for events with Ms>5.0 from 1990 to 1995.
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A Critical Analysis of Different Structural Models of the Eastern Cordillera, Colombia
Authors M.G. Rowan, R. Linares and T. VillamilFive published cross sections through the Eastern Cordillera show net shortening values that range from 68 km to 230 km for essentially the same transect through the area of Tunja. Each is compatible with surface exposures and geometrically balanced, but the subsurface structural styles are strikingly different. Low shortening values result from thick-skinned interpretations of an inverted rift basin, medium values are derived from thin-skinned interpretations dominated by fault-bend folds, and the highest value is from an interpretation showing overthrusting of basement-involved nappes.
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The Central Andean Transect GGT 6 - From Water Colors to the World Wide WEB
By H.J. GoetzeThe Global Geoscience Transects Program (GGT) was conceived by the Inter-Union Commission on the Lithosphere (ICL) in August 1985. The project encourages geoscientists in all countries of the world to produce cross-sections up to a few thousands of kilometers in length and drawn to the base of the crust and even deeper where data permit, using all available geological, geophysical and geochemical information. The latest transect compilation done by colleagues from Russia and China (Murmansk - Altai to Pacific ocean, length approx. 8.000 km) is a very impressive example. Transects are drawn to common scales and formats so that the earth's crust in different parts of the world can be directly compared.
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Seismic Acquisition in the Southern Bolivian Thrust and Fold Belt
Authors R. Dittz Chaves and J.R.S. Soares FilhoImaging in Trust Belt areas is a challenge to Seismic Surveying. On Anticlines, structural complexities associated to strong lateral velocity variations and rugged topography have close relationship with the deterioration of the quality of seismic data. Otherwise the seismic operation is plenty of hazards, demanding extensive use of helicopters and constant help of climbers. Petrobras Bolivia (PEB) and Partners face this challenge in two Blocks located on the Sub-Andean Zone of Bolivia. As these Blocks, were never object of seismic surveys, parameters design were biased by available structural and geologic data and similarities of these Blocks with areas of the Northwestern Basin of Argentina and other fold-belt areas. Additionally source tests were carried out aiming at defining optimum charge amount and depth. In this paper we show that the seismic lines acquired with the proposed parameters exhibit an overall improvement when compared to previous sections acquired nearby over structures in the same trend.
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Evidences of a Continous Wadati-Benioff Zone Beneath the Central Portion of the Andean Region
Authors J. Berrocal and C. FernandesImportant controversies exist about the seismotectonic characteristics in the Andean region, in relation both to the morphology of the Wadati-Benioff zone (WBZ) and to the focal mechanism that cause the very deep (h>500 km) South American earthquakes. These deep earthquakes define the deep structure of the subducted Nazca plate beneath the Andean region and, when correlated with the spatial distribution of shallower earthquakes, support the hypothesis of a twisted slab that is continuous laterally and in depth, at least in the central portion of the Andean region. The time distribution of deep South American earthquakes suggests also a lateral continuity of the slab, based on the probable temporal correlation between the activity in the southern and northern deep earthquakes' segments, especially in the case of the sequence occurred immediately before and after the Mw 8.2 deep southern Colombia earthquake of July 31, 1970. Engdhal et at. (1995) arrive to the same conclusion of a continuous slab in depth and laterally, by using tomographic images beneath western South America. The continuity of the slab under the central portion of the Andean region, supports the idea that the WBZ portion between 01°S and 200S, is being twisted relatively to the South, in such amount that its deepest corresponding extremes are, at present times, at latitudes 06°S and 29°S respectively. The hypothesis of a twisted slab, that may be provoked by the northwards component of a presumable slab translation, permits to infer a flat horizontal mechanism for the very deep South American earthquakes, similar than the focal mechanism determined for the large Mw 8.3 deep Bolivian event of June 1994. In this case, instead of only considering the dip-slip almost vertical plane as the solution, the other plane that indicates horizontal strike-slip motion for the very deep South American earthquakes, should also be considered.
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Justificación de la Longitud de Onda Andina del Geoíde en una Seccion en 24,5° de Lat. Sur
Authors M.C. Pacino and A. IntrocasoLa topografía de los Andes argentino - chilenos ha sido asociada con raíces corticales en la hipótesis de Airy. Con elias se calculó un modelo de geoide isostático que justifica la longitud de onda "andina" de la ondulación geoidal de un modelo regional. Los resultados obtenidos se aplican además bidimensional mente a una sección transversal en 24,5° de latitud sur.
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Modelo Preliminar de Cortaza para la Precordillera Argentina (300 30' S) a Partir de Datos de Gravedad
Authors S. Miranda and A. IntrocasoSe presenta un modelo de corteza para la Precordillera sanjuanina (300 30'S), que tiene en cuenta la gravedad positiva observada sobre ella, y las hipótesis de aloctonía propuestas para este terrane. En este modelo simplificado el bloque cortical que sustenta a la Precordillera requiere de un exceso de densidad con respecto a los bloques adyacentes, manteniendo el equilibrio isostático relativo. Así, sin descartar otras altemativas, la respuesta de gravedad es consistente con una corteza Precordillerana distintiva especto de las encajantes.
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Analisys Frecuencial de Cuatro Perfiles de "g" en la SA. de Valle Fertil-San Juan-Argentina
Authors M.P. Martínez, M.E. Gimenez and A. IntrocasoSe analizan cuatro perfiles gravimétricos que atraviesan a la Sa. de Valle Fértil-La Huerta en las latitudes de 30°, 30°30', 31° Y 31°30' S. EI rasgo más significativo de la seńal gravimétrica es el abrupto gradiente encontrado en el límite entre la Cuenca del Bermejo y la Sierra de Valle Fértil-La Huerta, coincidiendo con la megafractura Desaguadero-Bermejo. El método de filtrado frecuencial se extiende en este trabajo hacia el norte y hacia el sur, abarcando el total de la sierra. La gran consistencia en los resultados encontrados en todos los perfiles evidencian su caracter regional. Las residuales, significativamente positivas se interpretan con relación a densas masas anómalas emplazadas en corteza superior (a profundidades maximas inferiores a 20 km).
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Integraçăo de Dados Sísmicos, de Perfis e de Rocha no Estudo da Geometria do Campo de Garoupa, Bacia de Campos
Authors E.A. Thedy, P.R. Zarpelon and S.R.R. CarneiroA utilizaçăo de sísmica tridimensional no estudo de reservatórios vern contribuindo significamente no aprimoramento dos modelos geológico-estratigraficos. O projeto de reinterpretaçăo do Campo de Garoupa envolveu a integraçăo de dados de rocha e de perfis com os dados sísmicos, obtendo, como resultado, urn melhor entendimento da geometria intema e extema do reservatório.
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Estado Isostático de las Sierras de Córdoba (Argentina) con Manto Superior Anómalo
Authors S. Miranda and A. IntrocasoLa tectónica global implica en márgenes convergentes, subducción y descenso de la litosfera, proceso que afecta el régimen de temperatura local, evidenciándose en cambios de flujo termal, velocidades de ondas sísmicas y gravedad, entre otras variables geofísicas. EI efecto gravimétrico de la placa descendente ha sido evaluado en distintas regiones de convergencia (Grow y Bowin, 1975; lntrocaso y Pacino, 1988). Para las Sierras de Córdoba ('SC'), los análisis isostáticos realizados hasta el presente (con manto homogéneo), indican estado de subcompensación (lntrocaso et aI., 1987; Miranda-lntrocaso, 1996; 1997). Se analiza ahora el probable efecto gravimétrico del sistema de subducción sobre la estructura cortical bajo SC, y su relación con el estado isostático imperante.
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Análisis Isostático en una Seccíon de los Andes Peruanos a partir de Ondulaciones del Geoide
Authors I.R. Cabassi and A. IntrocasoLa sección de Nazca se denomina así pues conecta Nazca sobre la costa del Pacifico con Puerto Maldonado sobre la cuenca amazónica, atravesando las localidades de Puquio, Chalhuanca, Abancay, Cuzco y Mazuco (Fig. 1). Esta ruta cruza toda la sección desde la costa, Cordillera Oeste, Altiplano, Cordillera Este y Cuenca Amazónica.
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Storm-Time Low-Latitude Ionospheric Electric Fields
By B.G. FejerIonospheric plasma drifts often show highly complex and variable signatures during geomagnetically active periods due to the effects of different disturbance process. Low latitude disturbance electrodynamic drifts with times scales of about an hour are due to the prompt penetration of high latitude electric fields to lower latitudes, whereas longer lasting (up to about 30 hours) perturbation drifts results from disturbance dynamo electric fields driven by enhanced energy deposition into the high latitude ionosphere
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On The Reasons for Latin American Countries to Undertake a Joint Space Research Program
More LessAll Latin American countries share similar economical and social problems. Their governments face serious difficulties to compromise a small economical budget among the many social problems like: education, public health, security, transport and so on and the need to invest in science and technology to keep pace with the requirements of modem life. Present days life became so strongly dependent on space vehicles and equipments that one can not preclude the use of communication satellites nor the help of space meteorology. It is then time for the Latin American scientific community to look forward towards an economically feasible effective Space Research Program. This work presents some ideas on this matter. Also a proper balance is made on the requirements and benefits of the suggested alternative.
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Storm- Induced Equatorial Plasma Depletion in the Brazilian Southeastern Region: A Case Study
Authors J.H.A. Sobral, M.A. Abdu, J.H. Sastri and H. TakahashiIonospheric plasma depletions, or bubbles, occurring in the intertropical latidude range, have been studied by many authors. Their dynamics have been extensively studied at the Brazilian low-latitude southeastern station Cachoeira Paulista (CP, 22° 41 'S; 45°00W; dip 24 "S for the epoch of 1981), in the last 20 years, by means of spatial scanning photometers.
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Wave-Associated Sporadic Neutral Layers in the Upper Atmosphere
Authors B.R. Clemesha, P.P. Batista and D.M. SimonichSporadic neutral layers, with thickness between a few hundred meters and several kilometers, are observed by lidar in the same height range as ionospheric sporadic E. Ns layers were first observed in sodium, almost 20 years ago, and more recently have also been seen in iron and calcium.
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Latitudinal Investigations of Short Period 'Wavelength-Period' Trends Observed in Nightglow Image Data
Authors M.J. Taylor and W.R. Pendleton Jr.Over the past two decades radar and lidar observations have provided considerable information on the vertical and horizontal scale sizes of gravity waves propagating through the mesosphere and lower thermosphere (MLT) region.
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Electric Field. and Electron Density Irregularities Associated with Plasma Bubbles
Authors P. Muralikrishna, M.A. Abdu, M.G.S. Aquino and D.C. SantanaIn-situ measurements of the height variation of the ionospheric electric field and electron density variations were made with a rocket-borne double probe and two different types of electron density probes. A Brazilian made SONDA ill rocket launched on 18-th December, 1995 at 2117 brs (LT) from the equatorial rocket launching station, Alcantara reached an apogee altitude of 557km and covered a horizontal range of 589km. Several ground equipments were operated during the launch campaign with the specific objective of knowing the ionospheric conditions at the time of launch and thereby to launch the rocket into an F-region prone to the presence of large plasma bubbles. The rocket in fact passed through seserat medium scale plasma bubbles and the electric field double probe and the electron density probes detected the preseace of a wide spectrum of electric field and electron density irregularities. In the base of the F-region the electric field double probe measurements clearly indicated the presence of large amplitude fluctuations, closely associated with large amplitude electron density irregularities. But in the height region close to the rocket apogee though the electron density profile showed the presence of large scale spatial structures, the electric field measurements did not show fluctuations of similar amplitude. Being a nighttime launch one would expect the electron density irregularities, if generated by the well-known cross-field instability mechanism, in height regions where the electron density gradient is downward, i.e., in the same direction as the ambient Hall electric field. An FFT algorithm was then used to estimate the spectral distribution of the electric field and electron density fluctuations, thus estimating the height variation of the spectral variation. The results of this analysis are presented here.
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Equatorial F-Region Plasma Bubbles Studies
More LessEquatorial ionospheric plasma bubbles associated with equatorial range-type spread F, are regions where the plasma densities are much lower than the ambient plasma density and are aligned along the earth's magnetic field lines . The plasma bubbles can be observed by both the radio wave (e.g. ionosonde, radar, scintillation etc.) and optical techniques (e.g. by measuring nightglow emissions such as 01 630 nm, 01 557.7 nm and 01 777.4 nm, which come from the ionospheric F-region and their intensities are proportional to the electron density
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In Situ Measurement of High Altitude Spread-F Characteristics by Three Different Plasma Density Probes
Authors P. Muralikrishna, M.A. Abdu, J.H.A. Sobral, M.G.S. Aquino, D.C. Santana, J. La Belle, J.-M. Jahn and R. PfaffDuring the Guara campaign, conducted from Alcantara in Brazil, a Black Brant X sounding rocket was launched on 14-th October 1994 at 1955hrs (LT) to investigate into the phenomenon of high-altitude equatorial spread-F events. The rocket, as expected passed through an active topside spread-F event, monitored simultaneously by several ground-based instruments. The electron density height profile and the amplitude of the electron density fluctuations were measured simultaneously by three different plasm density probes; a High Frequency Capacitance (HFC) probe, a conventional Langmuir Probe (LP) and a Plasma Frequency Probe (PFP). While the PFP provided the absolute electron density, the LP gave the relative variation in the electron density. The electron density profile obtained from the HFC probe measurements is practically absolute except for a plasma sheath factor. But this technique does not provide the small-scale electron density fluctuation amplitude. Thus, the three experiments provided data, which could be used not only to obtain reliable electron density data, but also could be used to arrive at some of the inherent difficulties associated with each of these techniques. For example the electron density profiles estimated from the HFC and PFP experiments are almost identical except for a small factor varying with altitude. The amplitude of large-scale fluctuations provided by the LP measurements is considerably less than that provided by HFC and PFP. Characteristic features of the high-altitude spread-F as observed by these experiments are presented here.
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First Results on High Resolution Seismic Survey in the Arkona Basin, Baltic Sea
Authors F. Theilen, A. Ayres, G. Lange, M. Reidel, C. Papenberg, B. Jacobs, D. Fischer, A. Broser and C. MüllerThe marine geophysics group of the Kiel university has performed 3 high resolution seismic surveys (September/96, January/97 and March/97) at the Arkona Basin, Baltic Sea. The surveys were performed with the RN Alkor and aimed at the detection of sedimentary and structural features that would give more information about the development of the Baltic Sea. For the acquisition of the data two different seismic systems were used (3,5 kHz and Boomer). The main objective of the surveys was the localisation of geological structures that may provide more information about the development of the Baltic Sea, as well as the identification of gas-charged sediments. The results showed the occurrence of mud diapirs and paleo-channels of different sizes, sharp contacts, and the occurrence of several gas-charged areas.
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Numerical Estimation of Errors in the e and UT Parameters Calculation
More LessThe numerical estimation of errors in the E and UT parameters calculation has been done, considering a disturbed period ocurred on March 5, 1989. First, the coupling parameter, E, for the growing phase and the total energy dissipated by the magnetosphere, UT, have been calculated, using Simpson numerical method. However, the obtained solutions using a numerical method are only approximations for the correct one. Several errors sources can be mentioned:l- errors in the model built starting from the original physical problem, 2- errors in the empirical formula obtained adjusting experimental data, which include errors in the instruments measurements and in the used constants, 3- errors inherent to the used numerical methods, and 4- errors introduced by the computers, known as representation errors. The estimation of the errors by the numerical approximation are calculated considering the Simpson method error formula.The experimental errors have been also estimated. The results for both estimations are of the same order of magnitude.
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Airglow Studies in Brazil
Authors H. Takahashi, J. H. A. Sobral, P. R. Fagundes, D. Gobbi, S. M. L. Melo and B. R. ClemeshaStudies of aeronomic processes of the upper atmosphere in the equatorial and low latitude regions has been carried out in Brazil by means of airglow observation technique. In the Cachoeira Paulista airglow observatory (22.7 S, 45.0W), several airglow photometers, two imagers, one for or 6300 and one for IR-OH, and a Fabry Perot Interferometer for 01 6300 are in operation as a routine base.
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Modeling of Large Downward Vertical Plasma Drifts and Small Ionic Densities in the Premidnight Sector During June Solstice over the Southamerican Sector
Authors E.R. de Paula, B.G. Fejer and G.J. BaileyLarge downward vertical plasma drifts and simultaneous large ionic density decreases were measured by the AE-E satellite in the premidnight sector during June solstice over the southamerican region. Using the Sheffield ionospheric SUPIM model the vertical drifts and the ionic density were modeled for these conditions along one satellite track and for some specific latitudes and longitudes, over South America.
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A Space-Borne Retarding Potential Analyser for In Situ Measurement of Charged Particle Flux
Authors P. Muralikrishna and L.P. VieiraA standard technique to measure the flux of charged particles with different energies and masses is to use a Retarding Potential Analyser (RPA). The front end of the experiment is a window that allows the particles of all energies and masses to enter the detector. These particles pass through a set of grids, where the selection of particles in the required mass and energy range is made. Only particles in this certain energy and mass range finally reach the collector electrode. The current collected by the collector electrode is, therefore a direct measure of the number of particles collected by it. By varying the electric potentials applied to the set of grids one can measure the flux of charged particles in different energy ranges. An RPA experiment is being developed in the Aeronomy Laboratory of INPE, to be used for the measurement of the spectral distribution of thermal and suprathermal electron in the energy range of 0 to 32eV, on board rockets and satellites. Basic details of this experiment are presented here. The experiment can be adapted to measure the spectral distribution of negatively or positively charged particles including electrons in other energy ranges.
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Upper Atmospheric Observations in the Brazilian Geomagnetic Anomaly Region
Authors T. Kikuchi, K. Makita, S. Okano, N. Schuch and H. TakahashiThe joint study of the upper atmosphere in the Brazilian geomagnetic anomaly region was planned by the University of Santa Maria, INPE and National Institute of Polar Research, Takushoku University, Communications Research Laboratory and several other institutes of Japan. Our major concern is the atmospheric changes which could be caused by energetic particles precipitating from the inner radiation belt due to lower mirror height in the anomalously weak geomagnetic region.
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Pulsations of Energetic Particles in Atmosphere in the Brazilian Magnetic Anomaly Region
Authors I. Martin, A. Gusev, J. Kassovicova, K. Kudela and G. PugachevaIn the set of balloon flights in the Brazilian Magnetic Anomaly region (BMAr) there were observed the short time periodic variations, i.e. pulsations of the fluxes of secondary charged and neutral particles, x-and y-rays with the amplitudes of 2-4 %. They are accompanied with the geomagnetic Pc4 pulsations with similar periodicity. The phenomenon was observed during various local times and in quiet and disturbed magnetospheric conditions. The explanation of the effect is suggested.
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Temporal and Spectral Analysis of Electron Precipitation Events in South Atlantic NTIC Anomaly (SAA)
Authors U.B. Jayanthi, M.O. Pereira and K.A. JayanthiTo monitor electron precipitation events in the South Atlantic Anomaly (SAA) region, a stratospheric balloon experiment with an xray payload was conducted on December 20, 1994 at Cachoeira Paulista, SP. In the SAA region where the Earth's surface magnetic field is the lowest, electrons situated near the inner edge of the inner radiation belt dip into the atmosphere in their eastward drift and produce x-rays.
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Low-Latitude Geomagnetic Pulsations Recent Results from the 210 0 MM and Dip Equator Networks
By K. YumotoGeomagnetic pulsations had been studied by many researchers during the International Geophysical Year (IGY), and had been classified into two main types at the 1963 lAGA Berkeley Meeting; continuous pulsations (Pc) and irregular pulsations (Pi).
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Appraisal of the Electromagnetic Induction Effects on Geomagnetic Micropulsation Studies
Authors B.R. Arora, N.B. Trivedi, A.L. Padilha and I. VitorelloGeomagnetic micropulsations as recorded by a network of magnetometers are a useful guide to investigate the complex magnetospheric processes in general and to understand the generation and propagation mechanisms of the ULF waves in particular .(Hughes, 1994). In these studies, parametrization of the wave polarization properties is inevitably the first step. The geomagnetic field variations as recorded at the Earth's surface are the vector sum of external (source) and internal current systems, the latter resulting from the electromagnetic induction in the electrically conductive layers of the Earth.
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Ionospheric Modulation of the Pc5 Geomagnetic Pulsation
Authors N.B. Trivedi, B.R. Arora, L.E.A. Vieira, D.R.K. Rao and K. YumotoA sequence of globally coherent Pc5 geomagnetic pulsations that occurred between 1100-1325 UT on March 24, 1991 were earlier analyzed using the horizontal field variations from a chain of stations along the Brazil-American longitude line (Trivedi et al.,1997). The present paper, extends the analysis to the 39 worldwide observatories ( Fig. 1) to confirm and elaborate the role of the ionosphere in the wave-propagation path of the ULF waves to low and equatorial latitudes.
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Seismic Velocities of Near Surface Marine Sediments of the Continental Slope of the Barents Sea
Authors A. Ayres and F. TheilenDeep sea, unconsolidated near surface marine sediments are commonly composed by silt and clay with very small amounts of sand. In this environment the propagation and velocity of seismic waves are mainly influenced by lithology, porosity and water content. Most of the information about the in situ behaviour of seismic waves on near surface marine sediments come from the works of Hamilton (1979, 1980) and Hamilton and Bachman (1982). Recently, Briggs et al. (1996) studied the physical and geoacoustic properties of near surface, carbonatic marine sediments at the southern coast of Florida.
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Sun-Earth Relationship
By R.P. KaneSun is the most important source of energy for the Earth. Most of this energy reaches in the form of heat and light and sustains terrestrial weather and climate. However, the Sun also emits radiations in a wide range, from X-rays to radio waves. Though their contribution is only -5%, these are responsible for changes in the upper atmosphere of great importance.
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Electron Beam-Plasma Interaction: Experimental and Computer Simulation Results
Authors M. Virgínia Alves, F. do Prado and R.S. DallaquaThe excitation of Langmuir waves by a gentle bump-on-tail of the electron distributions has become the classic example of a kinetic instability. Clear manifestations of effects of this instability are observed in a number of space objects as solar wind, magnetotails, auroral regions and planetary foreshocks. The purpose of this work is to investigate the possible nonlinear phenomena associated with the Langmuir waves generated by the beam. This paper reports results obtained in a beam plasma experiment as well as some simulation results obtained using the particlein-cell code PDP1.
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Magnetosphere Electron Radial Transport with Accounting Synchrotron Radiation Losses
Authors G. Pugacheva, D.M. Boscher, A. Gusev, I. Martin and W.N. SpjeldvikA spatial 3-D distribution of the high energy component of the electron radiation belt obtained by the numerical solution of the diffusion equation taking into account both Coulomb interaction losses and synchrotronic radiation (SR) ones is presented. For boundary conditions, high energy 3 - 16 MeV electron fluxes observed at geosynchronous orb by GOES-7 were used. The comparison of electron distributions with and without SR losses shows that the losses of several MeV electrons substantially influence energy distributions of ornni and unidirectional fluxes at L<3. From the standpoint of steady state electron radial transport, the problem of multi-MeV electron origin for the inner zone remains unresolved, and may entail internal magnetospheric nonadiabatic electron acceleration, or, that is more probable, the results of impulsive unsteady cross-L transport.
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Cooperative Studies on the South Atlantic Geomagnetic Anomaly under Collaboration of UFSM, INPE and NIPR, Japan
Scientific investigation on the South Atlantic Geomagnetic Anomaly (SAGA) was started under collaboration of Federal University of Santa Maria, Instituto Nacional de Pesquisas Espaciais, INPE from Brazil and National Institute of Polar Research, Kyushu University, Communication Research Laboratory in addition to 5 institutions from Japan. It is aimed at investigating geomagnetic phenomena, such as micro pulsation, ionospheric phenomena, such as radio wave absorption, and aeronomic phenomena, such as airglow enhancement and ozone depletion due to particle precipitation. The cooperative studies were started on December 1996, with installation of a Riometer (32.8 MHz), ELFNLF receiver, VLFILF receiver, and high sensitive auroral TV camera. All of the instruments are installed at San Martino Space Observatory, RS. Preliminary ionospheric data were already produced and under analyzing phase. Some scientific interests and collaboration scheme will be discussed.
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Space Distribution of Energetic He Ion Flux in Magnetosphere: Ohzora Observations During 1984 - 1986
Authors I.M. Martin, A.A. Gusev, T. Kohno, G.I. Pugacheva and W.N. SpjeldvikThe results of measurements of absolute flux values and long term temporal evolution of the spatial distribution of trapped He ions in the energy range 1.2-9.2 MeV/nucleon below L=4 are reported. The observations were made with ion counter on board the Japanese OHZORA satellite during the period of January 1984 through March 1987.
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Solar and Anomalous Cosmic Rays in the Magnetosphere - Orcas Experiment on Board the Brazilian Satellite Saci-l
Authors U.B. Jayanthi, A. Turtelli Jr., I.M. Martin, T. Villela, K.A. Jayanthi and J.M.A. MaranhăoA satellite experiment "Observacoes de Raios Cósmicos Anomalos e Solares na Magnetósfera - ORCAS" is being developed to monitor the charged particle environment in the inner magnetosphere and will be deployed on board the Brazilian micro-satellite SACI-l in 1998. The satellite's minimum life time of 18 months is expected to span both quiet and active periods of solar activity and defines the scientific objectives. The satellite will orbit at - 700 Km in sun synchronous mode and spins at - 10 RPM. This experiment is a collaborative effort of scientists working at INPE and UNICAMP of Brazil, RIKEN and University of Tamagawa (UT) of Japan and Naval Research Laboratory (NRL) of USA.
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The Secondary Proton Radiation Belt: Origin and Dynamics
Authors A.A. Gusev, I.M. Martin, G.I. Pugacheva, A. Turtelli Jr., T. Kohno and W.N. SpjeldvikAt the interface between the upper atmosphere and the inner radiation, there exists a secondary radiation belt consisting mainly of energetic ions. The time history of its proton component was traced in the equatorial region for 3 consequent years. During most of this period a flux of -1.2 cm·2s·1s(1 was detected in the L range 1.05-1.15. A few surprisingly deep and rapid flux decreases and abrupt flux increases were also observed. Possible reasons for these unexpected phenomena are discussed.
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Modelling of Isotope Radiation Belts in the Inner Magnetosphere
Authors G.I. Pugacheva, A.A. Gusev, I.M. Martin and W.N. SpjeldvikAccording to current understanding, the major part of the Earth's radiation belt region is populated primarily (1) by external sources where particles from the outside of this region convectively and diffusively transport to the deep interior of the geomagnetic trapping region, and (2) by in-situ acceleration of pre-existing lower energy particles. But there are also secondary processes that contribute to the characteristics and composition of the trapped particle population, particularly in the inner radiation zone. These include not only the well-known Cosmic Ray Albedo Neutron Decay (CRAND) source of multi-MeV inner zone protons, but also sources of deuterium, tritium, Helium-3 and Helium-4 ions from tertiary nuclear reactions of the secondary CRAND-produced protons interacting further with the residual atmosphere. Indeed, unusually high 3He/4He flux ratios at MeV energies have been observed in the radiation belts near L=1.2 and may have their origin in such processes. In this report we present quantitative calculations of the source strength of deuterium, tritium and Helium-3 ions resulting from these interactions. And we extend diffusion theory for inner magnetosphere MeV ions by combining radial diffusive transport with local generation of deuterium and tritium and helium isotope ions due to nuclear reactions. Magnetospheric transport computations have been made in the range of L=1.0 to 1.6. The resulting MeV D, T and 3He ion flux distributions show a strong influence of the local nuclear source mechanism on the inner zone energetic ion content.
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Estudo de Micropulsacoes Geomagneticas PC3-5 em Baixas Latitudes, no Brasil
Authors A. Zanandrea, I.M. da Costa, S.L.G. Dutra, N. Trivedi and A.L. PadilhaAs micropulsaçőes do campo geomagnético observadas no solo, na ionosfera e na magnetosfera terrestre, devem-se a manifestaçăo de ondas de plasma (hidromagnéticas) de frequęncia ultra baixa (ULF) na magnetosfera. As ondas ULF com mais baixa frequęncia tem comprimentos de onda comparáveis ao tamanho da magnetosfera e amplitudes da ordem de centenas de nanoTesla. O limite superior do espectro das ondas de plasma causadoras de micropulsasőes é determinado pela frequęncia ciclotrônica do hidrogęnio na magnetosfera, que é tipicamente menor que 10 Hz (Samson, 1991). A interaçăo do vento solar com o campo magnético da Terra é o principal processo de geraçăo das micropulsaçőes geomagnéticas (Yumoto, 1986; 1988)
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Comparaçăo de Dados Experirnentais de Rotaçăo de Faraday para Natal-RN com Resultados Obtidos pelo Modelo Bonelli-90
Authors C.F.P. Oliveira, E. Bonelli, J.R. Souza, G.L. Borba and R.T MedeirosO estudo do Conteúdo Eletrônico Total através do método de rotaçăo de Faraday sempre foi de grande importância para o estudo da ionosfera, mas a comparaçăo de dados experimentais com modelos, principalmente modelos que tivessem boa resposta na regiăo equatorial, como é o caso do modelo Bonelli-90, raramente é feita. Esse trabalho consiste em comparar os dados de rotaçăo de Faraday obtidos pôr um polarimetro em Natal-RN com os resultados gerados pelo modelo Bonelli-90.
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Gas-charged Sediments in the Lowermost Amazon River?
Authors H. Vital, K. Stattegger, J. Posewang and F. TheilenSeismic evidence of the presence of shallow gas may take the forms of acoustic turbidity: chaotic reflections, caused by the scattering of the acoustic energy, and which looks like a dark smear on the record, obliterating all other reflections; Enhanced reflections: coherent seismic reflections which have an increased amplitude for part of their extent. It is thought that, in very shallow sediments, gas may occur either as accumulations within porous (silt and sand rich) sediments, or finely disseminated within impervious (clay-rich) sediments. It would seem that acoustic turbidity characterizes the later situation, and enhanced reflections the former (Judd & Hovland, 1992). Acoustic blanking: appears as patches on an inferred "gas" record where the reflections are faint or absent. They may result from the disruption of sediment layering by the migration of pore fluids (gas), or may be caused by the absorption of acoustic energy in overlying gas-charged sediments. It may also be caused by the reflection of a high proportion of the acoustic energy by an over-lying hard sediment; the reduction in the amount of energy penetrating the hard layer being represented by a relatively low amplitude return signal (Judd & Hovland, 1992). Usual bottom surface features are pockmarks, seabed domes and mud diapirs, giant gas mound, highly-reflective bed. In the lowermost Amazon River were identified acoustic turbidity, Enhanced reflections, Acoustic blanking, and highly-reflective bed.
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Modelagem Ionosferica de Baixas Latitudes no Brasil para Verao de Atividade Solar Minima
Authors J. R. Souza, M. A. Abdu, I. S. Batista and G. J. BaileyOs parametres foF2 e hmF2 (frequęncia e altura do pico da regiăo F ionosférica) medidos simultaneamente nas estaçőes de Fortaleza (4°S, 38°0, latitude magnética = 3°S) e de Cachoeira Paulista (22°S, 45°0, latitude magnética = 15° S) săo comparados com os resultados do modelo SUPIM (Sheffield University Plasmasphere-Ionosphere Model). Neste trabalho, tambem usando o modelo SUPIM e os dados observados, e apresentado um metodo para calcular deriva equatorial E x B, fluxo solar EUV (Extremo Ultra- Violeta) e ventos termosféricos. No código do modelo SUPIM foram incluídos os efeitos da Anomalia Geomagnética Brasileira (AGB) sobre a ionosfera de Cachoeira Paulista (CP) para explicar as variaçőes diumas de foF2 nesta estaçăo. Os resultados de foF2 e hmF2 calculados pelo modelo SUPIM năo apresentam concordância com os dados observados, mas fazendo-se ajustes nos parâmetros de entrada (deriva equatorial E x B, fluxo solar EUV e ventos termosféricos) as discordâncias sâo superadas. Os ajustes dos ventos săo feitos năo apenas sobre Fortaleza (FZ) e CP, mas também nos seus respectivos pontos conjugados.
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