3rd International Congress of the Brazilian Geophysical Society

Até o presente momento, o Sensoriamento Remoto tem se caracterizado por medidas de baixa resoluçăo espectral ao longo da faixa 400-2500 nm. Este é o caso das imagens TM-Landsat 5, adquiridas em bandas espectrais da ordem de 60 (Banda 3) a 270 nm (Banda 7) de largura, ou mesmo dos radiômetros multibandas construídos para obter dados de reflectância em faixas espectrais aproximadamente equivalentes ŕs imagens orbitais.

Com o advento de levantamentos sísmicos poço a poço e em perfis sísmicos verticais, a cobertura angula dos experimentos sísmicos passou a ser bastante variável, em contraponto ŕs observaçőes feitas nos levantamentos comuns, onde as ondas săo registradas em superfície, com ângulo de incidęncia quase vertical.

Existe um acervo de dados indiretos (de satélites, de radares, geofísicos, topográficos, etc) que cobre parte do território brasileiro e que muitas vezes năo é explorado em todo o seu potencial. A sua disponibilidade, aliada a algumas restriçőes existentes (financeiras e de infraestrutura) conduzem a um estado da arte, cada vez mais, direcionado ao uso de técnicas de processamento digital de imagens (PDI) e de sistemas de informaçőes geográficas (SIG) como opçőes para a manipulaçăo integrada destes dados.

In this work we summarize the major conclusions achieved in our search for improvements on knowledge of the crustal and upper mantle structure In South America using Rayleigh, waves group and phase velocity information. A new data set, consisting of group and phase velocity measurements using seismological stations located within Brazil made possible the current results. These data measured using surface wave records of the BDF' RDJ, and BEB stations, added to our previously' organized group and phase velocity global data base, represent a significant improvement on the study of the South American Plate using surface waves. Major results achieved up to this stage of the project show that the data collected at the brazilian stations are consistent and comparable to others measured at other regions with similar tectonic setting. We have also used the data for computation of regional models good for the South American Plate.

Ha sido seńalado que por debajo de los Andes Centrales existiría: (1) enfriamiento producido por la place de Nazca al subductarse por debajo de la litósfera continental (Grow-Bowin, 1975; Introcaso-Pacino, 1988), y (2) significativo calentamiento en el manto litosférico (Froideveaux-Isacks, 1985; Introcaso-Pacino, 1988; Isacks, 1988). Analizamos aquí los efectos tanto sobre la gravedad, como sobre el levantamiento andino, que producirían las anomalias de densidad provenientes de (1) y (2).

A existęncia dos esforços das mares no interior terrestre, os quais tęm se repetido periodicamente desde a origem do sistema Terra-Lua-Sol, sugerem as marés terrestres como uma das causas condicionantes de transformaçőes evolutivas do corpo planetário. Farrel (1972), quando do estabelecimento de uma metodologia de análise das deformaçőes da Terra, a partir de sobrecargas superficiais, evidenciou a possibilidade de estudo das propriedades da crosta a partir da observaçăo da resposta as mares terrestres, desde que disponível um modelamento adequado do efeito oceânico.

As forças de maré no corpo planetário e nas porçőes fluidas (atmosfera e oceanos), derivam de um mesmo potencial gravitacional variável espacial e temporalmente, o qual é prodominantemente produzido pela Lua e pelo Sol. Os efeitos resultantes săo completamente distintos nas porçőes sólidas (marés oceânicas e atmosféricas). No âmbito deste trabalho abordamos particularmente os efeitos da componente vertical das marés terrestres (maré gravimétrica) e os efeitos indiretos das marés oceânicas nas porçőes terrestres (efeito oceânico indireto).

Birch e colaboradores (1968) observaram em corpos plutônicos da Nova Inglaterra, noroeste dos Estados Unidos, uma relação linear entre a densidade de fluxo de calor (q˳ e a taxa de produção de calor na superfície (A˳).

Estimates have been made of the thickness of the lithosphere in Brazil, based on an empirical relation between surface heat flow and crustal to lithospheric thickness ratio in continental regions and making use of available geothermal data. The results show that cratonic areas such as Sao Fransisco, Amazon and Guiana are characterized by lithopshere with thickness in excess of 150 kilometers while in the northeastern and southern part~ of Brazil the lithosphere may have thickness of less than 150 kilometers. There are indications that the frequency of seismic events is high in areas where the lithosphere is thin. Rheological models, compatible with regional geothermal data, show that onset of brittle-ductile transition in the lower crust and upper mantle may have a significant role in the occurrance and distribution of seismic events in intraplate regions like Brazil.

A Bacia do Pantanal é a maior bacia sedimentar intracratônica quaternária do Brasil e está situada entre os paralelos 15° e 20°S e os meridianos 55° e 59,5° W (Fig. 1a). A origem desta bacia, que foi atribuida por Almeida (1959) ŕ orogenia Andina, através de soerguimentos reativando as falhas pré-existentes na Faixa Paraguai.

In this work we present several regularization techniques for the solution of geophysical ill-posed inverse problems, in the case of discrete data and discrete model parameters. In particular, the Twomey (1963) algorithm Is applied to Invert synthetic tomographic data, In both noise-free data and data corrupted by noise. This algorithm Is based on a smoothness criterion where the second differences of the model parameters are minimum. The selection of the regularization parameter Is also considered, and a practical trial and error procedure Is suggested for the selection of an optimal parameter, based on the behaviour of the RMS misfit curves for data and model parameters, the entropy of the solution, and the qualitative/quantitative analysis of the estimated model parameters.

As Bacias do Espírito Santo e de Campos (figura 1), localizadas na Margem Continental Sudeste do Brasil, podem ser consideradas, em termos gerais, semelhantes tanto estratigráfica quanto estruturalmente. No entanto, quando comparadas com relaçăo a ocorręncia de óleo e gás natural, verifica-se que, enquanto a Bacia de Campos apresenta-se como a mais prolífera de todas as bacias brasileiras, para a vizinha Bacia do Espírito Santo os resultados tęm sido bem mais modestos.

The seismicity and stress field of the Brazilian continental margin are studied and compared with those of the continental interior. Two different patterns are observed in the eastern and northern margins. In the eastern margin, the earthquakes are concentrated in the continental shelf and are due to reverse faulting in a weak, extended crust. Very little seismicity is observed onshore in eastern Brazil. In contrast, in the northern margin, earthquakes tend to occur onshore as a result of strike-slip stress regime, and almost no activity is observed offshore. Combination of regional stresses (such as ridge-push) with local flexural stresses due to sediment load in the continental shelf can explain most of the observed seismicity patterns.

O lineamento de Fernando de Noronha é uma feiçăo tectônica marcante na regiăo equatoriana do oceano Atlântico. E caracterizado por feiçőes morfológicas lineares e pela ocorręncia de montes submarinos que se estendem do arquipélago homônimo a 345 quilômetros da costa até atingir o sopé continental na altura dos estados do Ceará e Rio Grande do Norte (Gorini, 1981).

La Sierra de Córdoba es la entidad orográfica más oriental de las Sierras Pampeanas. Está constituida por varios cordones meridionales levemente orientados hacia el sudoeste, que se extienden desde 29° hasta 33.5° de latitud Sur. Las alturas rondan entre 700 y 3000 metros, mostrando una morfología asimétrica, con la ladera oriental tendida y la occidental escarpada. (Figura 1).

A model of lithosphere behavior under a compressional tectonic regime is proposed for Brazilian thrust belts. The thrust belts used in this paper are restricted to the central part of Brazilian Shield. The proposed model assumes that several faults cut the crust. The topography is created by simple slip to regain isostatic equilibrium in a regional manner by using the flexural model for a thin elastic plate. The predicted gravity signal is compared to the observed along six profiles crossing the study area. The lithosphere elastic thickness is between 40 and 50 km. The Araguaia belt is modeled by a thrust fault toward the Amazon craton, with a dip of 200. Uruacu and Brasilia belts are modeled with two thrust faults each towards the Sao Francisco craton. The Espinhaco belt is poorly modeled because of lack of gravity data.

In this paper, we will discuss a separate reconstruction of lateral velocity gradients and reflection Intorfaces using the method of reflection tomography. We will be limited by Inversion of traveltimes. As it Is known such inversion may suffer from a significant drawback: ambiguity. This ambiguity is referred to as depth/velocity ambiguity which is especially notable in the case of non-vertical angles of propagation. In tomographic experiments this ambiguity Is suppressed by finite angular aperture recording. The next problem is non-uniqueness. It will be shown that the lateral gradients of velocity and the depths to the reflection points can be uniquely recovered from surface seismic data in the reflection tomography experiments. Reflection data Inversion is done via an optimization process which can be formulated either in the physical space of seismic velocities or In the dual space of Lagrangian multipliers. We compare both methods and show the advantages of' the dual transform.

Hydrological balance of Lake Titicaca and its basin (located 10 Bolivia and Peru) was calculated in detail over the past 80 years. During this period, the Lake level has experienced intense variations which are mainly linked to the variation of the summer rainfall. These perturbations may be shown to be related to "EI-Nino Southern Oscillation" (ENSO) events. Moreover the isotopic 16O/18O ratio measured in the Quelccaya ice cap (Thompson, 1990) located in Peru at about one degree north of the Lake is also found to be correlated to the Lake water level. This isotopic signal is linked to the variation of the air mass stability over the amazonian region (Grootes et aI., 1989) and is therefore related to the precipitation level on the Titicaca basin. From these considerations, it is demonstrated that Lake Titicaca behave as a "natural large scale pluviometer" for its capacity of recording occurence of wet and dry episodes in the amazonian region.

Erupçőes vulcânicas (algumas das mais importantes estăo listadas na Tabela 1) săo em geral acontecimentos catastróficos que causam enormes prejuízos. No caso do Monte Pinatubo, Filipinas, que entrou em erupçăo em meados de 1991, năo foi diferente. Morreram 722 pessoas, e mais de 200.000 ficaram desabrigadas. Além dos efeitos dramáticos impostos ao meio ambiente próximo a superfície, estas erupçőes vulcânicas também causam enormes transtornos na atmosfera superior. Estima-se que 20 milhőes de toneladas de S02 foram injetadas na atmosfera (Bluth et al., 1992) o que parece ser a maior do seculo. As partículares de fumaça injetadas na estratosfera, por refletirem parte da energia solar que poderia ser absorvida pelo solo, contribuem para esfriar o planeta, e para reduzir a concentraçăo local de ozônio através de reaçőes químicas heterogęneas.

In most land 3D surveys (and in some marine 3D surveys) the low density of source and receiver points results in amplitude and phase perturbations which, in the presence of multiple reflections or radial noise, can hinder the extraction of stratigraphic information from the data. Analysis of the generation of these perturbations has been studied: it was shown that the amplitude of the perturbation was a direct function of the size of the elementary pattern which by translation generates the 3D grid. Two types of action can be taken to protect the data from these artefacts: at the acquisition stage, a design that minimizes artefacts may be preferred. To help select a design, we propose simulating 3D stack using either synthetic Of real data. At the processing stage, In Severe cases, conventional prestack processing techniques may not provide a satisfactory cancellation of residual perturbations. Furthermore, a poststack geometryderived filter can reduce these perturbations.

A evaporaçăo e a evapotranspiraçăo săo componentes naturais do ciclo hidrológico, pelas quais a água precipitada pelas chuvas e pela neve, retorna ŕ atmosfera. Do ponto de vista meteorológico săo entendidas como resultante da açăo combinada de uma série de outros elementos: balanço de energia, tensăo de vapor, temperaturo e vento. O conhecimento desses componentes constitui-se em parâmetro importante no estudo da disponibilidade hídrica de uma determinada regiăo. As taxas de evaporaçăo e evatranspiraçăo săo estabelecidas através o estudo do balanço de energia da superfície evaporante, o qual expressa de forma aproximada o princípio de conservaçăo da energia. Através deste estudo, pode-se avaliar os fluxos de calor latente e calor sensível.

O Leplac-Plano de Levantamento da Plataforma continental Brasileira - vem sendo realizado ao longo de toda a costa brasileira, coletando dados de sísmica de reflexăo multicanal, gravimetria, magnetometria e batimetria através dos navios da Diretoria de Hidrografia e Navegaçăo (DHN), em conjunto com a Petróleo Brasileiro S/A (Petrobrás).

Tradicionalmente, a aquisiçăo dos dados gravimétricos nas margens continentais tem sido realizada por gravimetros adaptados sobre plataformas estabilizadas ŕ bordo de navios. Entretanto, a altimetria do nível do mar realizada por satélites, iniciada pelo satélite GEOS-3 em 1975, está modificando radicalmente a forma de obtençăo da anomalia gravimétrica no mar. Enquanto os gravímetros medem o efeito do campo gravimétrico sobre uma pequena rnassa presa a uma mola, a altimetria por satelíte medem o efeito deste campo impresso na superfície do oceano.

One of the goals in planning and designing seismic surveys is to provide adequate spatial sampling. Spatial sampling has an Influence on many aspects of data processing and Interpretation. Since DMO and wave processes like DMO have become almost universal data processing procedures, the Interplay of spatial sampling and these wave-equation processes Is Important! Poor spatial sampling is often observed to affect waveequation processes such as DMO and migration. Inadequate spatial sampling can result for a variety of reasons. Offshore, irregularities are caused by obstructions, cable feathering, and the acquisition environment; on shore, obstructions and the inability to occupy desired stations are problems. However, a more fundamental problem occurs when the acquisition design itself is deficient. Conventional survey-design specifications such as CMP fold and offset distribution are aimed at achieving a uniform stack of the data. Here I develop similar attributes for evaluation of survey geometries that are based on the application of DMO and stack to the data. These new attributes can be anatyzed-as offset, fold, and azimuth now are in current practice. Both the survey design and acquisition quality control phases of seismic surveys can use these attributes to optimize the resulting DMO; stack, and migration. My method is based on previous work on DMO equalization (Beasley and Klotz, 1992). Uke conventional methods, the analysts requires only the survey geometry. It provides an accurate and practical means of assessing the suitability of the spatial sampling of a seismic survey for wave-equation processing.

A plataforma do Amazonas situa-se na porçăo norte da margem continental brasileira, sendo uma regiăo bastante afetada pelo sistema do Rio Amazonas. A margem continental norte brasileira é caracterizada por sua grande extensăo, o que contrasta com a margem continental do sul (Chaves et al., 1979). A plataforma continental apresenta largura razoável na área compreendida entre o Cabo Orange e o Rio Pará, atingindo 300 km em frente a desembocadura do Rio Amazonas, e ocupa mais de 200.000 km2 (Milliman, 1979). A quebra da plataforma ocorre a uma profundidade de 100 m.

Organic matter from plankton bloom, fishes and sewage outflow are common source for generating gaseous sediments. Presence of organic matter combined with high rate of sedimentation are common characteristics of estuaries though creating an appropriate setting for gas generation. The Amazon system at maximum discharge can reach of 354,793 m3/sec (Figueiredo et al. 1993) and sediment transport estimation 1.2 x 10 tons/yr (Mead et al. 1985). Because of the enormous discharge and associated sediment, a submarine delta is being constructed in the Amazon shelf (Figueiredo et al., 1972, Milliman et aI., 1975). Besides sediment Amazon river brings 3.1 x 10 12gC y*ą of organic carbon (Showers and Angle, 1986) and also great nutrients which are mainly used by planktonic community in the continental shelf. Rates of primary production in the shelf using C14 tracer technique measured by De Master et al. (1991) indicate mean values of 5.8 gCm-2d-1 in the high productivity zone. Organic matter incorporated in bottom sediment are reduced by bacteria generating methane gas that may- be trapped or liberated to the water column. Seismic surveys and core collection in the region during Amasseds cruises have indicated presence of gas in several areas as indicated in figure I. This paper presents results of seismic survey during Amasseds cruise III, leg. 2 and coring collection during others Amasseds cruises in the north Brazilian continental shelf. Amasseds is a cooperative, multi-disciplinary research between Brazilian and American universities.

O estudo dos mapas de isópacas da porçăo norte do Platô de Pernambuco e do trecho da Bacia Oceânica do Brasil adjacente, apresenta a evoluçăo geológica-sedimentar mesozóica-cenozóica desta regiăo. O trabalho teve como base a interpretaçăo de todas as seçőes de sísmica de reflexăo perfiladas na regiăo (Fig. I), e faz parte de uma série de pesquisas geofisicas que vem sendo realizadas na área em estudo pelo LAGEMAR/UFF nos últimos anos. As principais sequęncias sedimentares e fases erosivas e/ou deposicionais do platô foram definidas por Alves e Costa (1986) e as da bacia por Costa e outros (1991). Dos quatro grandes refletores detectados no plato e na bacia o mais profundo corresponde ao embasamento cristalino, que segundo Costa e Maia (1986) é de origem continenta/transicional no platô e de origem oceânica na bacia. Os outros tręs refletores do platô e da bacia associamse a discordâncias, que delimitam sequęncias sedimentares definidas em toda a regiăo (Fig.2).

The intracontinental rift basins of northeast Brazil constrain the chronology of breaking up the Gondowanaland between Brazil and West Africa. Besides constraining the timing of rifting, the basins demonstrate that during early stages of rifting, diffuse intraplate deformation may accommodate regional extension, as illustrated by the widespread distribution of rift basins, up to 600 km westwards from the actual passive margin. The basin distribution and geometry were basically controlled by the previous basement structural framework and by rifting kinematics, related to the spatial interaction between the Southern and Equatorial branches of the South Atlantic rift system. The northeast Brazilian rift basins show clear chronologie and tectonic links to progressive deformation that migrated, from south to north, in the Southern branch. Three syn-rift stages are associated with the rift evolution of the Southern branch, distinguished based upon the structural style and basin-fill characteristics. The main rift stage took pIace during the Neocomian-early Barremian Time, with the development of three main rift valleys: (1) the Gabon-Sergipe Alagoas trend (GSA),(2) the Reconcavo-Tucano-Iatoba trend (RTJ),and (3) the Carirl-Potiguar trend (CP).The influence of preeristing crustal weakness, a Proterozoic heritage, is demonstrated by the megashear zones of the Pernanbuco (Brazil) and Ngaundere (Africa), which behav~d like a huge accommodation zone, balancing ertensionat deformation along the RTJ e GSA trends with simultaneous enenslon along the CPtrend. The basins are typically 20-45 km wide, controlled by NESW trending normal faults, roughly perpendicular to the main extension direction. A major change in rifting kinematics occurred during the Neocomian when early rifting deformation developed in the eastern equatorial domain, meanwhile the CP trend was aborted, and the southern branch had the main stretching phase. During the Aptian, rifting occurred throughout the Equatorial branch and Benue trough (Africa), while a transitional eyaporitic sequence developed in the Southern branch (GSA),-and the RTJ trend was aborted. An Albian-Cenozoic marine drift megasequence overlies the northeast Brazilian rift basins eccept by the RTJ trend and some of the CP basins.

Transformada de Radon Parabólica via Transformadas de Fourier

O Prospecto LEPLAC-IV foi o terceiro programa geofisico levantado pelo Navio Oceanográfico Almirante Câmara (ES-500) no âmbito de Projeto LEPLAC, dando seqüęncia a uma açăo conjunta entre Petrobrás e Marinha do Brasil que, até o momento, permitiu a aquisiçăo de mais de 175 000 km lineares de batimetria de precisăo, sísmica de reflexăo multicanal, gravimetria e magnetometria.

O Leque Submarino de Săo Tomé está localizado na Bacia de Campos-RJ e corresponde a uma protuberância sedimentar notável no talude e elevaçăo continental adjacente. Nesta regiăo, expediçőes GEOMARES observaram cânions, descritos por Brehme (84), que identificou na porçăo norte do talude um grupo deles denominados de Grupo Nordeste de Cânions, compreendendo os cânions Săo Tomé, Itapemirim e Alte, Câmara; na porçăo sul do talude, foi identificado um outro conjunto denominado Grupo Sudeste de Cânions correspondendo a uma série de cânions que foram numerados de um a cinco de sul para norte.

O Complexo do Leque Submarino de Săo Tomé caracteriza-se por uma pronunciada protuberância sedimentar localizada ao largo do talude e elevaçăo continental da Bacia de Campos-RJ (mapa 1). Esta feiçăo submarina foi primeiramente definida por Brehme (1984) nos domínios do talude continental e do Plato de Săo Paulo, entre a foz do rio Itabapoana e o Grupo Sudeste de Cânions, sendo ŕ época denominado de Leque Submarino do Paraíba do Sul.

The rapid growth of exploration in the Brazilian and west African marginal basins in the last decade can be attributed to the recognition by oil companies that these basins constitute some of the last frontiers in the world for appropriation of large oil and gas reserves. In most Brazilian and West African basins, exploration is far from being mature, since deep water probes practically have not started. However, evidences have shown that similar oil types in the southern Brazilian Santos, Campos and Espirito Santo basins, are to be found in the West African Cuanza, Cabinda and Zaire basins. Such basins fit neatly in the gig-saw puzzle if the maps of the South America and Africa continents were to be brought together (Mello et al., 1993). A geochemical survey of the most productive Brazilian and West African sedimentary basins, using a wide selection of oils and source rocks ranging from Lower Cretaceous to Miocene in age, has been carried out. The aims were to identify and characterize the oil types, to assess the respective depositional paleoenvironments and the age of their putative source rocks, and to perform the oil-oil and oil-source rock correlations in order to compare and characterize oils derived from both sides of the Atlantic. This approach was based mainly on the distributions and concentrations of biological markers (Moldowan et al., 1985; . Mello et al., 1988, Mello and Maxwell, 1991).

Approximately 47000 line kilometres of 2-D seismics has been acquired along the west coast margin of South Africa. From this data and a limited number of boreholes an understanding of the tectonic and depositional history has been derived for this Atlantic divergent plate margin. Drilling results on the shelf area have had sufficient merit to establish the significance of this region as a hydrocarbon province. Aspects of the petroleum geology and the exploration techniques applied are discussed together with examples of typical trap types. Guided by inference from geochemical, seismic and gravity studies, exploration has recently moved into deep water areas where the potential exists for the future discovery of significant hydrocarbon reserves.

We analyze magnetic data collected during 1988 and 1989 cruises in the South Atlantic run jointly by the Naval Research Laboratory (NRL) and the Diretoria de Hidrographia e Navigacao (DHN). The cruises surveyed the Bahia Seamount group that lies off the Brazil coast between latitudes 11°S and 12°S and longitudes 31°W and 35°W (Figure 1). The seamounts are on oceanic crust formed during the Cretaceous Normal Superchron, and were found to be typically small, deep. and conical. We find paleomagnetic poles for 6 seamounts by inverting magnetic anomaly data using the seminorm minimization method. We find one paleomagnetic pole for 7 seamounts that share a volcanic platform using a least-squares inversion that models seamounts with rocks of opposing polarity. We could not find poles for other seamounts owing to reversals in magnetic polarity along the seamount group and the large number of seamounts sharing volcanic platforms. We construct a South American Tertiary apparent polar wander (APW) path using our seamount poles. One of the seamounts was radiometrically dated. We estimate ages for the other seamounts by comparing their paleomagnetic poles with Late Cretaceous South American poles, and with North American Tertiary poles. Our APW path begins with a Late Cretaceous pole at 76.4 °N, 178.4 °E, then moves toward the spin axis in a slight zigzag, through a Paleocene pole at 84.2°N. 194.3°E. and an Early Eocene pole at 85.7°N, 123.9°E. We estimate that the South American paleomagnetic pole reached the spin axis in the Middle Eocene.

In the Amazon basin extensive volcanism occurred during the Mesozoic with wide-spread intrusion of diabase sills into the Palezoic sedimentary infill. In some areas there exists a close relationship between hydrocarbon accumulations and distance from Mesozoic dikes and sills: Vitrinite reflectance, spore coloration and abundance of gas increase upwards towards the sills, contrary to what would be expected from sedimentary burial by subsidence alone. At greater distances away from the sill the geothermal effect due to burial becomes dominant and an increase in vitrinite reflectances away from the sill is observed. To investigate this relationship we developed both an analytic and numerical model of a finite width heat source (dike or sill) which takes also into account the geothermal heat flux due to sediment burial. This allows us to calculate the complete temperature history of the intrusive and the surrounding sedimentary rock. The model demonstrates that the temperature history is primarily sensitive to the thickness of the intrusive, the latent heat of fusion and the thermal conductivities and diffiuivities of the intruaive and the host rocks. The maturation of organic matter is evaluated using a vitrinite reflectance model proposed by Lerche (1988), for which we determined the relevant parameters through an inverse fitting procedure to actual well data. The resulting relationship can be used to evaluate hydrocarbon maturation in areas where sill intrusions are a major thermal factor.

The seismic reflection and drilling data show that the Southern Brazilian margin (Pelotas and Santos basins. Florianopolis High and Sao Paulo Plateau) is of a volcanic type. characterized by the effusion of flood basalt sequences (seaward dipping reflectors). recognized along a 70 to 380 km wide band parallel to the coast line. The flood basalt sequences. which may be as thick as 8 krn, was emplaced between the Early Valanginian and the Late Aptian. The volcanic rocks are partially overlain by clastic sediments. Both, volcanic rocks and clastic sediments are related to the synrift phase that preceded the openning of the Southern South Atlantic. The presence of the volcanic sequences along the Southern Brazilian margin. and of similar age and of similar seismic characteristics on the Argentinean margin and on the conjugate margins of South Africa and Namibia (where the seaward dipping reflectors have also been identified) indicates that the southern portion of the South Atlantic is borded by volcanic type margin.

Many magmatic provinces (mainly flood basalts) occur along continental margins and are associated with initiation and early development of continental rifting. The causes of this magmatic activity are yet unclear and frequently related with the early activity of hotspots. The hotspots would correspond to the anomalously warm material rising from the lower mantle through conduits, the so called mantle plumes. Many hotspots are supposed to have begun with massive outpourings of basaltic magmas over broad regions (e.g. Parana Basin). Subsequent volcanism over the plume conduit produces the tracks that connect flood basalts to presently active hotspots.

Cada material natural apresenta um estado intermediário entre os estados Iíquido e sólido cristalino. Em estado líquido, o material tende a apresentar uma reologia próxima a newtonian ŕ: isto e, esforços infinitesimais săo capazes de causar deformaçőes lentas (dependendo da viscosidade) mas infinitas, que terminam por deixá-Ios com sua superfície perpendicular ao maior esforço principal. Em estado sólido cristalino, ao contrario, o comportamento dos materiais e, idealmente, puramente elástico: a deformaçăo e proporcional ao esforço aplicado, e termina, sob a açăo de esforços desviados muito grandes, em fratura brusca.

A Margem Equatorial Norte Brasileira deve ser analisada ŕ luz das aberturas do Atlântico Norto o do Atlântico Sul. Para tal, suas macroestruturas devem ser observadas no contexto brasileiro e africano: e no que tem a nos ensinar as estruturas e processos geológicos atuais e do passado geológico recente, do meio do oceano Atlántico Equatorial. O presente trabaIho tem essa finalidade, para tentar compreender a relatividade da importância do arcabouço tectônico da margem versus a sedimentaçăo que moldou a morfologia atual de seu fundo submarino.

An extension of resistivity measurements in both the laboratory and field situations is demonstrated. Using high density computer-controlled measurements images of the electrical resistivity distribution in a core, or over an area of ground are produced. At both scales the variability in electrical properties over relatively small distances is evident. In a sediment-core the electrical image shows high resolution detail which complements a conventional photo, thus enhancing the usual interpretation. The electrical resistivity imaging is then used to monitor the passage of a saline intrusion through a volume of rock. Firstly this is demonstrated for a large volume of ground, and then separately for a core sample, This technique can be used to zone areas of investigation, assigning different characteristics to each zone, or to monitor the movement of fluids with time. Its application to both core and site investigations includes hydrocarbon and geotechnical characterisation, as well as aquifer and pollution monitoring.

The utility of the joint transient electromagnetic (TEM) and audiofrequency magneto-telluric (AMT) methods in stratigraphic and hydrogeological mapping has been evaluated in the southeastern part of the Parnaiba basin. The field experiments employed central-induction and coincident-loop TEM and tensorial AMT depth sounding techniques at several observational stations along a 400 Km traverse. The processed data are in excellent agreement with borehole data and known geology of the area. The results show that all the main stratigraphic formations can be distinguished on the basis of their resistivity characteristics and also allow proposing a zone for the Transbrasiliano lineament. A preliminary groundwater resource evaluation study of the Piaui State transect showed that the prolific aquifers in the basin are easily detectable using the TEM/AMT methods due to the favourable juxtaposition of electrically-contrasting permeable and relatively impermeable geological formations forming confined aquifers of regional extent. It is contended that these regional groundwater reservoir resources may be developed by machine-drilled boreholes to meet the water needs of the region but further detailed geophysical evaluation will be required for a cost-effective borehole siting program. Suggestions are also offered to guide any future detailed exploration for the more localized and shallower reservoir occurrences and resource development possibilities in the basement areas in the southeastern margin of the basin.

A geoelectric image of a portion of the Sao Fransisco sedimentary basin (Minas Gerais) was obtained based on magnetotelluric (MT) data from 43 MT soundings (400 Hz - 0.001 Hz). The soundings were carried out along 10 seismic lines. covering an area of 11.000 km˛ approximately. The distances between soundings were irregular and varied from 2.5 km to 20 km. Static shift corrections were applied to the data using the median resistivity of the first conductor. One dimensional (1D) Occam geoelectrical sections were obtained for all profiles. A sequence of conductors and resistors was found in the sedimentary package. followed by a geoelectrical basement. The MT sections show good agreement with both borehole information and seismic sections.

Pela sua potencialidade em discriminar camadas condutoras próximas ŕ superfície, o método audiomagnetotelúrico (AMT) vem sendo utilizado com sucesso na prospecçăo de depósitos maciços (Strangway e Koziar, 1979; Lakanen, 1986). A possibilidade de seu emgrego em estudos de sulfetos disseminados parece, porém, limitada a casos específicos nos quais a concentraçăo desses sulfetos se correlacione a algum outro parâmetro geológico que possa ser mapeado por levantamentos AMT.

Atualmente no Brasil a fonte sísmica para levantamentos VSP ( vertical seismic profiles) é o airgun. O pulso de um airgun possui duas deficięncias : baixa potęncia e o efeito causado pela oscilaçăo da bolha que gera pulsos secundários apos a emissăo do pulso primário (Ziolkowski, 1970; Schulze-Gatterrnann, 1972; Giles & Johnston, 1973 e outros).

This paper is a review of recent developments in the magnetotelluric method. Although there are many application areas to which MT may be applied, the increasing use of MT as a 'standard' survey method is emphasized. The review briefly considers the directions being taken to enable MT to 'become more useful' to the exploration community. A number of developments in the areas of data acquisition, response function estimation and modelling/inversion are discussed. Such developments should ultimately lead to MT surveys which consistently 'add-value' in the solution of exploration problems.

In the summer of 1990 a magnetotelluric (MT) survey was performed in southern Portugal to try to delineate the geoelectrical structure of a geothermal anomaly reported to exist there. Although the geological and electrical characteristics of the region of the MT study are apparently three-dimensional, a twodimensional modelling approach is being used for an initial interpretation of the data. The objective of this approximation is twofold. First, it allows a preliminary geoelectrical interpretation for the region; second, it will provide bounds for the construction of a threedimensional model for detailed interpretation and which will be closer to the actual geological character exhibited in geologic and tectonic maps of the area. The main features seen in the results of the numerical models are also seen in the results of the measured data. Low electrical resistivity zones which coincide with fault structures that cross the region are evident in both the model and data pseudosections.

A Bacia do Recôncavo situa-se no nordeste brasileiro, entre os paralelos 11°30 'S e 13°S, ocupando uma área de aproximadamente 11.000 km2. Em superfície ocorrem afloramentos que caracterizam tanto a borda flexural (a oeste) quanto a falhada (a leste), o que a torna especial em relaçăo aos demais riftes conhecidos, onde afloramentos desse tipo săo raros.

The Z/H-method uses the ratio of the vertical to the horizontal magnetic component of a non-uniform source field to determine a transfer function which equals the magnetotelluric (MT) impedance. At mid latidudes the Z/H-method can only be applied to the Sq-field with periods from 4 to 24 hours and to the On-field with even higher periods. In the vicinity of the dip equator the Equatorial Electrojet (EEJ) generates a non-uniform source field with periods from 100s to 10000s. Thus the transfer functions of the Z/Hmethod estimated from magnetic variations of the EEJ can be compared directly with the MT impedances. In this study the application of the Z/H-method to the EEJ-field and the comparison of the resulting transfer functions to MT impedances is investigated in numerical models and using electromagnetic data from the Northeast of Brazil.

The equatorial electrojet practically does not effect MT soundings at frequency higher than 10-2 Hz. However, below 10-2 Hz its influence can be very intense, increasing as frequencies decrease. Such influence can be felt at hundred of kilometers from either sides of the magnetic equator. The erratic behavior of the equatorial electro jet currents in the ionosphere and not its intensity seems to be the principal cause of such big effects. Since the electrojet currents are changing constant and erratically, the effects 011 the soundings can be very large and unforeseeable. To reach these conclutions we used in our numerical simulation the Onwumechillian model which is the simplest realistic model available for equatorial electrojet.