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An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.

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Abstract-Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5-D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies ⁄ structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short-wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.5-1 km below the current surface. These depths are also supported by 2.5-D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock-induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s )1 .

ABSTRACT. The reflection seismic method is extremely important for the hydrocarbon exploration. With more complex geological structures, as the pre-salt from Santos Basin, the reservoir exploration becomes more challenging and the multicomponent seismic investigation improves its mapping and characterization. To obtain multicomponent seismic data of an offshore survey it is necessary to use the OBN (Ocean Bottom Nodes) technology. The converted wave behavior, the large offsets and the difference of datum between source and receptors for layered media, results in strong nonhyperbolic travel-time events. Furthermore, the complexity and peculiarities of some unconventional reservoirs found in the pre-salt also increase the difficulty to perform the velocity analysis. For these reasons, it is necessary to use nonhyperbolic multiparametric travel-time approximations to control the nonhyperbolicity. Here we perform the comparison of nonhyperbolic travel-time approximations of seismic reflection events derived from geological models. The numerical study was considered as an inverse problem and it was treated according to an optimization criterion. The complexity analysis was performed in order to understand the behavior of each approximation concerning the unicity. After the computation of the relative errors between the observed curve and the calculated curve for each nonhyperbolic approximation, it was possible to find out the one with the highest accuracy for the events tested here.  Keywords:  multicomponent, OBN, nonhyperbolic.RESUMO.Ométodo sísmico de reflexão é extremamente importante para a exploração de hidrocarbonetos. Comestruturas geológicasmais complexas, como o pré-sal da Bacia de Santos, a exploração de reservatórios se tornamais desafiadora e a utilização da sísmica multicomponente promove o mapeamento e caracterização estrutural. Para obter-se dados de sísmica multicomponente em um levantamento offshore é necessário utilizar a tecnologia OBN (Ocean Bottom Nodes ). O comportamento de ondas convertidas, longos afastamentos e diferença de datum entre fonte e receptor pra meios estratificados resultam em uma forte não-hiperbolicidade dos eventos de tempos de trânsito. Além disso, a complexidade e peculiaridades, de alguns reservatórios não convencionais encontrados no pré-sal, também aumentam a dificuldade em realizar a análise de velocidades. Por estes motivos, é necessário usar aproximações não-hiperbólicas multiparamétricas de tempos de trânsito para controlar os efeitos da não-hiperbolicidade. No presente trabalho, foi realizada a comparação de aproximações não-hiperbólicas de tempos de trânsito de eventos sísmicos de reflexão provenientes de modelos geológicos. O estudo numérico foi considerado como um problema inverso e foi tratado de acordo com um critério de otimização. A análise de complexidade foi realizada para compreender o comportamento de cada aproximação com respeito a unicidade. Após computar os erros relativos entre a curva observada e as calculadas com cada aproximação, foi possível descobrir a aproximação que apresentou maior precisão para os testes realizado.Palavras-chave: multicomponente, OBN, não-hiperbólicas.