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10th International Congress of the Brazilian Geophysical Society
- Conference date: 19 Nov 2007 - 23 Nov 2007
- Location: Rio de Janeiro, Brazil
- Published: 19 November 2007
441 - 460 of 484 results
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Caracterização da sedimentação holocênica na plataforma continental em frente ao Rio Almada, utilizando levantamento com perfilador de sub-fundo
High-frequency seismic data (2 to 12 kHz) collected on the continental shelf offshore the Almada River Mouth
(Bahia State, Brazil) was used to map the Holocene sedimentary layers above the acoustic basement. The
produced isopach map was integrated with the bathymetry and with the superficial sediment distribution of of mud, sand and gravel fractions. Higher reflectivity of sands prevented the penetration of the geophysical signal while low reflectivity on mud permitted to map thicker mud deposits. The Almada Canyon is the most expressive geomorphic
feature in the area, filled essentially by mud.
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Using Sedimentary Markers to Evaluate Subsidence Rates: A Case study in the Gulf of Lion
Authors M. Rabineau, J-L Olivet, D. Aslanian, J. Baztan, C. Gorini and F. BacheThe sedimentary record of the offshore part of continental margins is characterized by important thermic subsidence, which favours a good preservation of sedimentary sequences (contrarily to the onshore part of margins). This sedimentation in turns enhances the subsidence because of loading effects. The quantification of this total postrift subsidence (and the relative effect of both factors) is not always easy, especially when no wells are available in the area. We present here a method based on sedimentary Markers, in the Gulf of Lion.
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Models of salt-sediment interactions influencing the architecture of the Plio-quaternary deep-water sedimentary systems, Gulf of Lions-Western Mediterranean
Authors A.T. Reis, C. Gorini, W. Weilbull, R. Perovano, E. Ferreira, M. Mepen and C. StratievskThe Plio-Quaternary architecture of slope and deep-water sedimentary systems of the Gulf of Lions is controlled by
thin-skinned extension induced by gravitational glidingspreading over a shallow décollement layer. Analysis of
about 30.000 km of 2D multichannel seismic data was integrated with isopach maps of the sedimentary cover
and morphological map of the subsalt relief to evaluate how salt deformation has influenced the sediment
organization during the Pliocene and the Quaternary. Our structural and stratigraphic interpretations show that salt
deformation had implications on the stratigraphic architecture of both the Pliocene (this work) and the
Quaternary sedimentary systems (Reis et al., 2005a) at different level. First of all, salt-sediment interactions can
occur at different scales: e. g., fault scarps can create local bathymetric relief that influenced the dispersal of
clastics and the configuration of depocenters; saltwithdrawal combined with residual topography below the saltlayer can provide subsidence for the development of local basins; syn-depositional halocknesis can create a series of isolated depocenters. As well as that, the degree and nature of salt-sediment intercations varied from the Pliocene to the Quaternary deposition as the turbidite deposition and salt tectonic mechanisms changed and evolved through time.
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Mapeamento das fácies sedimentares/tipos de fundo da plataforma continental entre as desembocaduras dos rios Itariri e Itapicuru utilizando sonar de varredura lateral
Authors Israel Silva Maranhão and José Maria Landim DominguezThis paper presents the results of a side scan sonar survey integrated to surface sediment sampling carried out at the continental shelf of Conde municipality, Northern state of Bahia. The integration of these two sources of information allowed the construction of a map of the continental shelf for this area depicting the main bottom types.
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Expressões sísmicas de correntes de fundo na Bacia do Pará-Maranhão
Authors Arnaldo Tanaka and Luiz Antonio P. GamboaThe continental margin of Pará-Maranhão basin is located in the Western Equatorial Atlantic and is characterized by a steep slope cut by canyons, and the proximity of seamounts of the North Brazilian Ridge. Geostrophic currents affect the slope and deeper areas and interact with gravitational processes that control the distribution of sediments into the basin. The objective of this study was to characterize structures produced by ocean bottom currents and downslope processes that are responsible for the construction of the continental margin morphology.
The methodology of the present study was based on interpretation of bathymetric maps, seismic sections along the slope and an extensive review about the processes related to geostrophic currents. The results of the study showed that the proximity and alignment of seamounts near the continental slope caused a confinement of bottom currents and consequently its acceleration. This scenario produced sedimentary drifts with mounded geometry,
sediment waves, upslope migration of the sediments against the continental slope, and channel related drifts. The onset of bottom currents activities were recognized in pre-Miocene interval. These observations agree with the global modification that has occurred in all ocean basins since Eocene times which is related to tectonic events and the growth of ice caps in high latitudes.
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Geomorphometric characterization of the foot of the continental slope in passive continental margins
More LessThe proposed work intends to make use of Geomorphometry – science that quantifies the forms of relief – in the characterization and distinction of passive continental margins. The main objective is to provide means to distinguish continental slope from continental rise in a non-subjective and morphologically coherent way. Geomorphologically smooth continental margins, that reach abyssal depths in a gradual way, with no sharp breaks in gradient, yields a specific geomorphometric approach which is proposed by the present work and applied to digital grids generated from the ETOPO2 bathymetric database and profiles extracted from multibeam
bathymetric data. No geological or geophysical arguments are used to corroborate the methodology.
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New geophysical evidences of erosion at the base of the Walvis Ridge by the Antarctic Bottom Water (AABW)
Authors M.A. Gorini, K. Owolabi, J.R. Derntl and C.G. SilvaNew single channel seismic and 3,5 kHz records obtained in 2004, coupled with pre-existent piston core data present strong evidences that the zone of erosion at the southern flank of the Walvis Ridge, southeastern Atlantic, can extend for up to 1.400 km in length, 30 km in breadth and can be relatively as deep as 150 meters. Data suggests that the area have been suffering strong erosion by the AABW that, forced against the Walvis Ridge produced the zone of scarving. Characterization of AABW physical properties from World Ocean Experiment (WOCE) data suggests that this water mass is confined to depths greater than 3.700 meters. Clockwise pattern
of circulation of this bottom water mass and the lack of terrigenous sediment input since the Upper Cretaceous, exposed the abyssal plain to the influence of bottom currents and pelagic sedimentation. At the base of the ridge, turbidity deposits were transported along bathymetric contours towards northeast. Sediments eroded at the
base of the ridge were deposited east of the scour zone in the Cape Abyssal Plain, forming broad drift deposits.
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3-D modeling of coast effects in MT soundings at the Borborema Province (NE Brazil)
Authors Marcelo Banik de Pádua, Ícaro Vitorello and Antonio Lopes PadilhaThe coast effect on magnetotelluric (MT) data acquired at the Borborema Province of Northeastern Brazil is investigated using three-dimensional modeling. The Atlantic Ocean was modeled as a layer of constant resistivity (0.3 ohm.m) and thicknesses up to 4 km at 300 km from the coast, according to bathymetry. Offshore Potiguar and Pernambuco-Paraíba basins were also included in the model using parameters derived from geological and geophysical information. The continental resistivity model was constructed based on the electrical structure inferred from previous MT studies and other geophysical data. The modeling exercise using a finite difference code
shows that the coast effect is very important for periods greater than 30 s at 100 km from the coast, extending landwards to large distances due to the high resistivity of the upper crust. To avoid contamination by off-plane structures, the available 700 km long MT profile across the Province can only be interpreted by 3D inversion.
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Imageamento da litosfera sob a Província Borborema por transectas geoelétricas – Primeira etapa
Electromagnetic induction studies of magnetotelluric (MT) and Geomagnetic Depth Soundings (GDS) have been conducted in the northeast of Brazil during the last two years, supported by the Millenium Program, the Litoborborema Project and INPE. Two MT profiles composed of 52 individual soundings were carried out in the Borborema Province along a 700 km long SE-NW transect, and more than 30 (GDS) were deployed in the northeast region, focusing on the geoelectric properties of the lithosphere. Preliminary results are discussed in this paper.
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Imageamento das estruturas geoelétricas da litosfera na Província Borborema pelo método magnetotelúrico :análises preliminares
This work presents preliminary results of a magnetotelluric study along a 135-km long NW-SE profile across the Borborema Province, northeast of Brazil. The data were processed using robust techniques deriving curves of apparent resistivity and phase in each station. The analysis of these data indicates the presence of a highly resistive layer (above 5000 ohm-m) in the crust at depths of about 20 km in the SE and central portions of the profile and of about 10 km in the extreme NW. This thinning towards the NW is accompanied by the presence of a large conductor at lower crustal depths, suggesting the existence of a geoelectric transition. Its origin, characteristics and relation with the regional geotectonic structures will be investigated in the later stages of this
work.
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Interpretation of magnetometric data from central region of Borborema Province, Northeast of Brazil
Results of an extensive processing, interpretation and geological integration of airborne magnetic field data are
presented. The study area is located in Borborema Province, a complex structural and geological region in the northeast Brazil, including specially the Paraíba and Pernambuco States. Products from linear transformation
and from interpretation allowed the mapping of magnetic units and structures. The integration of geophysical and
geological information leads to the detailed mapping of the geological features.
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Contribuição Geofísica à análise do arcabouço tectônico do domínio rio grande do norte, Província Borborema - NE Brasil
Airborne magnetic data are used as a tool in tectonic analysis of the Pre-Cambrian terrains with complex stratigraphic and structural relationships. This paper draw on the magnetic interpretation techniques for the identification of main tectonic structures, crustal blocks and tectonic boundaries present in the Rio Grande do Norte Domain (Borborema Province, NE Brazil). Geophysical data processing results in the production of residual magnetic field, allowing generating the vertical derivative, total horizontal gradient, amplitude and inclination of the analytical signal products. Our interpretation display magnetic lineaments (N70°E) as main tectonics structures in the Rio Grande do Norte Domain, which represent deep Pre-Cambrian lineaments, dividing different crustal blocks, later reactived during the Mezosoic to develop the Phanerozoic Potiguar basin. Magnetic anomalies characterize each crustal block, such as Maciço São José do Campestre, Caicó, Angicos and Rio Piranhas. The NW-SE lineaments cross the NE-SW features and can represent shear zones in tectonic deformational boundaries of these blocks. The E-W trends enhance mafic dykes present in the area.
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Instrumentação Magnetotelúrica (MT) para Sondagens em Grade Geográfica
A compact modular magnetotelluric (MT) system was designed for rugged field soundings under low magnetic latitude characteristics, having low power consumption, large data storage (128 GBytes) and simple operational procedures. For instance, when acquiring data at the rate of one sample per minute (slow acquisition rate), it continuously consumes only 100 mW, which sustain the battery for 40 days before recharging. Monitoring of the data with the use of a notebook and recharging of the battery can be carried out while the system is in operation. For very long-period telluric measurements, low-noise Pb-PbCl2 electrodes of high stability were constructed
according to Petiau (2000). Twenty complete MT units were constructed, tested and calibrated, after extensive field testing of two prototypes.
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Geological mapping and characterization of mafic rocks using airborne geophysical data from Umirim and Canindé regions, Central Ceará Domain, NE Brazil
More LessAdvances in airborne geophysical data processing techniques and their integration with geological data allow recovering valuable information from old geophysical surveys. This information represents an essential tool for lithostructural mapping of precambrian terrains, where the stratigraphic and structural relations are difficult to be established at the field work. The airborne geophysical data from Itatira Project (1977) was carried out in the central
region of Ceará State. Their processing led to useful products which indirectly facilitate the identification regional geologic features. The verification and applicability of this approach was tested in a pilotarea whose limits are within the Umirim and Canindé topographic sheets, at 1:100.000 scale. Integration of airborne geophysical and geological field data supported the mapping at the same scale.
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Geosteering in Faulted Fields with inter-bedded sands
This document presents examples of difficulties and the implemented solutions while geosteering in a Faulted inter-bedded turbidite Field. The existence of sub-seismic faults, limitations to represent reservoir model and True Vertical Depth versus True Stratigraphic Thickness Differences were overcome through proper planning of the landing section, Validation of the geological model through Resistivity Images, Integration of a 2D Geosteering Model, the 3D Reservoir Model and the use of a deep reading Azimuthal Resistivity tool.
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O efeito gravimétrico de borda da margem continental da Província Borborema – NE do Brasil: segmentações, arcabouço crustal e implicações para a evolução termo-mecânica das bacias sedimentares
Authors Roberto Gusmão de Oliveira and Walter Eugênio de MedeirosThe free-air gravity “edge-effect” is an expressive gravity signature at rifted continental margins. It’s defined by a high in the platform break and a low in the slope foot. This works presents an interpretation of the free-air gravity edge-effect of the Borborema Province margins. According to changes of shape in the edge-effect, we show that this margin presents significant segmentations which have good correlation with boundaries between sedimentary basins. The segmentations in the anomaly edge-effects might have connections with structures and Precambrian tectonic domains. Moreover, the shape of the anomalies reveals many evidences of rifting and post-rifting events. As an example, alongside of the margin, between Sergipe-Alagoas and Potiguar basins, the low in the slope foot is enlarged, suggesting post-rift magmatic underplating. This is coherent with a profuse magmatism in the oceanic region and with the uplift of the Borborema plateau in the contiguous continental area.
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Flexure of thin elastic plates using finite element method: the case of Brazilian Continental Margin
Authors Victor Sacek and Naomi UssamiThis work presents a numerical solution of flexure of a thin elastic plate that represents the South American Plate using the Finite Element Method (FEM) due to the load of the sediments that infill the Brazil’s marginal basins. For that purpose some softwares were developed based on the FEM using triangular elements in irregular meshes. Furthermore, the geographical variation of physical properties is considered. These numerical models were compared with analytical solutions and presented great agreement. Also it is made tests of sensibility for different geometries of the 3-D loads and lateral variation of rigidity, comparing with the solution expected in 2-D models, and the results showed significant differences between 2-D and 3-D models. Finally the method was applied to
the case of the Brazil’s marginal basins to observe the effects of the sedimentary load over the plate. The
application of the numerical solution in the context of the Brazil’s marginal basins considering lateral variation of rigidity resulted in bulges in some regions with amplitudes higher than 100 m: S of the Orinoco River, SW of the Amazon Cone, W of the Pelotas Basin and N of the Santos basin. Another important uplift is identified in the NE of Brazil with 80 m of amplitude due to the geometry of the contact between the portions of the plate with different values of rigidity and the distribution of the loads in the margin.
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Análisis Isostático Flexural en la transecta a la latitud 43°S que atraviesa el Macizo de Somuncura
An isostatic analysis was made on a profile centered at the 43° of south latitude that it unites the cities of Trelew in the Atlantic coast Argentina and the city of Chaiten in the coast of the Pacific in Chile. Standard crustal thickness was assumed as 35 km (Tn), while crustal and upper mantle densities were considered as 2.9 g/cm3 and 3.3 g/cm3 respectively. This crust is subjected to compression, and to the weight of the topographical and sedimentary load deposited on her, where the load is partially supported by the elastic strength inside the mentioned crust. The gravity effect produced by the elastic flexions that it causes the topographical load when varying the equivalent elastic thickness of the crust (Te = 0, 5, 10, and 30 km) they are smaller than the Bouguer anomaly in all the cases. The result of this study indicates isostatic sub-compensation for the whole profile pointing out a deficit of cortical thickness supporting the ideas of cortical attenuation in these latitudes.
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Inversão linear de anomalias do geóide utilizando camada equivalente
Authors Carlos Alberto Moreno Chaves and Naomi UssamiA new method of linear inversion of geoid anomalies, using the equivalent layer is proposed. This method estimates density distribution within a horizontally infinite plate with finite thickness. Three tests had been carried out in order to verify the effectiveness of the new method. Two tests used synthetic models one test used a shallow body (seamount) on oceanic plate, and the second test, a deep body within the mantle. The proposed algorithm was applied to a real case, the French Massif Central, where seismic tomography study has mapped a low velocity zone within the lithosphere. The estimated densities using the present algorithm were compared with estimates
obtained using an algorithm of linear inversion using absolute and relative constraints.
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Restringindo propriedades físicas da litosfera no sul da Província Ígnea do Alto Paranaíba com dados magnetotelúricos: evidência de metassomatismo no manto
Authors Mauricio S. Bologna, Ícaro Vitorello, Antonio L. Padilha and Marcelo B. PáduaInformation on electrical conductivity (or resistivity) of the subsurface is inferred by magnetotelluric (MT) data. Broadband and long-period MT soundings were collected along a 560 km E-W profile centered near the Araxá alkaline-carbonatite complex across the southwestern margin of the São Francisco craton, Southeastern Brazil. A very low resistivity wedge into the lithosphere is highlighted at lithospheric mantle depths beneath the volcanic complex, coincidental with a zone of low velocity defined by seismic tomography. The spatial correlation of mantle
conductivity with seismic anomaly is used as a firstorder constraint to assess the more plausible electrical conduction mechanisms that could be acting in that region of the mantle. Another geophysical feature sampled by this study is a significant gravimetric gradient that has been postulated as a suture zone between the Paraná
block and the São Francisco craton. This suture zone exhibits a relatively weak resistivity anomaly at crustal depths. The absence of a strong resistivity anomaly can indicate that no extensive organic material was involved in the collision process. The final resistivity model provides a vertical image that yields relevant new information on the current geophysical state of the upper mantle under the study area, from which inferences can be drawn on
different mantle processes.
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