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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
461 - 480 of 484 results
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Influence of lower crustal rheology on styles of continental collision: numerical modeling and observations
Authors Faccenda Manuele, Taras V. Gerya and Sumit ChakrabortySeveral numerical models of continental collision following a previous oceanic subduction were performed in order to investigate the role of lithospheric strength in determining the style of collisional zones. During oceanic subduction, the upper plate is upwarped due to the high frictional interface between the plates. When continents
collide, two different collisional zones develop depending on the rheology used for the lower crust: strong upper plate lithosphere produce the classical one-sided, asymmetrical collisional zone; medium-weak rheologies give rise to a two-sided, symmetric collisional zone where a large amount of continental crust can be recycled into the deep mantle.
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1-D Unversion of the rayleigh wave group velocity distribution in South America
More LessThis paper presents a map of the depth of the crust-mantle discontinuity (Moho) for South America. This map was obtained by inverting Rayleigh wave group velocity. The group velocity curves were inverted as a whole for an initial shear wave model in depth, 48 inversions were produced, one for each block that South America is divided in. The initial model for inversion was defined according to the position of the blocks with respect to the continent and to the ocean. The inversion results are the density and shear wave velocity maps for the depths of 15, 30, 35, 40, 50, 60, 80, 90 and 100 km. In order to obtain a map with the Moho depths in South America, a minimum value for shear wave velocity (4.6 km/s) was fixed and interpolation was carried out for all blocks. The result is the first map for the Moho discontinuity of South America. Keywords: Rayleigh waves, Moho discontinuity, Inversion, South America.
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Upper mantle S-wave velocity structure in South America from Rayleigh-wave tomography and definition of cratonic units
Authors Marcelo Assumpção, Mei Feng, Suzan van der Lee and Simon LloydA new tomographic S-wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S & Rayleigh waveforms and fundamental-mode Rayleigh-wave group velocities, to better constrain upper mantle S-velocity structure and Moho depth. We used about 5700 Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent. The joint inversion of this data set provided a new 3D upper mantle S-velocity model and a Moho depth model for South America, which fits both the group velocity and regional waveform data sets well (Feng et al., 2007).
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Considerations on th major lithosphere contacts on the Romanian territory and related dynamics as inferred from geophysical data
More LessPlate tectonics on the Romanian territory is still subject to debate. Based on geophysical data interpretation a three plate tectonic model is proposed. Geometry and nature of the major lithosphere contacts (plate boundaries) are documented and their dynamics emphasized. Gravity and geodetic information are used to emphasize their present dynamics. The role played by the Black Sea opening in geodynamic evolution of the SE Carpathians is also presented.
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Modelagem Gravimétrica direta 3-D do SE do cráton São Francisco
Authors Luiz Gustavo Rodrigues Pinto, Naomi Ussami and Carla Braitenberg3-D forward gravity modeling of SE São Francisco Craton data was carried out using the equilibrium condition of
topographic masses. Two bodies situated within the crust were modeled. The first body is located at the bottom of the crust with density of 2,755 g/cm3 and another body at surface with density 2,580 g/cm3. In order to interpret the geological significance of these bodies we used independent geophysical data, the broad-band MT soundings and crustal thickness provided by response function from previous studies in the region. Crustal gravity model are in accordance with geolectric model from MT soundings. High density and low resistivity lower crust is probably related to intrusions inside the crust and mantle, derived from the magmatism associated with the rupture of W Gondwana.
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Gravimetric and seismic data integration in a 2D forward gravimetric modeling for the crust and lid mantle beneath northern Brasilia Belt
Authors Munir Koosah, Roberta Vidotti, José Eduardo Soares and Reinhardt A. FuckThis work presents a 2D forward gravimetric model for the Tocantins Province, central Brazil, obtained by integrating gravimetric and seismic data in a 500 km long profile of the crust and lid mantle. Geometry and density of the initial gravimetric model were estimated taking into account an existing seismic model based on deep refraction data. The gravimetric model shows quantitative evidence that the gravimetric differences in central Brazil are mostly compensated in the lid mantle.
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Estudo da Estrutura da Crosta na provável Zona de Sutura entre o Cráton São Francisco e “Cráton Paranapanema”, usando a Função do Receptor
More LessThe limit between Paraná Basin and Brasília Belt provinces is a suture zone formed due to the convergence of São Francisco and Paranapanema cratons/plates during Neoproterozoic time. The Paranapanema Craton is nowadays under the Paraná Basin. The Moho depth, mean P velocity (Vp) and mean Vp/Vs ratio of the crust along Paraná Basin-south Brasília Belt surface limit is herein presented. They were obtained by receiver function results from NP4B and IGPB seismographic stations. The Paraná Basin NE border was poorly solved by refraction data Santa Juliana (Soares, 2007), but It suggests a Vp of 6.6 km/s and a Moho depth of 43 km. These results are used by receiver function results from NP4B and IGPB stations that converge to Vp/Vs of 1.72 and to Moho depth of 42.5 km, considering crustal Vp of 6.6 km/s. The mean Vp/Vs ratio of 1.72 is surprisingly low to Paraná Basin domain. It suggests a felsic composition to the crust of this region, although the expectation was finding high Vp/Vs values expressing the basaltic flows of Paraná Basin. Mean Vp/Vs of 1.72 suggests the presence of Brasília Belt/São Francisco Craton basement rocks under the Paraná Basin NE border.
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Lithosphere mechanical behavior inferred from tidal gravity anomalies: a comparison of Africa and South America
Earlier studies have shown that the amplitude difference of the M2 gravity tidal component (TGA) between the
measured and calculated response for a viscoelastic Earth is significantly correlated to the effective elastic thickness (Te) of the lithosphere.
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Crustal model of the Paraná Basin-Brasília Belt boundary: a deep refraction approach
More LessThis work presents a crustal model of the Paraná Basin-Brasília Belt boundary obtained through the modeling of the Santa Juliana deep refraction data. The seismic model presents a 43 km-thick crust, with mean P-wave velocity of 6.6 km/s in the Paraná Basin domain, and a 39 km-thick crust, with mean P-wave velocity of 6.25 km/s in the Brasília Belt. The model displays a tendency of the crust to become thicker under the Paraná Basin domain.
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Brazilian project of the Saint Peter Saint Paul Islands: the mantle exposure along the transform fault system of the Equatorial Atlantic region
This paper presents the Brazilian scientific program of the Saint Peter Saint Paul Islands, Equatorial Atlantic region. The program is composed of 24 individual research projects in biology, geology, geophysics, meteorology, architecture, and oceanography by financial and logistic supports of the CNPq and the SECIRM. The islands are about 1100 km to the ENE from Brazilian Northeast coast and sit on the Saint Paul transform fault zone, which offsets the Mid-Atlantic Ridge more than 600 km. In 1998, Brazilian Scientific Station was installed at Belmonte Island, in which four researchers stay alternatively. This area is free from continental urban pollution and a biological research group identified 120 species of fish. The migrant birds use the islands as a pausing point and nesting site. The studies of heavy metal contamination of bird eggs and the DNA of bacteria are important subjects. This place is also important for geology because of the mantle rock exposure and shallow earthquakes occurrence, being an ideal site for the observation of active mantle tectonism and related hydrothermal activities. The international cooperation with France realized 13 deep sea dives. The islands are composed of strongly deformed lherzolite, harzbergite and serpentinite of vertical configuration, which are cut by numerous hydrothermal veins. They are considered to be uplifted abyssal ultramafic rocks, being a rare mantle exposure on the Earth’s surface.
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Three-Dimensional Geophysical Structure of Oceanic Core Complexes Formed at the Mid-Atlantic Ridge, 30°N
Authors Donna K. Blackman and John A. CollinsGravity and seismic modeling of oceanic core complexes formed at the Mid-Atlantic Ridge near 30°N suggests that their shallow, domal 'core' may be dominated by mafic intrusive rocks. The youngest of the three core complexes in this area, Atlantis Massif, has been sampled to 1.4 km depth during Integrated Ocean Drilling Expeditions 304 and 305. The recovered sequence was gabbroic, in contrast to prior models of this domal core as an ultramafic mantle section, uplifted via long-lived detachment faulting. Rather than use a mantle Bouguer anomaly, as is typical for marine gravity studies, this study emphasizes anomalies after a seafloor Bouguer and a plate cooling contribution are removed from the Free Air Anomaly. Reanalysis of seismic refraction data at Atlantis Massif indicate that while a mantle velocity layer (~7.5 km/s) at subseafloor depths of less than 1 km provides a good fit to observed travel-times, models with greater along-strike variability in the structure of lower velocity units (<6.5 km/s) can fit the data equally well. These new gravity and seismic results support several structural and compositional interpretations. A 3-D gabbroic core with density 2900 kg/m3 and juxtaposed 3-D hanging wall of fractured basalt, density ~2600 kg/m3, can explain most of the Bouguer anomaly at Atlantis Massif. The detachment fault probably terminates along-strike or plunges northward beneath the surface where the corrugations end at the northern limit of the central dome. The southwest shoulder of the massif probably contains an upper crustal section of thickness ~1 km, whereas the peak and southeastern shoulder of Atlantis Massif have overall density more similar to the central dome. The older core complexes along the fracture zone in this study area are similar in size, depth, and distance of their peak from the transform fault. However, weathering probably has reduced their density somewhat compared to current values at Atlantis Massif.
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New constrains on ridge-hotspot interactions from the PLURIEL Cruise, Saint Paul-Amsterdam Plateau, Indian Ocean
Authors Marcia Maia, Claire Bassoullet, Cédric Brachet, Deborah Chavrit, Esther Courrèges, Pascal Gente, Christophe Hémond, Eric Humler, Kevin Johnson, Benoît Loubrieu, Christophe Martin, Abhay Mudholkar, Jean-Pierre Oldra, Martin Patriat, Ivo Pessanha, Aude Raquin, Manuel Richard, Jean-Yves Royer, Judith Vatteville and Walter RoestThe PLURIEL cruise, held on board the Marion Dufresne II from September 19 to October 31 2006, had as main
objective the study of the temporal evolution of the interaction between the St Paul-Amsterdam hotspot (SPA) and the Southeast Indian Ridge (SEIR) over the last 15 m.y.. The SPA-SEIR system is an ideal target to study the transition from the intraplate to the axial phases of activity of a hotspot, since it shows in the same interaction system, both an axial volcanic plateau and a seamount chain. The SPA plateau, corresponding to the axial phase of activity of the hotspot, is a wide platform with an average depth of 2500 m. On the plateau, west of the ridge axis, the Saint Paul and Amsterdam islands were active in the last million years. The most recent volcanic activity was recorded at the Boomerang Seamount, between Amsterdam Is. and the ridge axis, twenty years ago. A chain of volcanoes possibly marks the intraplate phase of activity of the SPA hotspot. The first results of the cruise show that the time evolution of the ridge-hotspot interaction in this area is very complex.
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A Simple Conduction-Advection Model of the Lithosphere and Possible Demise of the Hypothesis of Regional Hydrothermal Circulation in the Ocean Crust
Authors Roberto Rodrigues Cardoso and Valiya M. HamzaA simple conduction-advection model of heat transfer in the oceanic lithosphere (designated as CH model) is proposed and its implications for determining the thermal field of lithosphere at mid ocean ridge areas examined.
According to this model the young lithosphere is stratified, with a magma-rich segment beneath the magma-free upper layer. The unique feature of this model is that the transition from the layered magma-embedded structure to
non-magmatic stable configuration takes place on timescales much shorter than those predicted by the conventional boundary layer theory. In addition, it also retains the desirable features of the well-known plate model for the stable lithosphere. The model is thus found to be capable of reproducing the observed variations in heat flow, without the need to invoke the hypothesis of regional scale hydrothermal circulation in the ocean crust. Results of numerical simulations, based on the CH model, are found to reproduce regional-scale features in the thermal fields of the oceanic crust, identified in recent higher degree spherical harmonic representations of global heat flow (Cardoso, 2006; Hamza et al, 2007). Examples are presented illustrating the thermal structure of South Atlantic lithosphere.
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Space and time evolution of the interaction system between Tristan da Cunha hotspot and South Mid Atlantic Ridge
Authors Ivo Pessanha, Marcia Maia and Sidney MelloIn this work we have studied the space and time evolution of the Tristan hotspot and south MidAtlantic Ridge interaction system. The Tristan hotspot and its associated topographical features (Walvis Ridge, Rio Grande Rise) are one of the most significant features of the south Atlantic ocean. However, this system is not as well studied as the systems in the North Atlantic (Azores and Iceland).
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Increasing mid-ocean ridge research and exploration by developing countries
By Jian LinThe mid-ocean ridge system plays an essential role in the renewal of Earth’s surface, recycling of oceanic
lithosphere, release of heat and chemicals from Earth’s interior to oceans, and the deep-sea chemosynthetic
processes. International cooperation is essential in ocean ridge research. It is recognized that most ocean ridges are located in international waters, the scientific objectives and interests of researchers transcend national boundaries, and the scope of ocean ridge science is so large that it can never be covered fully by the resources of any single nation. This presentation will discuss the recent international advances in research of the deep-sea
geological, hydrothermal, and biological processes, focusing on the increasing cooperation between InterRidge member nations, especially with developing countries.
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Hydrothermal exploration using Autonomous Underwater Vehicles (AUVs)
More LessThirty years after the first discovery of seafloor hydrothermal venting no more than 90% of the global mid-ocean ridge crest hás been investigated for seafloor hydrothermal venting. Until recently entire ocean basins had remained unexamined. What was needed was a new, more efficient mode of exploration that could not only detect for the presence of hydrothermal venting but also produce first-order characterisation of the geologic nature, and extent of a vent-field’s setting and – equally important for first order characterisation – some indication of the nature of any chemosynthetic ecosystems these hydrothermal vents might host.
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Responsible Research of Deep-Sea Hydrothermal Vents Promoted by the InterRidge Program
Authors Rhian Waller, Jian Lin and Chris GermanHydrothermal vents are present in all of the world’s oceans in areas associated with tectonic and/or volcanic activity. This process results in extreme environments that are home to high densities of specialized microbes and animals, the study of which may lead to exciting new discoveries applicable to societal needs. However, with the increased scientific and potential commercial interests surrounding these hydrothermal vents comes the potential for large anthropogenic impacts. In 2006 the Biology Working group of InterRidge published a “Statement of commitment to responsible research practices at deep-sea hydrothermal vents”. This voluntary “code of conduct” statement encompassed the concerns of research scientists over the potentially harmful impacts of scientific collecting by multiple parties at the same vent sites. This talk will explain the voluntary Statement of Responsible Research as adopted by the InterRidge program and discuss issues concerning responsible research of deep-sea hydrothermal vents and their ecosystems.
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Multicomponent 4-D seismic reservoir characterization of tight gas sands, Rulison Field, Colorado
More LessThe Colorado School of Mines Reservoir Characterization Project (RCP) is conducting time-lapse multicomponent
seismic surveys at Rulison Field, Piceance Basin, Colorado, to identify and monitor fracture zones in tight gas sands. Multicomponent seismic monitoring is being conducted in a tight gas development project for the first time, as drilling to ten acre well spacing is occurring. The purpose of the study is to use dynamic reservoir characterization to develop reservoir characterization technologies and improve recovery efficiency in tight gas
reservoirs.
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Offshore High-Resolution 4D Acquisition
Authors Corinne Sagary and Mundy BrinkIn time-lapse seismic acquisition, many factors are critical for the 4D data quality. Some of these are discussed in this paper. Tidal effects and changes in water layer properties during the acquisition of the base and monitor surveys may have a dominating and detrimental effect on the quality of the 4D signal. Tidal statics can be compensated for using high-accuracy GPS measurements. Variations in the water layer velocities can be corrected for using new processing methods. Weather-related noise and other types of noise and acquisition anomalies like streamer depth variations and hydrophone sensitivity variations can be a limiting factor for the quality of the 4D signal. The impact of these types of noise and anomalies can be reduced significantly with the use of solid streamer technology. The gains in signal-to-noise ratio can be 15 dB for solid streamers in comparison with liquid streamers. High quality 4D signals require the accurate duplication of shot and receiver positions. This can be achieved with modern navigation methods adapted to 4D surveys.
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Critical reflection time lapse: sensibility and analysis of uncertainties of P-wave velocity changes caused by critical offset variation due to the reservoir fluid substitution
More LessWe present here, a survey of the time lapse critical reflection method in terms of evaluation of sensibility in the relationship among uncertainties of some parameters and elements that has participation in the mentioned method. The objective is to know, in the presence of fluid substitution, the type and level of the relationship between the perturbation (or change) of velocity in a reservoir and a dynamic parameter called critical offset variation that can be measured on the observation surface before and after reservoir fluid substitution of, for example, oil by water. Such relation is, also, considered with respect to the static model parameters such as: thickness of sedimentary rocks that cover the reservoir and its equivalent velocity. A performed analysis allows us to observe that: the constraint level between the critical offset and velocities in both media (where wave is
incident and where it is refracted, or critically reflected), varies with the velocity contrast. When the velocity
contrast increases, the mentioned relationship with the reservoir associated velocity becomes stronger. The
opposite occurs with respect to the associated velocity to the medium where the incidence has place. By the other
hand, the prediction process of the reservoir velocity variation, using the critical offset variation, has a strong sensibility with respect to the uncertainty associated: to the initial reservoir velocity (base reservoir interface) and
to the effective velocity of the sedimentary cover. Uncertainties in the estimation and quantification (or measurement) of reservoir top interface depth and critical offset variation do not affect, significantly, the reservoir
velocity variation prediction.
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