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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
451 - 484 of 484 results
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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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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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Improving the translation of 4D seismic attributes into reservoir properties
More LessThe optimization of the quantitative seismic data interpretation process is a key factor for reservoir
characterization and monitoring. In this paper, we show the results of a methodological investigation where Neural Network and Discriminant Analysis, two possible tools to incorporate the 4D seismic attributes in the quantification of saturation changes during the production of a reservoir, where analyzed. A special focus was applied to the evaluation of the impact of the quality and the amount of the calibration data to the final result, showing the
pseudo-well technology as a good alternative to improve the process.
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4D seismic monitoring of the Girassol field (Angola): enhanced interpretation of production mechanisms through impedance inversion
Authors Christian Hubans and Philippe Berthet4D-seismic is regarded with increasing interest as a strategic source of dynamic information, and as an added
value monitoring tool to optimise reservoir management decisions. The actual value of the 4D-seismic information relies on the possibility to bring it early enough in the project life to impact the development plan, and to
participate in the construction of a reliable Reservoir Model. Two seismic monitors have been acquired on Girassol-
Jasmim, as part of a seismic monitoring project. The 4D processing and interpretation workflow we have
developed has allowed us to start exploiting the essential of 4D information very early after the monitor acquisition.
This key dynamic information has been used to constrain the reservoir model history match. The integration of 4D
information has impacted fundamental decisions, like the drilling of a development well on a poorly drained area
identified on 4D data. Girassol-Jasmim 4D experience proves that time-lapse seismic brings added-value information all along the project life. 4D can effectively help optimising the development wells locations. The additional reserves associated to wells targeting undrained panels, the optimisation of development well locations and the increase in reliability of the Reservoir Model (basis for the reservoir management decisions) largely justify 4D expenses. The main goal of this work is to deconvolve, at least partially, pressure and saturation changes from 4D data. To advance in this direction, we have inverted seismic traces to extract reliable P and S time-lapse impedance changes through 4D-seismic multi-offset simultaneous inversion. Combined with the results of time shift
inversion process we have been able to split the P Impedance changes into Velocity and density parts. As Pressure does not impact the density in first approximation it is theoretically possible to approach pressure and saturation evolutions within the reservoir. A first step of well inversion tie have been performed and shows very consistent results..
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4D Seismic Interpretation in the Marlim Field, Campos Basin, Offshore Brazil
The growing application and acceptance of 4D or timelapse seismic technique as a key reservoir management
tool has been well documented. The technique has been successfully applied to numerous mature fields in order to
optimize reservoir production and recovery. This paper will describe such a case study on the Marlim Field where
the application of 4D seismic has contributed significantly to critical reservoir management decisions. The giant Marlim field is located on the eastern Brazilian coast in water depths varying from 600 to 1200m. The reservoir is characterized by an unconsolidated sandstone turbidite related to the regressive mega sequence of the Brazilian eastern continental margin. It is the largest producing oilfield in Brazil (average 450.000 bpd), with the original oil-in-place volume of 6.4 billion STB. Production started in 1991 and injection of water in 1994. Three seismic surveys cover the Marlim Field. The first one was Confidential Page 1 3/27/2007acquired in 1986, the others in 1997 and 2005. The most recent survey was acquired using WesternGeco´s Q- Marine acquisition system and was specifically acquired for reservoir monitoring and characterization purposes. Analyses of the 4D results started in June 2006 after parallel processing of the 1997 and 2005 datasets. This paper will highlight how the results of the 4D interpretation are being used to make critical reservoir management decisions with greater confidence and
reduced risk.
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Monitoramento Sísmico da Produção em Marlim Sul
Marlim Sul field, situated in Campos Basin, Brazil, applied its first time-lapse seismic acquisition in 2005. The main prime purpose of this 4D study is to understand the flow path of the injection water in the reservoirs. The seismic data were acquired by a streamer steering system and received special attention in terms of velocity analysis and crossequalization process. The 4D results have shown that the water paths are less regular than expected from flow simulation, which indicate the presence of barriers and flow baffles and trends in rock
permeability. Also, it was possible to identify swept and non-swept areas in the reservoirs, which allow studies of new well locations and the re-evaluation of present projects in the field.
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