Navegando por Assunto "Anisotropia"

Agora exibindo 1 - 20 de 21

Acesso aberto (Open Access)
3D raytracing through homogeneous anisotropic media with smooth interfaces
(2002-12) COSTA, Jessé Carvalho; SCHOENBERG, Michael; URBAN, Jaime Antonio
Two-point raytracing problem is solved for events in a piecewise homogeneous and laterally varying 3D anisotropic media by continuation techniques. In conjunction with the shooting method the algorithm can be used for computation of qP, qS1, and qS2 events. The algorithm has the same performance and robustness as previous implementations of the continuation method for tracing rays in isotropic models. Routines based on our algorithm have several useful applications. First, an efficient forward problem solver for traveltime inversion of elastic parameters in the presence of anisotropy. Second, Newton-Raphson iterations during two-point raytracing produce wavefront attributes, slowness and wavefront curvature. These attributes allows the computation of geometrical spreading and second order approximations for traveltimes. Therefore it can be used to investigate the effects of anisotropy on CRS, in simple velocity models.
Acesso aberto (Open Access)
Amplitudes e padrões de polarização de pulsos em meios anisotrópicos
(2011-06) PEREIRA, Rubenvaldo Monteiro; PROTÁZIO, João dos Santos; COSTA, Jessé Carvalho
Extracting subsurface lithological information from seismic data has become a great challenge to seismic exploration, as the hypotheses of stratification made by isotropic layers has been insufficient to represent the behavior of elastic field in surveys with great offset between source and receiver, multicomponent geophones, three-dimensional Vertical Seismic Profile (VSP) measures and so forth. Due to this reason, a more realistic model of subsurface is demanded. As a result, the seismic prospecting starts to consider the anisotropic models of subsurface to characterize reservoirs, for example. This paper aims at showing formalism to model the scattering of pulses from incident plane waves in horizontal plain interfaces which separate anisotropic media. This scattering is obtained primarily through the explicit formulation of traction and deformation fields as a function of propagator, polarization and impedance matrices of media. After that, this formalism is used to obtain the reflection and transmission coefficients matrices through a horizontal plain interface to be, subsequently generalized to a scattering through multiple layers. Finally, the amplitude of an analytical pulse is inserted in the incident wave to calculate the scattering of the pulse through stratifications.
Acesso aberto (Open Access)
Análise de sensibilidade para estereotomografia em meios elípticos e anelípticos
(Universidade Federal do Pará, 2005-12) BARBOSA, Brenda Silvana de Souza; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Stereotomography is extended to general anisotropic models and implemented for elliptical and anelliptical anisotropy. The elliptical and anelliptical models present only three parameters. This makes them less sensitive to the ambiguity due to limited coverage of surface seismic experiments than transversaly isotropic or orthorhombic models. The corresponding approximations of the slowness surface restrict the validity of the present approach to qP events and mild anisotropy. Numerical experiments show the potential and the limitations of stereotomography in estimating macro-velocity models suitable for imaging in the presence of anisotropy as well as the importance of transmission events from multiple-offset VSP experiments for the success of the approach.
Acesso aberto (Open Access)
Análise de um esquema de inversão para estimativa de anisotropia local a partir de dados de VSP
(Universidade Federal do Pará, 2012-05-18) BARRETO, Adriano César Rodrigues; GOMES, Ellen de Nazaré Souza; http://lattes.cnpq.br/1134403454849660
Inversion scheme for the determination of arbitrary in a vicinity of a receiver situated inside a borehole from the data obtained during a multi-offset multi-azimuthal VSP experiment is studied. The data consist of vertical components of the slowness vector and all components of the polarization vector of P waves direct and reected. The inversion is made from a first order approximation around an reference arbitrary isotropic medium, that does the relation between the data of slowness and polarization of P waves parameters and the weakly anisotropic medium (WA parameters). This inversion scheme applies to arbitrary anisotropy medium, provided that the anisotropy is weak and independent of downhole type (vertical, horizontal or inclined). The results show that reversing is sensitive to the number and orientation of the proles of sources that are distributed on the surface. An analysis of the design of an experiment and discussion about the choice of parameters of the reference isotropic medium are presented. Within the realistic assumptions about the level of noise is possible to estimate the phase velocity for a limited angular aperture around the well.
Acesso aberto (Open Access)
Análise de um esquema linear para estimativa de anisotropia local a partir de dados de onda P em experimentos de VSP multiazimutal
(Universidade Federal do Pará, 2013-08-08) MACAMBIRA, Raiza de Nazaré Assunção; GOMES, Ellen de Nazaré Souza; http://lattes.cnpq.br/1134403454849660
This study presents an analysis of the linear inversion scheme for estimating anisotropy in the neighborhood of a receiver placed in a well using the vertical components of the slowness and polarization vectors of P-waves measured in multi-azimuth walkaway vertical seismic profile (VSP) surveys. Independently of the medium above the geophone (homogeneous or heterogeneous) and the shape of the well (directional or curved, vertical or sloped), an inversion is performed from a first-order approximation around a reference isotropic medium. The analysis of the inversion scheme considers factors such as the noise level of the data, the type of P-wave, the degree of the anisotropy of the medium, the choice of parameters in the reference isotropic medium and the degree of heterogeneity of the medium.
Acesso aberto (Open Access)
Caracterização de reservatórios fraturados através de dados de ondas qP em levantamentos VSP Walkaway
(Universidade Federal do Pará, 2008) SILVA, Saulo da Costa e; GOMES, Ellen de Nazaré Souza; http://lattes.cnpq.br/1134403454849660
This dissertation presents a method to estimate the orientation of the axis of symmetry of a medium assuming it behaves effectively as a transversely isotropic medium (TI). The fracture orientation is then achieved from the estimation of the axis of symmetry of a TI medium. This estimative is done with the slowness and polarization vectors from qP waves, measured in VSP Walkaway experiments. The inversion process is based on linearized equations of the slowness and polarization vectors from qP waves and the weakly anisotropic parameters of the medium. Numerical tests are presented, on which the sensibility to factors like the strength of anisotropy, survey geometry, type of wave utilized and noise level are analyzed. Test results for a set of real data are also shown.
Acesso aberto (Open Access)
Espalhamento elástico em meios anisotrópicos estratificados
(Universidade Federal do Pará, 1994-06-09) PROTÁZIO, João dos Santos; SCHOENBERG, Michael
AVO analysis is an importante tool for extracting lithological information from seismic data using the contrast in acoustic impedance at the lithological boundaries. The isotropic assumption behind this analysis does not hold in many cases. The advent of large offset surveys and multi-component data has revealed the presence of subsurface anisotropy. To interpret such data, the AVO analysis must include anisotropy. This work presents an AVO theory and numerical results for an anisotropic layered medium. This thesis contains three contributions. First, a new approach to the study of reflection-transmission at a plane interface between anisotropic media with a horizontal mirror plane is presented. The Zoeppritz equations are generalized to include anisotropy by the introduction of impedance matrices which greatly simplifies the previous formalisms. Second, the study of the P-wave reflection at an interface between an isotropic and a transversally isotropic medium is described and it is show that the reflected P-wave does not have information about the underlying anisotropic subspace for pre-critical incidence. Finally, the behavior of post-critical reflected and transmitted pulses through a stack of anisotropic layers is discussed. The post-critical pulses are show to carry valuable information on the anisotropy of the structure through which the waves propagate.
Acesso aberto (Open Access)
Estimation of fractures orientation from qP reflectivity using multiazimuthal AVO analysis
(2002-12) GOMES, Ellen de Nazaré Souza; COSTA, Jessé Carvalho; PROTÁZIO, João dos Santos; SIMÕES FILHO, Ivan de Araújo
We investigate the estimation of fractures orientation, strike and dip, through multiazimuthal AVO analysis of qP and its converted waves qS1 e qS2. We assume weak impedance contrast, weak anisotropy and that the fractured medium behaves as an effective transversally isotropic (TI) medium. Under these assumptions, the estimation of fractures orientation is reduced to the estimation of the orientation of an axis of symmetry from qP reflectivity data. Linearized approximations of qP reflectivity are used for inversion.
Acesso aberto (Open Access)
Estimativa de parâmetros elásticos em meios anisotrópicos
(Universidade Federal do Pará, 2003-06-20) GOMES, Ellen de Nazaré Souza; PROTÁZIO, João dos Santos; http://lattes.cnpq.br/4210442535067685
Amplitude, polarization and the slowness vector measurements carry information about the medium where wave propagation occurs. This thesis investigates these data aiming at the recovery of elastic properties in anisotropic media. Reflection coefficients can be estimated from amplitude data and depend nonlinearly on elastic and density contrasts across an interface. When the impedance contrast is weak, the linear approximations for the qP reflectivity are more convenient for inversion of density and elastic parameters using analysis of amplitude versus the angle of incidence (AVO) and amplitude versus the direction of the incidence plane (AVD). Partitioning the linear system defined by Zoepprittz equations allows one to write the solution of these equations in terms of impedance and polarization matrices. Using this solution, linear approximations for the qP reflectivity are derived for weak impedance contrasts and arbitrary symmetry classes of anisotropy. The linear approximations are evaluated for different acquisition geometries and choice of the reference medium. The approximations for the reflection coefficients of the reflected qP and the converted waves are in good agreement with the exact solution for incidence angles up to 30° for media that satisfy the weak impedance assumption. If a single oriented set of fractures is represented by a transversely isotropic effective medium, the linear approximations for qP reflectivity can be used to estimate the fractures orientation. Under these assumptions this problem is reframed as the estimation of the symmetry axis orientation from qP reflectivity data. This work shows the requirement of multiple components and multiple azimuthal data and quantifies the minimum amount of data for stable estimation. Also it is shown that the reflection coefficients of converted waves qS and qT only are sensitive to fractures dip. The inversion of polarization and slowness from multiazimutal VSP data are investigated for the estimation of local anisotropy. We use measurements of the vertical component of the slowness vector and the qP polarization data of direct and reflected waves. The inversion algorithm is validated in synthetic data sets for different choices of the wave front normal, reference medium and acquisition geometries. This analysis shows that only a subset of elastic parameters is recovered. An important application of this approach is its potential to determine the class of anisotropy. The application of this methodology to the Java Sea data set shows that isotropy and transversely isotropic models are inadequate to fit the data.
Acesso aberto (Open Access)
Estimativa de parâmetros em meios VTI usando aproximações de sobretempo não hiperbólicas
(Universidade Federal do Pará, 2015-09-30) PEREIRA, Rubenvaldo Monteiro; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023
Transversely isotropic (TI) media is a more realistic model for processing seismic data, for example, fractured media with preferred fracture direction, or composite by periodic thin layers. In particular, TI media with vertical symmetry axis (VTI) are widely used as models for P-wave propagation in shales, abundant rock in hydrocarbon reservoirs. However, the P-wave propagation in homogeneous media VTI have as their main characteristics, depend on four parameters of rigidity and also to possess: complicated algebraically phase velocity equation, difficult group velocity equation to explain and moveout equation nonhyperbolic. Therefore, several authors have presented parameterization and obtained approximations to these equations depending on three parameters only. Among these, the moveout approximations have been widely used in inverse methods to estimate lithological parameters in homogeneous media VTI. Such methods have generally been successful in estimated stacking velocity vn and the anellipticity parameter η, since these are the only ones required for generating initial models for the steps of seismic processing in the time domain. One of the most used methods for estimating parameters is the basedsemblance velocity analysis, though, because this method is limited to sections with small offset-depth ratio, adaptations for anisotropic media, considering nonhyperbolic moveout approximatios are required. In this paper, based on anelliptical approximation shifted hyperbola, anelliptical rational approximations are presented for: phase velocity, group velocity and moveout nonhyperbolic in homogeneous VTI horizontally layered media. The validity of these approximations is made by calculating their relative errors by comparing with other known approximations in the literature. To semblance-based velocity analysis is performed to measure the accuracy of the rational moveout approximations to estimate parameters in VTI media. The results demonstrate the great potential of rational approximations in inverse problems. In order to adapt to VTI media, we modify two coherence measurements by semblance which are sensitive to amplitude and phase variations. The accuracy and robustness of the adapted coherence measurements are validated by estimation of in anisotropic parameters in VTI media.
Acesso aberto (Open Access)
Experimental verification of Hudson and Eshelby-Chen’s effective crack theory
(Universidade Federal do Pará, 2015-12-04) HENRIQUES, Jéssica Pena; FIGUEIREDO, José Jadsom Sampaio de; http://lattes.cnpq.br/1610827269025210
Physical modeling of cracked / fractured media in downscaled laboratory experiments has served as a great alternative for understanding the anisotropic media behavior. In this work, it was performed ultrasonic measurements on samples with low crack densities and different aspect ratios. The main goal was to investigate the experimental behavior of elastic parameters, such as: waves velocities and Thomsen parameters ε and 𝛾 and elastic stiffness coefficients for transverse isotropic media. Comparison of the results with the predictions made by the effective models of Hudson (1981) and Eshelby-Cheng (1993), it was also investigated in this work. Twelve samples were prepared with two types of cracks density, 5 and 8%. The cracks that have three different aspect ratios (0.133, 0.1778 and 0.2667) were formed by rubber inclusions in a homogeneous isotropic matrix resin. Moreover, an isotropic matrix sample was constructed by only epoxy resin. Among all samples, six (three for each density) have only one aspect ratio type (samples with single crack), while another six (three for each density) have three types of different aspect ratio (mixed samples). Among the predictions of the models, the Eshelby-Cheng (1993) shows a better fit in relation to the experimental results for samples with single crack (for the two densities of inclusions). However, none of the models predicts accurately with minimal tendency for the mixed samples.
Acesso aberto (Open Access)
Inversão vinculada de dados de EMAP e MT-1D anisotrópico
(Universidade Federal do Pará, 1999-11-26) RÉGIS, Cícero Roberto Teixeira; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
We present a technique to invert magnetotelluric data. Two kinds of MT data are treated: data generated by anisotropic one-dimensional models and two-dimensional data from EMAP (ElectroMagnetic Array Profiling) surveys. In both cases we perform the inversions using approximate equality constraints to stabilize the solutions. We show the advantages and the limitations of those constraints in the inversion process. Even when the inversion is constrained, it can still be unstable. To invert the two-dimensional EMAP data we present a process that consists of three steps: 1 – The interpretation model and the initial guess for each case are derived from the filtered apparent resistivity sections which are the result of the filtering process of the EMAP method; 2 – One layer formed by small outcropping bodies (termed static shift layer) is added to the interpretation model in order to resolve the sources of static distortions that contaminate the data; 3 – The inversion is constrained by approximate equality constraints. The first two steps enable us to extract the largest possible amount of information from the data, while the use of the approximate equality constraints allow us to incorporate a priori information which has true physical or geological meaning. With these steps we obtain solutions that are stable and meaningful. We study the method in two-dimensional models as well as in real data from an EMAP line surveyed at the Paraná Basin.
Acesso aberto (Open Access)
Linearização dos coeficientes de reflexão de ondas qP em meios anisotrópicos
(2001-04) GOMES, Ellen de Nazaré Souza; PROTÁZIO, João dos Santos; COSTA, Jessé Carvalho; SIMOES FILHO, Ivan A.
The reflection coefficients at a planar interface separating two anisotropic media have a nonlinear dependence on the elastic parameters and densities of both media. Linear approximations on the elastic parameters for the qP wave reflectivity are more convenient for AVO/AVD analysis. We present the solution of the Zoeppritz equations in terms of impedance and polarization matrices. Using this approach and assuming weak impedance contrast and weak anisotropy, a simple derivation of linearized approximations for qP the reflectivity is presented for general anisotropy. The linear approximations of reflection coefficients, qP and converted waves, for qP incidence are very close to the exact results for incidence angles up to 30 degrees considering moderate impedance contrast and anisotropy.
Acesso aberto (Open Access)
Migração FFD 3D em profundidade usando aproximação de Padé complexa
(Universidade Federal do Pará, 2011) CAMPOS, Itamara do Socorro da Silveira; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Fourier finite-difference (FFD) migration implementations use splitting techniques to accelerate performace and save computational cost. However, such techniques introduce numerical anisotropy which leads to mispositioning of dipping reflectors along directions not used for splitting the migration operator. We implement 3D FFD continuation migration without splitting in the frequency-space domain using the complex Padé approximation and implicit finite differences. This approach eliminates numerical anisotropy at the expense of a computationally more intensive solution of a large banded linear system. Numerical experiments in homogeneous and heterogeneous models show that splitting techniques produce noticiable positioning erros for models with strong lateral velocity variation. We compare the performance of the iterative stabilized biconjugate gradient (BICGSTAB) and the multifrontal massively parallel direct solver (MUMPS). It turns out that the use of the complex Padé approximation provides an effective preconditioner for the BICGSTAB, reducing the number of iterations relative to the real Padé expansion. The iterative BICGSTAB method is more efficient than the direct MUMPS method when solving for a single term in the Padé expansion. For wide angle approximations more terms are required to represent the migration operator, in this case direct methods are required. The algorithm is validated and the properties evaluated computing the migration impulse response in the SEG/EAGE salt model.
Acesso aberto (Open Access)
Migração por equação de onda em meios anisotrópicos com correção de amplitude
(Universidade Federal do Pará, 2010) AMAZONAS, Daniela Rêgo; SCHLEICHER, Jörg; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Standard real-valued finite-difference (FD) and Fourier finite-difference (FFD) migrations cannot handle evanescent waves correctly, what can lead to numerical instabilities in the presence of strong velocity variations. A possible solution to these problems is the complex Padé approximation, that avoids problems with evanescent waves by a rotation of the branch cut of the complex square root, and we apply it to the acoustic wave equation for vertical transversely isotropic (VTI) media to derive more stable FD and hybrid FD/FFD migrations. Our analysis of the dispersion relation of the new method indicates that they can provide stable migration results with less artifacts, and higher accuracy at steep dips. These conclusions are confirmed by the numerical impulse responses of the migration operator, and by the migration of synthetic data in strongly heterogeneous VTI media. Wave-equation migration in heterogeneous media, using standard one-way wave equations, can only describe correctly the kinematic of the propagation. For a correct description of amplitudes, we must use the so called true-amplitude one-way wave equations. In vertically inhomogeneous media, the resulting true-amplitude one-way wave equations can be solved analytically. In laterally inhomogeneous media, these equations are much harder to solve, and even numerical solutions tend to suffer from instabilities and other artifacts. We present an approach to circumvent these problems by implementing approximate solutions based on the one-dimensional analytic amplitude modifications. We use these approximations to modify split-step and Fourier finite-difference migrations in such a way that they take better care of migration amplitudes. Simple synthetic data examples demonstrate the recovery of true migration amplitudes. Applications to the SEG/EAGE Salt model, and to the Marmousi data, show that the method improves amplitude recovery in the migrated images. We also show that the method for amplitude correction can be applied to migration algorithm for VTI media, and the algorithm was applied to the HESS synthetic data.
Acesso aberto (Open Access)
Modelagem 1D e 2,5D de dados do método CSEM marinho em meios com anisotropia transversal inclinada
(Universidade Federal do Pará, 2014-02-28) SANTOS, Walleson Gomes dos; RÉGIS, Cícero Roberto Teixeira; http://lattes.cnpq.br/7340569532034401
In this work I present the solution to the electromagnetic field generated by a horizontal electric dipole in transversally isotropic media with vertical (TIV) as well as inclined (TII) symmetry axis. In one-dimensional models the electromagnetic field was obtained with two distinct methods: (1) For the TIV case, I have written a semi-analytical solution to the Maxwell’s equations, by using a vector potential formulation; (2) For the TII case, the field was represented as the composition of primary and secondary fields, where primary fields are those found in an underlying isotropic layered medium, and the secondary field is calculated numerically via the finite element method in the spatial Fourier transform domain (kx, ky, z). This last methodology was also used to calculate the fields in two-dimensional structures, including inclined anisotropy in any region of the models. In this 2,5D case, I have applied the finite element method in the (x, ky, z) do main. Here I have used unstructured meshes to discretise the media, and parallel programming to solve the linear systems of equations. The responses were used to study the effects of electrical anisotropy in marine CSEM data.
Acesso aberto (Open Access)
Modelagem de dados MT 2D em multicamadas com anisotropia arbitrária
(Universidade Federal do Pará, 2011-12-19) COSTA, Marcus Danilo Ferreira Borges da; RÉGIS, Cícero Roberto Teixeira; http://lattes.cnpq.br/7340569532034401
We present in this work the bidimensional Magnetotelluric method modelling with arbitrary anisotropy in the electrical conductivity for horizontally stratified multilayers models, separating the total field into primary and secondary fields. The primary field is the 1D MT field in the absence of heterogeneity. The secondary field is the field generated by the heterogeneity, in other words, it is the difference between the total field and the primary field. The numerical technique of Finite Elements (FE) is used for this 2D Modelling. In addition we include the MT 1D modelling case with arbitrary 3D conductivity anistropy using the propagation matrix method. This result is used as the source in the bidimensional formulation. When we studied the horizontal fields in layered media; there seems to exist sources in each anisotropic layer, because there is an increase of amplitude in these layers, also we verify the appearence of the normal component Ez of the field, without a current density in this direction. The arbitrary anisotropy allows the principals anisotropic axes to have any orientation in relation to cartesian referencial system choosen for the problem. This orientation is defined by three consecutive rotations, the first being strike, the second dip and the third slant. These rotations simulate common geological environments in the earth. This type of anisotropy changes the pseudo-sections of apparent resistivity, making data interpretation more challenging.
Acesso aberto (Open Access)
Modelagem e imageamento 2.5D no domínio do tempo através de diferenças finitas
(Universidade Federal do Pará, 2010) SILVA NETO, Francisco de Assis; SCHLEICHER, Maria Amélia Novais; http://lattes.cnpq.br/4767998352165705; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
This thesis discuss modeling and imaging of seismic wavefields in 2.5D using finite-differences to solve numerically the wave equation. Modeling in 2.5D is extended to anisotropic elastic media with an arbitrary class of symmetry. The sources of the wavefield are generalized to simulate of explosive, dipole and double-couple distributions. The acquisition geometry is not required to coincide with a symmetry plane. Reverse time migration in 2.5D is implemented in conjunction with a new imaging condition based on the asymptotic analysis of the classical correlation imaging condition. The new imaging condition is designed to improve the amplitudes in reverse time migration (RTM) images, and to reduce back-scattering artifacts. Numerical experiments indicate that 2.5D RTM improves the resolution of the migrated images when compared to its 2D counterpart, and that the proposed imaging condition was effective improving the amplitudes and reducing back-scattering artifacts.
Acesso aberto (Open Access)
Modelagem sísmica e inversão na presença de anisotropia
(Universidade Federal do Pará, 1993-11-04) COSTA, Jessé Carvalho; SCHOENBERG, Michael
The assumption of anisotropic elastic models, in the context of exploration seismology, has been increasing since the advent of new aquisition tecniques: VSP, walkway VSP, crosshole tomography and large offset seismic surveys. Effective anisotropic elastic models can account for patterns of inhomogeneities at a scale much lower than the wavelength of the propagating seismic energy. Particularly, effective media can account for the most robust seismic data, i.e., traveltime measurements. This work investigates some aspects of wave propagation, raytracing and traveltime inversion in anisotropic media. The propagation of SH waves in a layered anisotropic medium in the most general case where these waves can occur, i.e., propagation in the mirror symmetry plane of a monoclinic medium, is studied. It is shown that SH reflected field from a stratified half-space is 'blind' to the possible anisotropy below. Fast ray tracing procedures for stratified anisotropic media in 3D are presented. Such ray tracing codes are a first step towards the development of traveltime inversion algorithms for non-flat anisotropic layers in 3D. Traveltime inversion of VSP and walkway VSP is a promissing approach to 3D velocity model building, starting from a well location. These velocity models are necessary for the migration of seismic data in the presence of anisotropy. The analysis of the nonlinear tomographic inversion, for a vertically inhomogeneous transversally isotropic medium with vertical axis of symmetry (TIV), is presented. The limitations of qP traveltime data are pointed as well as the consequences of the lack of full angular ray coverage for tomographic inversion. An algorithm for tomographic inversion is presented and evaluated in synthetic data seis. Application to real data is presented. This approach is attractive for cases where the formations are known a priori to be relatively flat, and where crosswell data itself shows a high degree of left-right symmetry. It also may be of use for preliminary surveys, where the layered estimate can be used as a background model to carry out more detailed analysis, e.g., as an anisotropic velocity model for migration, or as a calibration model for AVO analysis.
Acesso aberto (Open Access)
Modelos de evolução e colocação dos grantitos paleoproterozóicos da Suíte Jamon, SE do Cráton Amazônico
(Universidade Federal do Pará, 2006-10-27) OLIVEIRA, Davis Carvalho de; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
The 1.88 Ga, anorogenic, A-type Jamon suite and associated dikes intruded 2.97 – 2.86 Ga-old Archean granitoids of the Rio Maria Granite-Greenstone Terrane which lies to the south of Serra dos Carajás, in the southeastern domain of the Amazon Craton, northern Brazil. Petrographic and geochemical aspects associated with magnetic susceptibility and gamma-ray spectrometry data showed that the Redenção and the northern part of Bannach plutons are normally zoned, with mingling relationships that indicate multiple magma injections in their construction. Both were formed by two magmatic pulses: (1) a first magma pulse which fractionated in situ after shallow crustal emplacement and generated a series of coarse, evengrained monzogranites with variable modal proportions of biotite and hornblende; (2) a second, slightly younger magma pulse, localised in the center of both plutons, and composed of a more evolved liquid from which even-grained leucogranites were derived. Seriated and porphyritic biotite monzogranite facies intruded the coarse (hornblende)-biotite monzogranites and formed anellar structures within the Redenção pluton. The magmatic zoning is marked by a systematic decrease in mafic mineral modal content, plagioclase/potassium feldspar and amphibole/biotite ratios, and anorthite content of plagioclase. TiO2, MgO, FeOt, CaO, P2O5, Ba, Sr, and Zr decreased, and SiO2, K2O, and Rb increased in the same fashion. Magmatic differentiation was controlled by fractionation of early crystallized phases, including amphibole±clinopyroxene, andesine to calcic oligoclase, ilmenite, magnetite, apatite, and zircon. The Jamon suite is subalkaline, metaluminous to mildly peraluminous, ferroan alkali-calcic, and displays geochemical affinities with within-plate A-type granites. The ubiquitous occurrence of magnetite and titanite as well as high magnetic susceptibility values demonstrate that granites of the Jamon suite are oxidized in character. Oxidized A-type granites have high FeOt/(FeOt+MgO), TiO2/MgO, and K2O/Na2O ratios and low CaO and Al2O3 compared to calc-alkaline granites. The oxidized character of the Jamon suite makes it more akin to the USA Mesoproterozoic magnetiteseries A-Ttype granites but differs from the reduced rapakivi granites of the Fennoscandian Shield, and Serra dos Carajás and Velho Guilherme suites of the Carajás province, probably because of different magmatic sources. The Jamon suite probably crystallized near or slightly above the nickel-nickel oxide (NNO) buffer and an Archean sanukitoid biotite-hornblende quartz diorite source was proposed for the oxidized Jamon magmas. Gravity modelling indicates that the Redenção and Bannach plutons are sheeted-like composite laccolithic intrusions, ~6 km and ~2 km thick, respectively. These plutons follow the general power law for laccolith dimensions and are similar in this respect to classical rapakivi granite plutons. Gravity data suggest that the growth of the northern part of the Bannach pluton was the result of the amalgamation of smaller sheeted-like plutons which successively intruded in sequence from northwest to southeast. Jamon suite plutons were emplaced in an extensional tectonic setting with the principal stress oriented approximately along NNE-SSW to ENE-WSW, as indicated by the occurrence of diabase and granite porphyry dike swarms, orientated WNWESE to NNW-SSE and coeval with the Jamon suite. The 1.88 Ga A-type granite plutons and stocks of Carajás are disposed along a belt defined by the general trend of the dike swarms. The inferred tabular geometry of the studied plutons can be explained by magma transport via dikes and it is supported the high contrast of viscosity between the granites and their Archean country rocks. Mechanisms responsible for emplacement of granitic plutons, and in particular of anorogenic A-type plutons, are still debated. A magnetic fabric study derived from anisotropy of magnetic susceptibility (AMS) measurements was applied to the Redenção pluton in order to understand its emplacement history. High magnetic susceptibilities (K from 1 x 10-3 SI to 54 x 10- 3 SI) indicated that magnetic fabrics are primarily carried by ferromagnetic minerals (magnetite). Low P' values and absence of intracrystalline deformation features indicated that the magnetic fabric is of magmatic origin. The magnetic fabric is well organized and characterized by concentric steep foliations associated with moderately to gently plunging lineations. The lack of a well-defined unidirectional linear fabric at pluton scale suggests the reduced or null influence of regional stresses during granite emplacement. Three stages are proposed for construction of the Redenção pluton, which reconcile the tabular shape of the intrusion with the occurrence of steep magnetic foliations: (1) ascent of magmas in vertical, northwest-striking feeder dikes and accommodation by translation along east-west-striking regional foliation planes; (2) switch from upward flow to lateral spread of magma with space for injection of successive magma pulses created by floor subsidence; and (3) in situ inflation of the magma chamber in response to the central intrusion of late facies, accompanied by evacuation of resident magmas through ring fractures.