Programa de Pós-Graduação em Geofísica - CPGF/IG
URI Permanente desta comunidadehttps://repositorio.ufpa.br/handle/2011/2355
O Programa de Pós-Graduação em Geofísica da UFPA (CPGF) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA). Foi o segundo no Brasil a formar recursos humanos em Geofísica em nível de pós-graduação stricto sensu. Criado em 1972, funcionou até 1992 junto com os Cursos de Pós-Graduação em Geoquímica e Geologia.
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Item Acesso aberto (Open Access) Aproximações não-hiperbólicas do tempo de trânsito utilizando aproximantes de Padé(Universidade Federal do Pará, 2017-07-03) NEVES, Rodolfo André Cardoso; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023Traveltime approximation is a fundamental tool of the stack and migration steps in seismic data processing. To increase the accuracy of these approximations, we propose new traveltime approximations based on Padé approximants, to CMP and CRS gathers. Hyperbolic approximations such as normal moveout (NMO) and comom reflection surface (CRS) are taylor series approximations of second order of the reflection traveltime. Padé approximants appear as an alternative to Taylor series, because they converge quickly to the desired function, and they have a major radius of convergence improving approximations acuracy. They can be obtained through the proper Taylor serie of the approximated function. This new approximation is obtained from the [2/2] Padé approximation of the generalized moveout equation; and from [2/2] Padé approximation of the Taylor series expansions of fourth order of the CRS surface. The acuracy of Padé approximation is superior when compared with other convencional approximations: normal moveout, shifted hyperbola and Transversal isotropic medium with vertical symetry axis (VTI). CMP gather Padé approximations depend just only one more parameter than normal moveout approximation and they keep the acuracy for long offsets. CRS gather non hyperbolic approximations, non hyperbolic CRS, fourth order CRS and Padé CRS, have major acuracy than hyperbolic CRS, increasing the convergence of the approximation for offset and CMP domain. The quadratic approximation of fourth order CRS is superior than non hyperbolic CRS approximation, producing less error in least square CRS parameter inversion.Item Acesso aberto (Open Access) Conversão tempo-profundidade de seções sísmicas empilhadas por raio imagem e raio normal(Universidade Federal do Pará, 2014-06-24) REZENDE, Diogo Pena; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023In general, the oil industry makes the time-to-depth conversion of seismic data by the image ray tracing method. This method takes time into the depth, point to point, the amplitudes of the time migrated seismic section. For each point of the migrated time section, it is necessary to trace a ray perpendicular to the surface. After this, the amplitude of the migrated point of the section takes place in depth. The seismic migration method pre- or post-stack consists of placing seismic events in the correct positions in time or depth sections. Seismic depth sections provide an image near of the subsurface, in order to facilitate the identification of possible oil accumulating geological structures. The conversion of sections from the time to the depth domain is an intermediate step in the construction of seismic images in depth. This work developed and tested a method of converting time to depth the zero-offset seismic sections. In this case, the construction of sections in depth uses normal ray tracing method. The proposed method makes use of the (slowness versus time of intersection) transformation on the zero-offset section. Each point in the domain provides initial conditions for the normal ray tracing: a start position of the initial rays and initial angles formed with the normal to the surface, i.e., the slowness initial parameter. Unlike ray image method, several rays use the same travel time and the same initial position, defining an isochronous curve. The amplitude of each point in the zero-offset section takes place to depth from the distribution of values along each isochronous curve in depth. The image ray based time-to-depth conversion has good recovery of the depths of reflectors as well as low computational cost, since it is necessary only one ray to convert each point of the section in time. However, the reflector continuity may be damaged in case of sharpened curvature. In turn, by normal ray the time-to-depth conversion correctly approximate the depth of the reflectors, since the same point in time assigns several times in depth. However, it has a higher computational cost, because it is necessary many rays to convert one point in time.Item 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/8498743497664023Transversely 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.Item Acesso aberto (Open Access) Interpolação de eventos de reflexão em traços sísmicos de dados pré-empilhamento usando aproximação de tempo de trânsito SRC-AF(Universidade Federal do Pará, 2014-06-24) FERNANDES, Alexandre Sodré; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023The seismic method is a well known technique for revealing details of the geological subsurface structures because the seismic data are processed and produce seismic sections, which are images of subsurface structures. The quality of the recorded seismic data depends on factors such as the complexity of the subsurface, level of noise, surface topography of acquisition, heterogeneities in the mantle of weathering, among others. Irregular acquisition, short offsets, low coverage in common points in depth lead to a low quality imaging in subsurface and low resolution in seismic sections. In common reflection surface imaging, the hyperbolic approximation of traveltimes for paraxial rays in the vicinity of a central ray reflected with finite source-receiver offset, function of five kinematic attributes of the wave field, represent better the time field reflection compared to conventional methods. The Common Reflection Surface for Common Offset (CRS-CO) approaches reflection events within the vicinity of the trace with a fixed offset that we want to interpolate, adding correlated events, where the output is defined as the weighted average of the amplitudes along the traveltime approximation SRC-AF, assigning the result to its trace to all points of an common offset section. The specific objective of this work is to model the regularization of temporal sections filling areas lacking seismic data, increasing the signal to noise ratio by interpolation of reflection events in seismic traces based on the approximation of traveltime for Common Reflection Surface for Common Offset (CRS-CO). The algorithm was applied to the seismic acquisition settings Common Source, Common Offset and Common Mid Point for a set of 2D synthetic data, modeled by ray tracing.Item Acesso aberto (Open Access) Migração Kirchhoff paraxial pré-empilhamento em profundidade com amplitudes verdadeiras(Universidade Federal do Pará, 2015-08-31) MOREIRA, Marcio Fernando de Andrade; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023The seismic depth migration is one of the most important steps in the exploration of geologically complex process areas of the oil industry interest. To obtain a more realistic image of the structure, it needs is a great settings recovery process of the reflectors in the subsurface and efficient determination of reflection coefficients to characterize the oil and gas reservoirs. One of the techniques used to obtain the seismic image is the Kirchhoff migration method which can be greatly enhanced by appropriate weights, which when applied to amplitudes during the stacking diffraction provides an estimate of the reflection coefficients. In this work, we used the 2-D paraxial ray prestack Kirchhoff depth migration method. In the calculation step traveltimes makes use of second-order paraxial approximation, by which one can get a good approximation of the traveltimes in the mesh migration. The weights used during migration were calculated using the equations of dynamic radius and the extrapolation to the points of the mesh migration. Compared with the conventional Kirchhoff method, the results were quite satisfactory as regards the increase in image resolution of the reflectors as well as in obtaining the reflection coefficients. The efficiency of the proposed technique was tested on synthetic data type anticlinal geological structure and seismic data Marmousi.Item Acesso aberto (Open Access) Migração Kirchhoff pré-empilhamento em profundidade usando aproximação paraxial do tempo de trânsito(Universidade Federal do Pará, 2014-08-28) CUNHA, Antonio Rizimar de Andrade; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023In order to get an accurate image of the subsurface we need efficient techniques for evaluating the wave field. In literature we find several geophysical methods based on the numerical solution of the seismic wave equation. Among the various techniques of seismic imaging, Kirchhoff prestack depth migration remains widely used because of its flexibility in processing data through several geometries of acquisition, and its practicality in solving problems related to imaging. Assuming this view, we seek to create more efficient alternatives for the accurate calculation of the parameters involved in migration processes. We consider the efficient calculation of the traveltimes as a critical factor, focusing on the fundamental amplitude in their respective points in depth using the Paraxial Ray Theory through the Runge-Kutta method of fourth order. The paraxial extrapolation of the traveltimes belongs to the class of dynamic ray tracing, where it is possible to determine information in complex geological environment on regions influenced by shadow zones. The application of this technical demands amounts previously arranged in a central reference beam that is obtained from the kinematic ray tracing, furthermore a macro velocity model of the medium is necessary for the traveltimes calculation. For comparison, we consider two different ways to calculate the traveltimes tables: The first was performed by routine RAYT2D from SEISMIC UNIX (SU) package, which is considered a robust method of evaluation; the second is based on Paraxial05 method. The depth migration required the seismic data input and the traveltimes. The images were obtained from an algorithm written in SHELL and a third image is obtained through the difference between the two initial results.Item Acesso aberto (Open Access) Modelagem sísmica acústica e elástica por diferenças finitas e imageamento do depósito de minério de ferro N4WS no estado do Pará(Universidade Federal do Pará, 2017-07-03) HOLANDA, Rafael Mansano; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023The finite difference method consists in the numerical resolution of differential equations and one of its applications is a solution of the wave equations or elastodynamics, by replacing the partial derivatives in space and time by their approximations of finite differences. The present work consisted in the accomplishment of the seismic modeling using the method of the finite differences, to acoustic and elastic case. Then, we performed the seismic imaging, to acoustic case, in a model that simulates an iron ore deposit area of N4WS, located in the mineral province of Carajás, in the state of Pará. We found a heterogeneous, isotropic and bidimensional velocity model to acoustic and elastic cases. In order to generate the synthetic seismogram, we executed the program FDSKALAR, to acoustic case, and SOFI2D, to elastic case, using fourth-order approximation of the equation of acoustic and elastic wave in space and time. For a validation of the programs of finite differences and aid in the interpretation of the events, we compared the results with the travel times obtained by the ray theory. After the seismic modeling, we accomplished the processing of the data generate, using the open-source software package Seismic Unix, to obtain an image of the reflector contained in the mineral model. In this way, we searched a better understanding and control on the problems of seismic modeling and imaging, contributing to an interpretation of seismic data and understanding about a seismic wave propagation in environments where there are significant heterogeneities. In addition, we sought to demonstrate the application of seismic methods in the study and delimitation of mineral bodies.Item Acesso aberto (Open Access) Processamento e imageamento sísmico usando o CRS(Universidade Federal do Pará, 2014-02-04) PENA, Felipe Astur Valdes; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617This work aimed at the application of the common-reflection-surface stack methods (CRSconventional), of the CRS-partial method, and of the NIP-tomography inversion method, to generate seismic data images for the interpretation related to geologically complex areas. The constructed model, and named Duveneck-Astur, was used to simulate a geological ambient formed by layers limited by smooth surface reflector interfaces, but that the paraxial ray theory was attended, differently from other synthetic common models where the presence of geological faults, and of high horizontal and vertical gradients exist, like in the Marmousi and the Sigsbee models, among others. To analyze comparatively the resolution of the applied methods, two tests were performed with the synthetic data. One test consisted of decimated data with random muting of traces in the CMP families, and another test with addition of noise. It was computer analyzed the behavior of the different stack methods to obtain a depth velocity distribution by NIP-tomography inversion, that uses the kinematic wavefield attribute constraints to estimate a velocity model consistent with the data. NIP-tomography results were mutually compared, and also to the velocity model obtained from semblance velocity analysis. The velocity distribution were used in the PSPI migration to verify consistency in the results.Item Acesso aberto (Open Access) Processamento, imageamento, interpretação e predição de pressão de dados sísmicos na bacia sedimentar do Jequitinhonha(Universidade Federal do Pará, 2016-08-18) SILVA, Aucilene de Nazaré Pimenta da; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617This present work aimed to compose a seismic-stratigraphical framework for part of Jequitinhonha basin (marine east part of the State of Bahia), with data released for funded academic projects in progress. The study aims at the exploration of oil and gas, and corresponds to a proposal for for sedimentary basin revaluation. For this, the methodology is composed of velocity analysis, CRS stacking, migration, culminating in pressure prediction for the subsurface, where the aim is to map low (reservoir) and high (generator) pressure zones that act as natural pumps for fluid accumulation. The seismic data used in this study was provided by PETROBRAS for the Postgraduate Course in Geophysics (CPGF), of the Institute of Geosciences of the Federal University of Pará. The data were acquired by the PETROBRAS seismic team 214 in the offshore part or the basin. The available lines used in this study were the L214-266, L214-268, L214-270 and L214-297. Being able to separate them into two groups, we have three lines in the NE-SW directions (L214-266, L214-268, L214-270) and one in the direction NW-SE (L214-297). The velocity distributions used for the seismic sections were based on petrophysical information and empirical models, instead of using the subjective event picking in common-midpoint, stack or migration sections. The methodology presents as the first part the application of techniques based on the theory of the common reflection surface stack (CRS), which aims to generate seismic images of good quality for the interpretation of real data, and related to geologically complex media. The seismic-stratigraphic interpretation were performed using as a basis the geological information, making a correlation between the main reflectors (interfaces of higher impedance) and the stratigraphic units of the area. In this way, we constructed an empirical model for the velocity distribution (𝑣𝑅 and 𝑣𝑆) and density (𝜌) for the sections of the studied block. A higher study project aims to predict stress in sedimentary basins, as a contribution to the methods and techniques of geology and oil and gas engineering exploration. This subject is based on the knowledge of the compressional (𝑣𝑃 ) and shear (𝑣𝑆) velocities, and densities (𝜌), in order to locate areas low and high pressure zones in the subsurface, which serve as natural suction pumps for gas and oil accumulation. The theory is based on the elastodynamic equations, where the gravitational weight of the overload is responsible for the effects of strain-stress deformation in the subsurface. Therefore, to organize this problem requires the generalized Hooke’s law of linear elasticity. We presented details of the theoretical model, and an example to show how the pressure varies in the subsurface, where we highlight that pressure prediction does not necessarily increase linearly, but in a complex way that requires specific numerical formulas to be able to see important details. The applied theoretical model poses as pressure agent the vertical gravity load of the geological formations, and does not take into account the effects of curvatures, faulting and diagenesis. Also, the complex lateral tectonic events are not accounted for. The prediction of pressure and tension is an important issue for the analysis of sedimentary basins, aiming at mapping and extending potentially oil and gas productive areas. But an accurate prediction needs a 3D model for a significantly complete practical application.Item Acesso aberto (Open Access) Seismic amplitude analysis and quality factor estimation based on redatuming(Universidade Federal do Pará, 2015-04-25) OLIVEIRA, Francisco de Souza; FIGUEIREDO, José Jadsom Sampaio de; http://lattes.cnpq.br/1610827269025210Amplitude correction is an important task to correct the seismic energy dissipated due the ineslasticity absortion and the geometrical spreading during the acoustic/elastic wave propagation in solids. In this work, we propose a way to improve the estimation of quality factor from seismic reflection data, with a methodology to estimate de quality factor based on the combination of the peak frequency-shift (PFS) method and the redatuming operator. The innovation in this work is in the way we correct travel times when the medium is consisted by many layers. In other words, the correction of traveltime table used in the PFS method is performed using the redatuming operator. This operation, which is performed iteratively, allows to estimate the Q-factor layer by layer in a more accurate way. A redatuming operation is used to simulate the acquisition of data in new levels, avoiding distortions produced by near-surface irregularities related to either geometric or material property heterogeneities. In this work, the application of the true-amplitude Kirchhoff redatuming (TAKR) operator on homogeneous media is compared with conventional Kirchhoff redatuming (KR) operator restricted to the zero-offset case. Our methodology is based on the combination of the peak frequency-shift (PFS) method and the redatuming operator (TAKR with weight equal 1). Application in synthetic and in seismic (Viking Graben) and GPR (Siple Dome) real data demonstrates the feasibility of our analysis.