Teses em Geofísica (Doutorado) - CPGF/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/2357

O Doutorado Acadêmico pertente a o Programa de Pós-Graduação em Geofísica (CPGF) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA).

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Agora exibindo 1 - 20 de 33

Acesso aberto (Open Access)
Aplicação de redes neuronais artificiais ao tratamento e interpretação de perfis geofísicos de poço aberto
(Universidade Federal do Pará, 1997-02-13) ANDRADE, André José Neves; LUTHI, Stefan Moritz
The analysis of openhole wireline logs is of great importance for the subsurface mapping of geological layers and the identification and quantification of hydrocarbon and mineral deposits. An importants aspects are the determination of geological boundaries, the mineralogical identification and the well-to-well correlation, which can be a tedious and time-consuming task for the geologist. Automating this procedure is complicated but potentially rewarding because it may save the production geologist and log analyst substantial amounts of time. Artificial neural networks have been shown to handle this task efficiently including in cases where sequential algorithms have problems. We show in this thesis that neural networks can be used to determine layer boundaries, the mineralogical identification and the well-to-well correlation, on well logs, and we present the new networks archtectures. These procedures are tested on synthetic as well as actual field data.
Acesso aberto (Open Access)
Atenuação de múltiplas e compressão do pulso fonte em dados de sísmica de reflexão utilizando o filtro Kalman-Bucy
(Universidade Federal do Pará, 2003-01-24) ROCHA, Marcus Pinto da Costa da; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
The main objective of this work is the study and the application of the Kalman-Bucy method in the processo f deconvolution to the impulse and deconvolution with prediction, considering the observed data as no stationary. The data used in this work are synthetic and, with this, this Thesis has characteristics of a numerical and search. The operator of deconvolution to the impulse is obtained from the Crump theory (1974), doing use of the solution of equation of Wiener-Holp presented by Kalman-Bucy in the continuoun and discrete forms considering the stacionary process. The prediction operator (KBCP) is based the Crump (1974) and Mendel et al (1979) theorics. Its structure resembles the Wiener-Hopf filter, where the coefficients of the operator are obtained through the autocorrelation, in the case (KBCP) are obtained from the function bi(k). The problem is defined in two steps: the first consists of the generation of the signal, and second of its evaluation. The deconvolution performed is classified as statistics, and is a model based in the properties of the registered signal and its representation. The method were applied only in synthetic data with common-shot section obtained from models with continuous interfaces and homogeneous layers.
Acesso aberto (Open Access)
Atenuação de múltiplas pelo método WHLP-CRS
(Universidade Federal do Pará, 2003-01-28) ALVES, Fábio José da Costa; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
In the sedimentary basins of the Amazon region, the generation and accumulation of hydrocarbons is related to the presence of diabase sills. These rocks present a great impedance contrast to the host rocks what turns to cause the generation of internal and external multiples with similar amplitudes the primary events. These multiples can predominate over the information originated at the deeper interfaces, making more difficult the processing, interpretation and imaging of the seismic section. In the present research work, we conducted de multiple attenuation in synthetic commonsource (CS) seismic sections by combining the Wiener-Hopf-Levinson for prediction (WHLP) and the common-reflection-surface-stack (CRS) methods. We denominated this new combination under the name and label of WHLP-CRS method. The deconvolution operator is calculated from the real amplitudes of the seismic section trace-by-trace, and this strategy represents efficiency in the process of multiples attenuation. Multiples identification is carried out in the zero-offset (ZO) section simulated by the CRS-stack applying the periodicity criteria between the primary and its repeated multiples. The wavefront attributes, obtained by the CRS-stack, are employed to move the shifting windows in the timespace domain, and these windows are used to calculate the WHLP-CRS operator for the multiple attenuation carried out in the CS sections. The development of the present research had several intentions as: (first) avoid the inconveniencies of the processed ZO section; (second) design and apply operators in the CS configuration; (third) extend the WHL method to curved interface; (fourth) use the good results obtained in the new CRS-stack technology whose application extends to migration, tomography, inversion and AVO.
Acesso aberto (Open Access)
Avaliação de métodos eletromagnéticos aplicando campos polarizados e focalizados
(Universidade Federal do Pará, 2006-01-17) BARBOSA, Antônio Vinícius Corrêa; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
In the near surface investigations employing advanced electromagnetic induction methods, the low induction number – LIN targets produce very low anomalies which are difficult to interpret. To overcome this difficulty, in this thesis I have studied the applicability of the Polarized Focused Electromagnetic – POLFOCEM fields as a primary inductive source. The horizontal end vertical E.M. field polarizations are obtained by the vectorial combinations of the fields due to the coil pairs in the central region between them; and the focalization occurs in this region at a depth 0.25 of the spacing between the transmitting coils – L. Therefore, maximum coupling can be obtained by selecting the field polarizations depending upon the geometry of the target and focusing the magnetic flux on it by varying L; consequently producing higher anomalies. This study has been realized by computing anomalies employing the Finite Element Technique for 2.5D problem. In all these numerical experiments a comparative qualitative and quantitative study has been done between the POLFOCEM anomalies and the conventional system anomalies where only one transmitter is used (dipole-dipole system). The POLFOCEM system anomalies, wherein both transmitters T1 and T2 are energized simultaneously, correspond to the sum of the anomalies produced by the T1-R and R- T2 dipoles independently; obeying in this way the linearity the electromagnetic fields. These numerical experiments have been carried out for a bidimensional prismatic model in a resistive half-space at three different inclinations and using radio frequencies. The asymmetrical anomalies obtained in a conventional coil system become symmetrical in POLFOCEM system, and have their amplitudes reduced in relation to that of the conventional system. On the other hand conventional anomalies, symmetrical or asymmetrical, become anti-symmetrical in POLFOCEM, and have their amplitude increased. As a consequence, the Argand diagrams of LIN targets for conventional coil system suffer respectively anticlockwise or clockwise rotation for POLFOCEM system. The experiments, to verify the quality in identifying the two nearby targets, shows that the conventional system is superior in relation to the POLFOCEM system.
Acesso aberto (Open Access)
Determinação das velocidades intervalares usando a teoria paraxial do raio: aproximação de segunda ordem dos tempos de trânsito
(Universidade Federal do Pará, 1998) MONTES VIDES, Luis Alfredo; SÖLLNER, Walter Franz
In this work a method was developed to solve the inverse seismic problem in models consisting of isotropic and homogeneous layers separated by smooth interfaces, which determines the interval velocities in depth and calculates the geometry of the interfaces. The traveltime is expressed by a function with parameters referred to a coordinated system fixed at the central ray, and numerically estimated at the superior surface of the model in the vicinity of the normal ray. The function is later recalculated at the anterior interface limiting the unknown layer, through a process which determines the characteristic function in depth. The characteristic function of the traveltimes evaluated at the anterior interface allows to know the interval velocity of the layer and the geometry of the posterior interface where the normal reflection takes place. The procedure is repeated recursively at deeper layers getting the complete solution without a priori knowledge but the upper determined layers. Computer’s programs expressing the algorithm of the method were developed and tested with synthetic seismic data, generated through models with structural factions very common in geological sections, obtaining the interval velocities in depth with considered acceptable errors and reconstructing the interfaces. A sensibility analysis was done in order to verify the stability of the two methods. The empirical range of applicability of hyperbolic dynamic corrections was taken for the range of applicability of the developed method.
Acesso aberto (Open Access)
Empilhamento pelo método superfície de reflexão comum 2-D com topografia e introdução ao caso 3-D
(Universidade Federal do Pará, 2003-01-27) OLIVA, Pedro Andrés Chira; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023; HUBRAL, Peter; http://lattes.cnpq.br/7703430139551941
The CRS stacking method simulates ZO seismic sections from multi-coverage data and does not dependente on a macro-velocity model. For 2-D medium the stacking traveltime depends on three parameters: the emergence angle of the normal ray (with respect to the measurement surface normal) and the wavefront curvatures of two hypothetical waves, called Normal-Incidence-Point (NIP) wave and Normal (N) wave. The CRS method consists of summing the amplitudes of the seismic traces in the multicoverage data along the surface defined by CRS stacking traveltime which that fits best the data set. The result of the CRS stack is assigned to points of a grid pre-defined in the ZO section. As the result obtain a simulated ZO section. This means that for each point of the ZO section must be estimated the three optimal parameters that yield the maximum coherence between the events of seismic reflection. In this Thesis I present formulae for the 2-D CRS method and for the NMO velocity that consider the topography of the measurement surface. The algorithm is based on the optimization strategy divided into three steps: 1) To search for the emergence angle and the curvature of the NIP wave, by applying a global optimization, 2) to search for the curvature of the N wave, by applying global optimization, and 3) to refine the initial parameters estimated in first two steps by applying local optimization. In the first two steps is used the Simulated Annealing (SA) algorithm and in the third step the Variable Metric (VM) algorithm. For the case of a measurement surface with smooth topography the curvature of this surface is included in the 2-D CRS stack formalism. This CRS algorithm implemented was applied to synthetic data set. The result is a simulated ZO section of high quality, with a high signal-to-noise ratio, and the estimative of the parameter triplet. It is performed a sensibility analysis for the new CRS stacking traveltime with respect to the curvature in several points of the curved measurement surface. This study showed that the CRS traveltime is more sensitive for fast midpoints of the central points and larger offsets. The expressions for the NMO velocities presented here is applied to estimate the interval velocities and the depth of the reflectors for 2-D model with a smooth topography. For the inversion of the velocities and the depth of the reflectors is considered the Dix-type inversion algorithm. The NMO velocity for a curved measurement surface deserves to best estimate the velocities and the depths of the reflectors than NMO velocities referred to planar surfaces. Also, I present an introduction to 3-D stack. In this case, the stacking traveltime depends on eight parameters. These parameters can be obtained by using some parameter-search strategies that I have showed in this Thesis. The combination of the strategy of the Traveltime Approximations and the strategy of Arbitrary Curvatures is used to apply 3-D CRS stack successful in synthetic and real data sets, respectively.
Acesso aberto (Open Access)
Empilhamento sísmico por superfície de reflexão comum: um novo algoritmo usando otimização global e local
(Universidade Federal do Pará, 2001-10-25) GARABITO CALLAPINO, German; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023; HUBRAL, Peter; http://lattes.cnpq.br/7703430139551941
By using an arbitrary source-receiver configuration and without knowledge of the velocity model the recently introduced seismic data stacking method called Common Reflection Surface (CRS) simulates a zero-offset (ZO) section from multi-coverage seismic reflection data. For 2-D acquisition, as by-products provides three normal ray parameters: 1) the emergence angle (β0); 2) the radius of curvature of the Normal Incidence Point Wave (RNIP); and 3) the radius of curvature of the Normal Wave (RN). The CRS stack is based on the hyperbolic traveltime paraxial approximation depending on β0, RNIP and RN. In this thesis is presented a new algorithm of the CRS stack based on two-parameters and one-parameter search strategy combining global and local optimization methods for determine the three parameters that define the stacking surface (or operator). This is performed in three steps: 1) two-parameters search by applying global optimization to determine β0 and RNIP; 2) one-parameter search by applying global optimization to determine RN; and 3) three-parameters search by applying local optimization to determine three parameters, using as initial approximations the parameter triple of the earlier two steps. In the first two steps is used the Simulated Annealing (SA) algorithm and the Variable Metric algorithm is used in the third step. To simulate the conflicting dip events, for each ZO sample where there are interference of intersecting events is determined an additional parameter triple corresponding to a local minimum. The stacking along the respective operator for each particular event allows to simulate their interference in the simulated ZO section by means of their superposition. This new CRS stack algoritm was applied to synthetic data sets providing high-quality simulated ZO sections and high precision determination of the stack parameters in comparison with the forward modeling. Using the hyperbolic traveltime approximation for identical radii of curvature RNIP = RN, an algorithm called Common Diffraction Surface (CDS) stack was developed to simulate ZO sections for diffracted waves. In a similar way to the CRS stack procedure, this new algorithm also uses the SA and VM optimization methods to determine the optimal parameter couple (β0, RNIP) that define the best CDS operator. The main features of the algorithm are the data normalization, common-offset data, large aperture of the CDS operator and positive search space for RNIP. The application of the CDS stack algorithm in a synthetic dataset containing reflected and diffracted wavefields provides as main result a simulated ZO section containing diffracted events clearly defined. The post-stack depth migration of the ZO section locates correctly the discontinuities of the second interface.
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)
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)
Imageamento da porosidade através de perfis geofísicos de poço
(Universidade Federal do Pará, 2004-01-27) MIRANDA, Anna Ilcéa Fischetti; ANDRADE, André José Neves; http://lattes.cnpq.br/8388930487104926
Porosity images are graphical representations of the lateral distribution of rock porosity estimated from well log data. We present a methodology to produce this geological image entirely independent of interpreter intervention, with an interpretative algorithm approach, which is based on two types of artificial neural networks. The first is based on neural competitive layer and is constructed to perform an automatic interpretation of the classical Pb - ΦN cross-plot, which produces the log zonation and porosity estimation. The second is a feed-forward neural network with radial basis function designed to perform a spatial data integration, which can be divided in two steps. The first refers to well log correlation and the second produces the estimation of lateral porosity distribution. This methodology should aid the interpreter in defining the reservoir geological model, and, perhaps more importantly, it should help him to efficiently develop strategies for oil or gas field development. The results or porosity images are very similar to conventional geological cross-sections, especially in a depositional setting dominated by clastics, where a color map scaled in porosity units illustrates the porosity distribution and the geometric disposition of geological layers along the section. The methodology is applied over actual well log data from the Lagunillas Formation, in the Lake Maracaibo basin, located in western Venezuela.
Acesso aberto (Open Access)
Imageamento homeomórfico de refletores sísmicos
(Universidade Federal do Pará, 1994-10-06) CRUZ, João Carlos Ribeiro; HUBRAL, Peter; http://lattes.cnpq.br/7703430139551941
This thesis presents a new technique for seismic stacking called homeomorphic imaging, which is applicable to the imaging of seismic reflectors in a bidimensional, inhomogeneous and isotropic medium. This new technique is based on ray geometrical approximation and topological properties of reflection surfaces. For this purpose the concepts of wavefront, incidence angle, radius and caustic of wavefront and ray trajetory are used. Considering a circle as the geometrical approximation of the wavefront in propagation, it is possible to define diferent homeomorphic imaging methods, depending on processing configuration. In this way, the following methods are possible: 1) Common Source (Receiver) Element (CS(R)E), which relate to a set of seismograms with a single source (receiver) and a real reflected wavefront is considered; 2) Common-Reflecting-Element (CRE), which relate to a set of seismograms with a single reflection point and a wavefront hipotetically generated in the same reflection point is considered; 3) Common Evolute Element (CEE), which relate to a set of seismograms with each pair of source and geophone located in the same point on the seismic line and a wavefront hipothetically generated in the curvature center of the reflector is considered. In the first method is obtained a stacked seismic section using arbitrary central rays. In the last two methods the result is a zero-offset seismic section. These methods give also other two sections called radiusgram and anglegram, the latter being emergence angles and the former radii of wavefront in the moment that it reaches the observational surface. The seismic stacking is made using a local correction-time applied to the travel time of a ray that leaves the source, and after reflection, is registered as a primary reflection at a geophone, in relation to the reference time which is the travel time of the central ray. The formula used for the temporal correction depends on the radius, the emergence angle of the wavefront and the velocity which is considered constant near the seismic line. It is possible to show that in this new technique the registered signal is not submitted to stretch effects as a consequence of the temporal correction, furthermore there is no problem with reflector point dispersal as a consequence of dip reflectors, in contrast with the techniques that are based on NMO/DMO. In addition, considering that no a prori knowledge of a macromodel is necessary but the velocity near the seismic line, the homeomorphic imaging can be applied to inhomogeneous models without losing the strictness of the formulation.
Acesso aberto (Open Access)
Influência de estruturas geológicas bidimensionais no campo geoeletromagnético na presença do eletrojato equatorial
(Universidade Federal do Pará, 2005) SILVA, Marcos Welby Correa; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The Earth acts as a large magnet, whose field resembles one that is generated by a magnetic dipole. This field presents intensity changes that vary with observation location and the local time. The main part of the geomagnetic field is created within the Earth by electromagnetic processes. Extensive studies showed that there are also contributions from outside the Earth, mainly from solar origin. Among these outside sources there are anomalies of the magnetic field that arise from an diurnal increase of the electric current in a narrow strip located in the ionosphere, with east-west direction, centered above the magnetic equator and denominated Equatorial Electrojet (EEJ). Occasionally these currents present flow reversions, therefore denominated Counter-Electrojet (CEJ). Several authors have been studying the effects of the EEJ on the geomagnetic observations. They are interested in the combined effect of the equatorial electrojet and the 1-D e 2-D conductive geological structure underneath. In these works the 2-D structure is always considered parallel to the electrojet, which is a quite restrictive hypotheses in view to realistic geological situation, in that two-dimensional structures can have any direction in relation to the electrojet. We present in this work the solution of this problem without this restriction. Thus, here we present the geomagnetic fields due to a two-dimensional structure that possess oblique strike in relation to Equatorial Electrojet, through profiles of the electric and magnetic fields, calculated on the surface and forming arbitrary direction to the 2-D conductive heterogeneity. Further, we also evaluate the influence of an arbitrarily oriented two-dimensional structure would cause on the Magnetotelluric data, under the quatorial Electrojet. In the development of this work, we applied the method of finite elements with the EEJ and CEJ as electromagnetic source, that was represented by a sum of gaussians distributions of current density. This source was decomposed in the parallel and the perpendicular directions to the 2-D structure, resulting in the mode TE1 and the coupled mode TE2 and TM, respectively. We solved the coupled mode applying a Fourier Transform in the Maxwell equations and one Inverse Fourier Transform in the transformed-domain solution. According to the numerical experiments on a interpretative model of Parnaíba Basin Conductivity Anomaly, formed by a large 3000 ohm-m body inside a very conductive (1ohm-m) external structure, we conclude that the presence of the CEJ causes an inversion in the anomaly. We also conclude that at high frequencies the components of the electric field present smaller influence of the internal part of the 2-D body than the external part. Otherwise, we observed this behavior in the magnetic field at low frequencies. Varying the frequencies, we saw the effects of the “skin-depth” mainly in the magnetic field. Besides, there are situations where electric field is insensitive to the internal structure of the model for all values of the frequency used. With regard to the angle θh between the strike of the conductive heterogeneity and the EEJ direction, we observe the TM mode naturally when θh is different from 0°. In this case, the TE mode is composed of two components, one decomposition of the EEJ parallel to the heterogeneity and the other perpendicular to it. As consequence, the E and B fields have all their three components. When we analyzed the influence of the angle between the direction of the profile of fields and the strike of the 2-D heterogeneity, we conclude that its variation causes an asymmetry on the anomalies, which give an idea about the profile’s direction. Finally, we conclude that one of the influences that the distance between the center of the electrojet and center of the 2-D structure causes on the fields is related to the reverse currents, because at 500 km from the EEJ’s center, these currents have their maximum intensity. In the MT soundings, we also used the EEJ and CEJ as primary source and we compared our results with the plane wave response. We noted that the components of the geomagnetic field, used to evaluate the impedance, have an influence from the coupling factor between the TE2 and TM modes. Moreover, this influence become greater with decreasing of the frequency and for resistive host. However, the coupling factor do not affects the MT response at frequencies higher than 10-2 Hz. For lower frequencies, about 10-4 Hz, we detect two kinds of pertubations on the MT data with respect to the plane-wave one: the first is due the presence of the 2-D electromagnetic source (EEJ and CEJ) as primary field, which violates the plane wave hypothesis; and the second is caused by the coupled TE and TM modes because additional electric currents arise in the heterogeneity’s direction when it is oblique to EEJ. These efects increase with the resistivity of the environment. In conclusion, the strike of a large conductive 2-D structure relative to the direction of the EEJ or CEJ do have influence on the geomagnetic field. On the other hand, for shallow magnetotelluric studies (frequencies higher than 10-3 Hz) the effect of angle between the strike of the 2-D geological structure and the direction of the EEJ is not so important. However, for litospheric studies (frequencies lower than 10-3 Hz) the coupling between the two modes can not be ignored.
Acesso aberto (Open Access)
Inversão da forma de onda orientada ao alvo
(Universidade Federal do Pará, 2016-09-16) COSTA, Carlos Alexandre Nascimento da; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
We propose a new target-oriented waveform inversion to estimate the physical parameters from a specific target in the subsurface from observed data from deviated-VSP acquisition or surface seismic data. Furthermore, we investigate a strategy to estimate the impulse responses from a local target in the subsurface from deviated-VSP acquisition or surface seismic data as an iterative sparse inversion approach, where the main feature of this strategy is that all multiple scattering in the data is used to enhance the illumination at target level. In these approaches we fit the upgoing wavefields observed at a specific level near the local target with the upgoing wavefields estimated at same depth level through convolution-type representation for the Green’s function. The main feature of the target-oriented waveform inversion is that we just need to know the up- and downgoing wavefields at the depth level above the target area to estimate the physical parameters for the area of interest. We show through numerical tests that the iterative sparse inversion approach does not require dense sources sampling to estimate the impulse responses from a target below a complex overburden, because of all the extra illumination via multiples. The physical parameters above the target area is not necessary to know if we use the data from deviated-VSP geometry of acquisition, but for surface seismic data we need to know a smooth physical parameter above the target area to estimate the up- and downgoing wavefields at depth level nearby the local target. For surface seismic data we used Joint Migration Inversion to estimate the up- and downgoing wavefields at depth level near the target area.
Acesso aberto (Open Access)
Inversão de dados eletromagnéticos com o regularizador Variação Total e o uso da matriz de sensibilidade aproximada
(Universidade Federal do Pará, 2012-12-20) LUZ, Edelson da Cruz; RÉGIS, Cícero Roberto Teixeira; http://lattes.cnpq.br/7340569532034401
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)
Migração 3-D Kirchhoff-Gaussian-Beam (KGB) pré-empilhamento no domínio da profundidade
(Universidade Federal do Pará, 2013-06-24) PEREIRA, Glauco Lira; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023
The Gaussian Beam (GB) is an asymptotic solution of the elastodynamic equation in the paraxial vicinity of a central ray, which approaches better the wave field than the standard zero-order ray theory. The GB regularity in the description of the wave field, as well as its high accuracy in some singular regions of the propagation medium, provide a strong alternative to solve seismic modeling and imaging problems. In this thesis, i presenty a new procedure for pre-stack depth migration with true-amplitude, combining the flexibility and robustness of Kirchhoff migration type using superposition of Gaussian beams to represent the wave field. The proposed migration algorithm comprises in two stacking process: the first is the beam stack is applied to subsets of seismic data multiplied by a weight function defined such that stack operator has the same formulation of the integral of the Gaussian beams superposition; the second is a weighted diffraction stack by means of the Kirchhoff type integral having as input the GB stacked data. For these reasons it is called Kirchhoff-Gaussian-Beam (KGB) migration. The main characteristics that distinguish the KGB migration, during the first stage stacking, with other migration methods that also use the theory of Gaussian beams, is the use of the first Fresnel zone projected to limit the width of the subset of seismic traces (beam) using a second-order approximation of the reflection travel time. Examples are shown for applications on two-dimensional (2-D) and three-dimensional (3-D) synthetic seismic data, respectively, to the models Marmousi and SEG/EAGE salt dome data sets.
Acesso aberto (Open Access)
Migração em profundidade pré-empilhamento utilizando os atributos cinemáticos do empilhamento por superfície de reflexão comum
(Universidade Federal do Pará, 2007-11-12) LUZ, Samuel Levi Freitas da; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023
The Common-Reflection-Surface (CRS) stack is a new seismic processing method for simulating zero-offset (ZO) and common-offset (CO) sections. It is based on a second-order hyperbolic paraxial approximation of reflection traveltimes in the vicinity of a central ray. For ZO section simulation the central ray is a normal ray, while for CO section simulation the central ray is a finite-offset ray. In addition to the ZO section, the CRS stack method also provides estimates of wavefield kinematic attributes useful for solving interval velocity inversion, geometrical spreading calculation, Fresnel zone estimate, and also diffraction events simulation. In this work, Its proposed a new strategy to do a pre-stack depth migration by using the CRS derived wavefield kinematic attributes, so-called CRS based pre-stack depth migration (CRS-PSDM) method. The CRS-PSDM method uses the CRS results (ZO section and kinematic attributes) to construct an optimized stack traveltime surface along which the amplitudes of the multi-coverage seismic data are to be summed and the result is put in a point of the migration target zone in depth. In the same sense as in Kirchhoff type pre-stack depth migration (K-PSDM), the CRSPSDM method needs a migration velocity model. Unlike the K-PSDM method, the CRS-PSDM needs only to calculate the zero-offset traveltimes, i.e, along only ray conecting the considered point in depth to a given coincident position of source-receptor at surface. The final result is a zero-offset time-to-depth converted seismic image of reflectors from pre-stack seismic data.
Acesso aberto (Open Access)
Migração Kirchhoff pré-empilhamento em profundidade modificada usando o operador de feixes gaussianos
(Universidade Federal do Pará, 2007) FERREIRA, Carlos Augusto Sarmento; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023
The Gaussian Beam (GB) concept was introduced in the seismic literature by Russian and Czech researchers in the begining of the 80’s. This theory, which by its turn was based on the scalar electromagnetic diffraction theory, is in fact a (zero order) complex paraxial ray theory, designed to satisfactorilly describe the seismic wavefield propagation beyond the standard zero order ray theory, up to then the only theory used to describe the high frequency seismic wavefield propagation in smoothed velocity models. As an imaging tool, the first works to deal with GB’s were published in the end of the 80’s and in the begining of the 90’s. The regularity in the description of the wavefield by GB’ s, as well as its high accuracy in some singular regions of the velocity model, transformed the use of GB’s into a viable hybrid alternative in the migration theory. In this work, we unite the flexibility in imaging of the true amplitude prestack Kirchhoff depth migration with the regularity in the description of the wavefield by a superposition of GB’s. As a way of controlling in a very stable way some quantities used in the construction of the beams, we have made use of some informations based on the Fresnel volume elements, more especifically speaking the Fresnel zone radius around the reflection point in depth and its counterpart, projected towards the acquisition surface. This information is centred around the recording point of the seismogram and is also present in the seismic data reflection traveltime curves. Our migration process can be named a true amplitude prestack Kirchhoff depth migration using GB’s as Green function, namely KGB-PSDM.
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)
Migração pré-empilhamento Kirchhoff feixes gaussianos 2,5D nos domínios afastamento comum e ângulo-comum
(Universidade Federal do Pará, 2012) COSTA, Manuel de Jesus dos Santos; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023; CALLAPINO, German Garabito; http://lattes.cnpq.br/6064981270181319
A Kirchhoff-type migration is considered in the geophysics literature as one of the most fundamental tools in seismic data processing, the base for solution of several imaging problems. In this respect, it must be considered its wide use and its successful history for the oil and gas industry, associated with its low computational cost and flexibility to deal with non-wavefield datasets when compared to other methods. However in 3D, even when compared to other existing and most effective methods, its computational cost and implementation is still considered high, due to several reasons: new acquisition technologies, data storage and burden, azimuth richness, etc. Thus the main objective of the present work is to implement and simulate migration results (i.e., images) with high signal-to-noise ratios and with a less computer burdens in 2.5D media, using the theoretical framework of Gaussian Beams (GBs). By considering one implementation of a superposition of GBs integral operator studied by Ferreira and Cruz (2009) and by the use of the stationary phase method (Bleistein, 2000), a new integral superposition migration operator using paraxial fields (i.e., GBs) was implemented and studied. Theoretically speaking, the present migration operator was inserted in the kernel of a conventional, 2.5D, true-amplitude, prestack Kirchhoff migration integral operator, thus defining a 2.5D prestack Kirchhoff-Gaussian Beam (KGB) migration operator. The present migration operator was later configured to hold commonoffset (CO) and common-angle (CA) seismic acquisition configurations. I remark that in the present thesis one flexibility of the GB migration operator was idealized in order to handle its effective application in more the one sorting configuration, i.e., common-offset and commonsource.