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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Agora exibindo 1 - 20 de 49

Acesso aberto (Open Access)
Análise da resolução do modelo de Hughes em sondagens magnetotelúricas
(Universidade Federal do Pará, 1996-02-13) SANTOS, Elinei Pinto dos; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
In most geophysical exploration methods, the interpretation is carried out assuming an uniformily stratified Earth model. Due to the lack of uniqueness in the inversion of eletromagnetic data, all of the methods require theoretical modeling techniques to aid interpretation. In the literature data has usually been interpreted in terms of a one-dimensional conductivity structure; commoly the Earth is assumed to be horizontally uniform so that the conductivity is a function of depth only. In this work a new technique was developed to model a generic two-layer Earth in which the interface separating the layers can be represented by a Fourier series. The technique involves a pertubation method to find the effects of a sinusoidal boundary with small ondulations. Because the first-order pertubation solution is a linear one, we can use the overlap principle and combine solutions for several different sinusoids to form the solution for any two-layer in which the boundary is represented by a Fourier series. Comparison with the finite elements technique, the following conclusions can be drawing: • For one model of a flat, two-layer Earth, the layers separated by an interface whose depth varies sinusoidal in one diretion, the eletromagnetic response are more strong when the first layer tickness is of the order of skin depth of the eletromagnetic wave in the medium; • On the other hand, the aparent resistivity for this model is not affected by the change in the spatial frequency (v) of the boundary; • In case of using the general solution for any two layer Earth that can be developed in Fourier series, this technique produced good results when compared to finite elements technique. The lineralization restrict the application of the technique to small structure, nevertheless, a lot of structure can be model in simple way and the computational time is very fast; • When the dimension of the first layer has the same order of magnitude of the structure, this technique is not recommended because for some particular sounding position, the apparent resistivity curves obtained show a small shift when compared to the curves obtained by finite elements.
Acesso aberto (Open Access)
Uma análise de métodos de supressão de múltiplas de superfície livre aplicados a um dado real
(Universidade Federal do Pará, 2011-10-14) OLIVEIRA, Andrei Gomes de; GOMES, Ellen de Nazaré Souza; http://lattes.cnpq.br/1134403454849660
The seismic record in 2D linear events are common features in the seismograms. These events are overlap with interesting reflections (early reflections) complicating the stages of seismic processing and identification of possible subsurface geological horizons. It possible highlight multiple surface that distinguished from others by a low frequency, high amplitude and low velocity of propagation. The identification and removal of multiple reflflections in seismic data is a real step of great importance in seismic processing, because the attenuation can prevent many of these errors in the interpretation of geological images seismic. This study is the determination of a processing flow for multiple attenuation free surface. Methods of elimination of multiple surface (SRME, Radon filtering, predictive deconvolution domain τ - p and fk filltering) applied to a real data 2D were used in different combinations. These are analyzed and presented the results are the best combinations.
Acesso aberto (Open Access)
Análise de velocidade por otimização do semblance na reflexão sísmica
(Universidade Federal do Pará, 2010) VIEIRA, Wildney Wallacy da Silva; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
This work had as a general aim to develop a systematic methodology for the inversion of seismic reflection data organized in common-midpoint gathers (CMP), starting from 1D vertical variation of velocity and thickness that allows to obtain interval velocity, vint,n, in time, the correspondent interval thickness, zn, and the correspondent mean-square velocity, vRMS,n in individualized CMP gathers. A direct consequence of this work the transformation of these values from time to depth. Two methods were developed to attack the problem defined as velocity analysis based on the estimation of interval velocity. The first method was based on manual picking of reflection events on CMP gathers, and inversion by curve fitting in the least-square sense. The second method was based on the otimization of the semblance function to obtain an automatic picking. The methodology combined two types of optimization: a Global Method (Price or Simplex), and Local Method (second order gradient or cojugate), subject to a priori information and constraints. The picking of events in time-distance section is of fundamental importance in the process of inversion, and the picked points are the input data along with a priori information of the model to be adjusted. The picking must, in principle, avoid events that represent multiples, diffractions and intersections, and in a section over 50 pickings can be made, while in a semblance map not more than 10 events could usually be picked by eye. The application of this work is focused on seismic data of marine sedimentary basins to obtain a distribution of velocities for the subsurface, where a plane-horizontal model is applied for individual CMP sections, and that the solution can be used as an initial model in subsequent processes. The real data used in this study were collected by Petrobras in 1985, and the selected seismic line was of number L5519 of the Camamu Basin, and the CMP presented is of number 237. The line consists of 1098 shot points with right-lateral arrangement. The sampling interval is 4 ms. The spacing between the geophones is 13.34 m with the first geophone located at 300 m from the source. The spacing between the sources is 26.68 m. As a general conclusion, the method for estimating interval velocity in this work stands as an alternative support to velocity analysis, where it is necessary a control over the sequential inversion of CMP gathers along the seismic line such that the solution can be used as an initial model for imaging, and further tomographic inversion. As future work, we can be propose studies directely and specifically related to seismic velocity analysis by extending the 2D semblance optimization method to 3D, and extending the present studies to the method based on the image ray, aiming at producing a continuous velocity map for the entire section in an automatic way.
Acesso aberto (Open Access)
Anomalias eletromagnéticas de corpos tubulares inclinados em contato com o manto de intemperismo usando diferentes arranjos de bobinas-modelamento analógico
(Universidade Federal do Pará, 1997-03-16) SILVA, Roberto Cesar Alves; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
Electromagnetic methods are mainly applied in the prospection of massive sulphide deposits. These deposits are frequently long in extension and thin, therefore the interpretation can be carried out considering a half-plane model in a resistive ambient. However, in Amazon region or in semi-arid and tropical regions where a partialy conductive to conductive overburden is encountered, interpretations using a resistive ambient model give erroneous results. The objective of this work is to study the influence of the conducting overburden on electromagnetic anomalies of inclined sheet-like bodies in ohmic contact with the overburden, using coil configurations - horizontal coplanar (HCP), vertical coplanar (VCP), vertical coaxial (VCA) and minium couple system (PERP). These studies are realized with the aid of analog modelling where the ore-body is simulated by a graphite sheet and the conducting overburden by ammonium choride solution. Generally, the diferente coil systems studied here show the similar characteristics of the overburden effects, reserving some peculiar effects of each coil system. The effects of a conductive overburden in ohmic contact with the orebody could be briefly described as follows: (i) There is an anti-clockwise phase rotation initially with the increase in the overburden induction number, but with further increase the phase rotates clockwise; (ii) The form of the quadrature anomaly profile is altered due to the appearence of an extra peak on the down-dip side of the ore-body, and (iii) The peak to peak quadrature anomaly on the up-dip side increases uniformly with increase in the overburden induction number. Highest anomalies are obtained with the horizontal coplanar system and lowest with vertical coplanar. No extra-peak in the quadrature profile is observed with the vertical coaxial system in this study. Also, the overbuden effects are more prominent in the horizontal coplanar system compared to vertical coil systems, and in the anomalies of the less inclined half-planes and at lesser depths.
Acesso aberto (Open Access)
Aplicação dos métodos eletromagnéticos VLF e HLEM na prospecção hidrogeológica no município de São Domingos do Araguaia-PA
(Universidade Federal do Pará, 1996-04-25) FARIAS, Valcir João da Cunha; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work deals with the geological mapping using the electromagnetic methods VLF and HLEM to hidrogeological study of the county São Domingos do Araguaia in the southest of the State of Pará. It was carried out within the cooperation agreement between the Fundação Nacional de Saúde and the Universidade Federal do Pará, through the Geophysics Department and the Curso de Pós-Graduação em Geofísica. Also, we studied the quantitative performance of the VLF method, applied to ground water prospecting. First, we analysed the VLF response, by means of the parameters (tilt angle and ellipticity) of the ellipse of polarization of various geological situations likely to be found in the local under study, and for hidrogeological studies of other areal as well. Also, we evaluated the influence of different physical parameters of selected models using finite elements method. The quantitative analyse of the field data was carried out by trial and error using theoretical curves obtained by means of model simulation suggested by the qualitative interpretation using Fraser's filter and profiles of the HLEM method. The results showed the existence of structural features with trends N-S and NE-SW which correlated with the regional geology are normal and transference faults, respectivily. Based on these results, it was selected several sites for drilling of well with maximum probability of sucess.
Acesso aberto (Open Access)
Avaliação geofísica e geológica de um porção de quebra de talude da Bacia do Jequitinhonha
(Universidade Federal do Pará, 2013) SANTOS, Renata Vieira dos; SARGES, Roseane Ribeiro; http://lattes.cnpq.br/9737447758598999; GOMES, Ellen de Nazaré Souza; http://lattes.cnpq.br/1134403454849660
The seismic reflection methods use at the exploration and development hydrocarbon reservoirs is due to its vast and dense sampling in both area and depth, combined with techniques refinement to processing seismic reflection data, from these data, are generated seismic sections, which after applying appropriate treatment, are used in the interpretation of the strata and/or the subsurface geologic structures. At this paper have made one Geophysics Geological analysis at two real marine seismic lines 2D of break portion embankment Jequitinhonha Basin. Therefore, we performed a set of seismic processing in order to mitigate the multiple reflections common marine data, moreover, were estimated velocity models in depth, used to determine the depth migrated seismic sections. At these were identified reflective surfaces. Through analysis at these surfaces markings of sismofacies. have made based on the initial concepts seismic stratigraphy in order to evaluate the product quality derived from the seismic processing used at this study for seismic stratigraphic interpretation, which is based on the analysis termination patterns of reflectors and internal standard of sismofacies.
Acesso aberto (Open Access)
Comparação dos efeitos do eletrojato equatorial nos dados magnetotelúricos bi e tridimensionais
(Universidade Federal do Pará, 1995-11-01) PONTES, Altem Nascimento; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work deals with the comparison between the effects of the equatorial electrojet in two and three-dimensional magnetotelluric data. It is composed of two parts: in the first part, which consists in comparing the effects of two-dimensional with three-dimensional models (current line and gaussian electrojet), we determine the dimensions that a three-dimensional structure should have so that the MT response of that structure, in YX and XY polarizations, can be replaced by the MT response of a 2D structure in TE and TM modes, for apparent resistivity and phase. The results show that the dimension of the 3D structure in the main direction must be over 16 times its length in the initial dimension, so that the MT response of that structure in YX polarization can be replaced by the MT response at a 2D structure in TE mode. If the polarization is XY the main dimension must be 26 times the initial dimension. In the second part, which consists in determining the effects of the Onwumechillian equatorial electrojet, we placed the 3D structure at 10 km, 100 km, 300 km, 500 km, 1000 km and 1500 km, from the electrojet. In this way we determine the distance at which the effects of the Onwumechillian equatorial electrojet on the MT soundings are practically negligible. We observe that, for a conductive basement, the equatorial electrojet does not affect the MT soundings if the 3D structure is placed at 1500 km from the source. When the basement is resistive, the electrojet affects the MT soundings even if the structure is at 1500 km away from the source. We verified that the equatorial electrojet affects the MT soundings in frequencies under 10-2Hz, in both resistivity and phase and in both polarizations. We also observed that the galvanic effect is stronger on the YX than on the XY polarization.
Acesso aberto (Open Access)
Construção de banco de dados petrofísicos para determinação automática de litologias
(Universidade Federal do Pará, 1994-04-14) COUTO, Nuno André Pomar; LUTHI, Stefan Moritz
In this work we develop a solution for the classic problem of automatic lithology determination from wireline logs. A data bank generated using the discriminant analysis technique, applied over cored intervals of logged wells from a deep water field situated in the Campos basin. The clustering technique, although preferred by many workers in this field, was found to give inferior results compared to discriminant analysis. The log data, after going through environmental corrections, were placed on depth with the cored data, and a discriminant function was computed. This function classifies the log response according to the facies described from the cores. Thus we obtain a data bank, whitch allow us to perform lithologic discrimination on cored or uncored wells, in the same area. We show that the litology has to be clustered into four facies in order to get optimum results. The final product is a discrete (discriminated) curve which reproduces, reasonably well the subsurface litology. As an aplication of this method, we use the discrete (discriminated) curves to correlate three wells using an automatic correlation algorithm.
Acesso aberto (Open Access)
Decomposição em valores singulares aplicada a dados de campo magnético
(Universidade Federal do Pará, 1992-12-15) MOURA, Helyelson Paredes; O'BRIEN, Douglas Patrick
The singular value decomposition of a matrix A, n x m, which represents a magnetic anomaly, can be seen as a bidimensional coherence filtering method which separates the correlated information from noncorrelated information in a magnetic data matrix A. The filter is defined by expansion of matrix A into eigenimages and singular values. Each eigenimage is constructed by the scalar product of the base vectors and eigenvectors, which are associated with the eigenvectors and eigenvalues of the covariance matrices ATA and AAT. This filtering method is based on the fact that the eigenimages, which are associated with large singular values, concentrate the major part of the correlated information present in the data, while the noncorrelated part, including noise caused by external magnetic sources, compilation errors, and shallow magnetic sources comprise the remaining eigenimages. This method was employed on many examples of synthetic and real data from PETROBRÁS' Carauari-Norte project (Solimões Basin) in order to investigate the utility of the method in the identification, elimination and attenuation of noise present on magnetic data and as a possible method for enhancing certain features generated by anomalies of shallow and deep origin. This work suggests the desirability of introducing both static and dynamic shift on magnetic lines to enhance the correlation (coherence) between the magnetic lines. This shift concentrates the correlated signal in the first few eigenimages. Another important aspect of this decomposition into eigenimages and eigenvalues is the savings gained in storage of a matrix A of n x m units. Memory requerements can be diminished considerably by using p autoimages, i. e. p x (n + m + 1) units without altering the form of the anomaly. We conclude that an appropriate choice of eigenimages generated by SVD decomposition shows good promise as a processing method in magnetic data.
Acesso aberto (Open Access)
Deconvolução de perfis de poços através do ajuste de energia
(Universidade Federal do Pará, 1994-04-14) GUERRA, Carlos Eduardo; LOVELL, Michael Anthony
Resistivity measurements are of fundamental importance for the calculation of oil saturation in potentially producing reservoirs. The combined measurement of shallow and deep resistivities enables the determination of the parameters Rt, Rxo and di. But in complex reservoirs we have difficulty in obtaining a confident reading of Rt, due to the low vertical resolution of deep reading tools. In laminated reservoirs, for example, the deep induction reading ILD can be interpreted erroneously with the belief that the measurement refers to one bed. This may be true for extreme case of thick beds, but more often is not. This problem can be partly resolved by enhancement of the vertical resolution of the deep reading log through comparison with the high resolution (shallow resistivity) log. One approach is to use a high resolution log where there is good correlation with the deep reading log. This correlation can be better evaluated if we apply a filter to the high resolution log such that theoretically the resultant log has the same vertical resolution as the low resolution log. However, this assumes that the vertical response of the high and low resolution tools are available, and in practice this is often not the case. In this study we attempt to demonstrate an alternative approach where the filter can be obtained from consideration of the frequency domain. The technique compares the spectral energy of the high and low resolution logs. It is shown that the vertical resolution depends fundamentally on the spectral energy of the actual log based on Parseval Theorem. Next a linear regression is applied to the filtered high resolution and low resolution logs and for each depth a minimisation routine is applied to determine the best correlation interval between the logs. Finally a correction factor is applied to each point on the low resolution log. This factor is considered by the correlation coefficients over the interval minimised for each point. The results obtained with induction logs are promising and the metodology shoud be aplyed on diferent logs techniques.
Acesso aberto (Open Access)
Detecção do contato entre camadas através do perfil de indução
(Universidade Federal do Pará, 1996-08-21) MARQUES JÚNIOR, José; HOWARD JUNIOR, Allen Quentin; http://lattes.cnpq.br/6447166738854045
Two of the main objectives in petrophysical log interpretation are to determine geologic bed boundaries and fluid contacts. For these, the induction log has several important properties: it is sensitive to fluid type and distribution in the pore space, the induction measurement is accurately modeled as a convolution of formation conductivity and the tool response function. The first property assures good discrimination of the reserves and at the same time delineates oil-water contacts. This information permits a fundamental zoning of the well log. The second property follows from the quasi-linear relationship between the induction log and formation conductivity. Thus it is possible to use linear system theory, and in particular digital filter design to adaptively deconvolve the original signal. The idea in this work is to produce an algorithm capable of identifying contacts between beds traversed by a borehole, given the apparent conductivity read by an induction tool. To simplify the problem, the formation model is assumed to be a distribution of plane-parallel homogeneous beds. This model corresponds to a rectangular formation conductivity profile. Using the digitized input log, inflexion points are obtained numerically as extrema of first derivatives. This generates a first approximation of the real formation profile. This estimated profile is then convolved with the tool response function giving an estimated apparent conductivity log. A conditioned least-mean-square cost function is defined in terms of the difference between measured and estimated apparent conductivity. Minimizing the cost function yields the bed conductivities. The optimization problem of finding the best rectangular profile for induction data is linear for amplitudes (bed conductivities), but non-linear estimation for bed boundaries. In this case amplitudes are estimated by linear least-squares maintaining fixed contacts. A second pass maintains fixed amplitudes and computes small changes in bed boundaries using a linearized approximation. This processes is iterated to obtain successive refinement until a convergence criteria is satisfied. The algorithm is applied on synthetic and real data showing the robustness of the method.
Acesso aberto (Open Access)
Efeitos do manto de intemperismo sobre anomalias “VLF” dos corpos tabulares inclinados
(Universidade Federal do Pará, 1997-03-16) ROZAL, Edilberto Oliveira; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
Application of eletromagnetic methods for mineral exploration in tropical regions is complicated due to a presence of a conducting overburden encountered in these regions. In the Amazon region, overburden is well developed and conductive; while in semi-arid regions there may be a fine layer containing salts. Consequently, considerable error is introduced in the interpretation of EM data if the presence of the conducting overburden is ignored. In order to study the overburden effects, on VLF anomalies due inclined tabular bodies in contact with the former (Ohmic - overburden), a number of analog model experiments were conducted for varying response parameters of the overburden and the ore body. Overburden is simulated by an amonium chloride solution (NH4Cl) and the ore body by inclined grafite sheets of different thicknesses. Three overburdens of varying conductance were used to simulate diverse geological situations. Both, tilt-angle and elipsity anomalies decrease in amplitude with the increase in depth of the ore body and the position of the peak in the profile is removed further away from the crossover irrespective of the presence of the overburden or not. There is some increase and tilt-angle anomaly profiles. Eliptisity anomaly increases with the increase in the condutance of the overburden while tilt-angle anomaly decreases a little bit. However, at higher overburden condutances both of then are attenuated. Phase angle are rotated anti-clockwise with the overburden. This rotation is more intense at higher induction number of the conductor. In the presence of the ohmic-overburden and at moderate values of their condutance, the conductor appears (i) at lower depth than the real, due to the increase in the anomaly amplitude, and ii) less conducting due to the anti-clockwise rotation of the Argand diagram.
Acesso aberto (Open Access)
Eletroresistividade aplicada à caracterização hidrogeológica para o abastecimento da cidade de São Domingos do Araguaia-Pará
(Universidade Federal do Pará, 1996-02-13) SILVA, Arnaldo de Queiroz da; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The use of geophysical methods (electrical, eletromagnetics and well logging) for groundwater exploration in many areas in the State of Pará has been possible due to a program of cooperation between the Fundação Nacional de Saúde and the Universidade Federal do Pará, in which the Department of Geophysics and the Graduate Programe in Geophysics are respossible for the research and techinical work. This thesis has been conducted under such program. Our objective in this work is to find geoelectrical criterious for caracterization of the subsurface geology for hydrogeological exploration, based on apparent resistivity, data observed in cristaline rock environment. First, we analysed the influence of some 2D structures on resistivity data using vertical electrical sounding with Schlumberger array. This study was carried out with the computer program SEV2D, developed in the Departament of Geophysics, based on the finites elements technique. The results show clearly the ambiguities which are obtained when 1D interpretation is used for two-dimentional geological environment. The pratical aspect of the thesis is concerned with the processing and interpretation of resistivity data obtained in São Domingos do Araguaia in the southeset of the State of Pará. The Schlumberger array was used in the vertical electrical sounding survey. By correlating the geophysical results with geological informations it was possible to define the best zones for groundwater exploration in the prospected area.
Acesso aberto (Open Access)
Eletroresistividade na prospecção hidrogeológica no município de Ourilândia do Norte-PA
(Universidade Federal do Pará, 1996-05-22) SOUSA, José Antonio Ferreira de; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
Geophysical prospecting for groundwater in crystalline regions is a difficult task due to the presence of faults, fractures and lateral variations of resistivity in the overburden. To analyze the influence of bidimensional (2D) variation of resistivity on Schlumberger vertical electrical soundings several layer models were studied containing 2D vertical and dipping structures. These 2D structures are located in the alteration mantle at varying distances from the sounding position. The numerical modeling was done employing the finite element method using an algorithm developed for this purpose (EGSLIB/SEV2D). The results of simulation show that the Sclumberger vertical electrical sounding technique over bidimensional media is not very effective as in sedimentry and alluvial regions due to the phenomenon of ambiguity which complicates the interpretation of the curves. Among the geophysical methods used for hydrogeological investigation, electroresistivity is simplest, economical and efficient. 32 Schlumberger vertical electrical soundings and several kms of dipole-dipole profiling were realized in the Ourilândia do Norte town in order to help locate sites for tube-wells. The apparent resistivity curves were treated and processed considering an isotropic, homogeneous, horizontal layers model, using algorithm EGSLIB/SEV1D and EGSLIB/SEV1DINV program. This permitted to establish approximate geoelectrical models for the subsurface geology of the town. Two distinct geological zones could be recognized based on interpreted geoelectric sections and available geological information. One of the zones is formed by soil/embankment followed by a soil horizon containing sandy sediments over a sandyclay layer and finally the bedrock. This geological sequence predominated in the town. The other zone is formed by soil/embankment overlying a sandyclay bed which is underlain by granitoids type Rio Maria. This second zone is limited in the central part of the town. The sand and sandyclay stratum posses good potential for water storing.
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)
Estimativa de porosidade em lâminas petrográficas através da morfologia matemática binária
(Universidade Federal do Pará, 2013-08-02) CASTELO, Fernando Walleson Louzada; ANDRADE, André José Neves; http://lattes.cnpq.br/8388930487104926
Oil exploration in offshore basins needs for drilling boreholes with high angle and horizontal wells, aimed at optimizing the number of exploration targets hit by a single platform. In these cases, it is technically impossible to carry out the coring operations, which prevents core analysis for direct measurement of porosity. In some situations in formation evaluation the geological knowledge of the area may help when there is low confidence in the porosity values. For the semi-submerged basins, the lateral continuity of geologic layers may allow sampling of outcrops in the immersed part of the basin. In the case of offshore basins, may be collected hand samples in outcrops of analogous formations. A relatively common problem in a petrophysical laboratory is the extraction of plugs adapted to the measuring equipment, directly from rock samples collected from outcrops. On the other hand, for this type of rock sample is trivial to obtain thin sections. The objective of this work is to estimate porosity directly on the petrographic images using the image processing method, known as mathematical morphology, which aims to describe quantitatively the geometric structures (forms) in this image.
Acesso aberto (Open Access)
Estimativas da condutividade térmica dos minerais e rochas e influência de parâmetros térmicos e petrofísicos na resistividade aparente da formação
(Universidade Federal do Pará, 1995-08-09) COZZOLINO, Klaus; HOWARD JUNIOR, Allen Quentin; http://lattes.cnpq.br/6447166738854045
The present study carries out estimates of thermal conductivity in the principal rock-forming minerals, as well as estimates of the average conductivity of the solid phase of five common lithologies (sandstones, dolomites, limestones, anhydrites, clay lithologies). Several thermal models were compared, permitting the verification of one as the most appropriate to represent the aggregate of minerals and fluids of which rocks are composed. The results of this study can be applied to a wide variety of thermal models. The chosen methodology is based on a non-linear regression algorithm denominated Random Search. The algorithm's behaviour is evaluated with sinthetic data before being applied to real data. The geometric mean model is used in the regression to obtain the values of thermal conductivity in these rock-forming minerals. The regression method used in each lithological sub-group gave the following values for average thermal conductivity in the solid phase: sandstones 5.9 ± 1.33 W/mK, limestones 3.1 ± 0.12 W/mK, dolomites 4.7 ± 0.56 W/mK anhydrites 6.3 ± 0.27 W/mK and for argillceous lithologies 3.4 ± 0.48 W/mK. In the sequence the fundaments for the study of heat diffusion are presented in cylindrical coordinates. The effects of invasion of mud filtrate into the formation are considered using an adaption of simulation of well injection techniques originating in theories developed in reservoir engineering. Assuming the original temperature of the formation as a reference, the relative errors in apparent resistivity can be estimated. In this phase of the work the finite differences method is used to measure distribution of the well-formation temperature. Simulation of the invasion is carried out in cylindrical coordenates via an adaptation of the Buckley-Leverett equation into carthesian coordenates. Effects such as the appearance of mudcakes in the borehole, gravity and capilliary pressure are not taken into consideration. The radial distribution of resistivity is obtained via the distribution of saturation and temperature, and is convolved with the radial geometrical factor of the induction tool (transmissor-receiver), resulting in the apparent resistivity of the formation. Admitting as reference the original temperature of the formation, the relative errors in apparent resistivity are obtained at each time. Through variation of certain parameters, it becomes clear that the porosity and original saturation of the formation can be responsible for serious errors in the measurement of resistivity, especially if such readings are taken immediately after drilling (MWD). The difference in temperature between well and formation is the principal cause of such errors. In situations where this difference is large, therefore, profiles with- induction tools should only be carried out between 24 and 48 hours after the well has been drilled.
Acesso aberto (Open Access)
Estudo da resposta de perfilagem de indução de camadas finas com diferentes arranjos de bobinas: modelamento analógico
(Universidade Federal do Pará, 1993-12-16) CARVALHO, Paulo Roberto de; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
Analog model studies were carried out comparing the eletromagnetic responses of various two-coil systems in a borehole, in order to improve the vertical resolution of the indution tools. For this purpose geoeletric models, simulating well-logging situations in the stratified beds of varying thicknesses, with or without fluid invasion, were constructed at a reduced scale of 20. The sensitivity of the system to measure relative fields (secondary/primary) is of the order of 0.01 %. Following conclusions were drawn after analysing the response profiles obtained for a coaxial and a coplanar coil systems: • In case of thin conducting beds placed in relatively resistive beds, such as shale beds in sandstones containing hydrocarbons, the coaxial system shows a better resolution than coplanar system both in determination and estimating the thickness of thin beds; • On the other hand, in the presence of thin resistive beds placed in relatively conductive zone, such as sandstone containing hydrocarbons lying in a shale, the coplanar coil system gives a better vertical resolution than the coaxial system; • "Shoulder effect" is much more pronounced in the coaxial system than the coplanar coils; • In case of thick beds, bed-boundaries are well defined in the coplanar coil system response. However, when the thickness is reduced the small oscillating signal indicating the interfaces disappears.
Acesso aberto (Open Access)
Estudo de geração de campo uniforme para modelagem eletromagnética
(Universidade Federal do Pará, 1996-10-25) CHAVES NETO, Antonio Maia de Jesus; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
In order to study the electromagnetic prospecting problems in geophysics through analog modelling, the full scale conditions are represented in the laboratory at a reduced scale according to the theory of similitude. Therefore, to investigate the problems of VLF, AFMAG and MT techniques, its necessary to create a uniform field in the experimental set up. Various physical systems for generating the uniform fields were analysed theoretically in this thesis. The systems studied here are a circular coil, a Helmholtz coil system, a solenoid, a rectangular coil, a plane of currents and a two parallel planes of currents. Analytical relations were obtained for the magnetic fields at a point in space and subsequently the conditions for a uniform field. In cases where the conditions for the uniform field could not be obtained analytically, the percentage deviation of the field with reference to a pre-selected point was computed. The magnetic field maps as well as the percentage deviation maps were presented for all the systems studied here. Also, volume of the space avaliable for various percentage deviation of the uniform field have been calculed. A comparative study of these systems shows that the solenoid is the most efficient way to create a uniform field followed by a Helmholtz coil system. Nevertheless, the field created within a solenoid is a closed space where it is difficult to place models in order to carry out the experiments. Therefore, it is recommended to use a Helmholtz coil system to create a uniform field, because it provides sufficient open space of a uniform field that facilitates the experiments.