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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Acesso aberto (Open Access)
Análise de processos oceanográficos no estuário do rio Pará
(Universidade Federal do Pará, 2016-11-04) ROSÁRIO, Renan Peixoto; ROLLNIC, Marcelo; http://lattes.cnpq.br/6585442266149471
This thesis investigated physical oceanographic processes in the Pará River estuary, focusing on saline intrusion and hydrodynamic process. The choice of this topic arose from the motivation to consolidate the understanding of hydrodynamic and hydrographic issues in the Pará River estuary since this region of the Amazon Coastal Zone still a challenge to researchers. The first step was to define the methods and parameter to get better data in time and space difference. In this context, direct observations were conducted in the estuary in two moments, the low and high river discharge, using velocity, salinity profile (longitudinal and vertical), and temperature profile. Furthermore, in an unprecedented way, it was conducted over a year and ten months salinity and water level (tide) monitoring at strategic points of the estuary. The main conclusions of this research obtained from this data set was the identification of salt water intrusion in the estuary of the Pará River, entering about 100 km from the mouth. The sensitivity of salinity intrusion is affected by river discharge (seasonal variability), and tide energy (daily variability). The Stokes drift generated by tidal propagation in the estuary was the responsible for the net salt flux landward. The innermost portion of the estuary (more than 60 km from the mouth) does not show gravitational circulation and the estuary salt transport above is performed entirely by turbulent diffusion; and the outer portion of the resulting stream reverts to the depth and advective and diffusive processes are important to contribute to the salt transport in the estuary.
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)
Caracterização de fraturas em imagens de amplitude acústica utilizando morfologia matemática
(Universidade Federal do Pará, 2013) XAVIER, Aldenize Ruela; GUERRA, Carlos Eduardo; http://lattes.cnpq.br/7633019987920516; ANDRADE, André José Neves; http://lattes.cnpq.br/8388930487104926
Fractures analysis is of particular interest in the characterization of carbonate reservoir since the fractures are the classic geological setting for stock and produce hydrocarbon in this kinds of reservoirs. Particularly in Brazil is growing the interest in the characterization of carbonate reservoirs, with the recent discoveries in pre-salt. The acoustic imaging tools provide valuable information about the amplitude of the reflected waves in the borehole wall, which can be interpreted to allow the characterization of fractures. However, some problems arise due to the qualitative interpretation of these images that are basically performed with the use of vision and experience of the interpreter. This work presents a methodology that performing the fractures analysis of acoustic images and can be divided into three steps. The first one presents the image modeling, which is used to infer the aspect of the fractures in different geological settings. In the second step, the mathematical morphology is used as an edge detector and performs the fractures identification in the acoustic image. The last step deals with the extraction of geometric attributes of the fractures with the adoption of a four degree polynomial according to the least square criterion. The evaluation of this methodology is performed with synthetic images generated by the presented modeling, which supports the characterization of fractures performed in real images.
Acesso aberto (Open Access)
DesG: uma nova metodologia para intervenção e coleta de dados magnetotelúricos
(Universidade Federal do Pará, 2006-08-29) SILVA, Lúcia Maria da Costa e; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
This thesis presents two contributions to magnetotelluric data interpretation. The first one is a new MT inversion method, named Geologic-Descriptive Method (DesG) because it explicitly incorporates a priori information, keeping a close correspondence with the standard description of geological features. The user defines by means of geometric elements (points and axes) the skeleton of geological features, and establishes a priori values for the resistivity of the assumed geological bodies. The method estimates the resistivity distribution in the subsurface, which are closest to the specified geometric elements and produce an acceptable fit to the observations. The analysis of an obtained solution may help in modifying the a priori outline for the sources, allowing, in this way, that successive inversions be performed until the solution fit the data and make geological sense. Among the attractive features of the presented method are: (i) the anomalous sources may present a resistivity larger or smaller than the resistivity of the host rocks, (ii) several host rocks, enclosing or not anomalous sources, may be crossed by the traverse, and (iii) the resistivity contrast between the anomalous source and the host rock may be either abrupt or gradational. Tests on synthetic data reveal, among other relevant features, excellent results in estimating the attitudes of variable-dip faults, which are of particular importance in regional tectonic studies, and in delineating diabase sills within sedimentary basins, which represent a severe obstacle in oil prospecting. The method also allows the joint interpretation of the target sources and the sources causing static shift. Tests using data from COPROD2 shows a estimative of the resistivity distribution in agreement with the geological knowledge about the area. The second contribution relates to the design of geophysical experiment. By means of several indicators, particularly the density information matrix, it is shown that one can access the theoretical resolution of the data, which guides the explorationist in carrying out a survey design. The survey design optimization allows determining the most suitable observation locations and periods to produce a more precise delineation of sources, whose locations are approximately known.
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)
Estimativa de parâmetros em meios VTI usando aproximações de sobretempo não hiperbólicas
(Universidade Federal do Pará, 2015-09-30) PEREIRA, Rubenvaldo Monteiro; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023
Transversely isotropic (TI) media is a more realistic model for processing seismic data, for example, fractured media with preferred fracture direction, or composite by periodic thin layers. In particular, TI media with vertical symmetry axis (VTI) are widely used as models for P-wave propagation in shales, abundant rock in hydrocarbon reservoirs. However, the P-wave propagation in homogeneous media VTI have as their main characteristics, depend on four parameters of rigidity and also to possess: complicated algebraically phase velocity equation, difficult group velocity equation to explain and moveout equation nonhyperbolic. Therefore, several authors have presented parameterization and obtained approximations to these equations depending on three parameters only. Among these, the moveout approximations have been widely used in inverse methods to estimate lithological parameters in homogeneous media VTI. Such methods have generally been successful in estimated stacking velocity vn and the anellipticity parameter η, since these are the only ones required for generating initial models for the steps of seismic processing in the time domain. One of the most used methods for estimating parameters is the basedsemblance velocity analysis, though, because this method is limited to sections with small offset-depth ratio, adaptations for anisotropic media, considering nonhyperbolic moveout approximatios are required. In this paper, based on anelliptical approximation shifted hyperbola, anelliptical rational approximations are presented for: phase velocity, group velocity and moveout nonhyperbolic in homogeneous VTI horizontally layered media. The validity of these approximations is made by calculating their relative errors by comparing with other known approximations in the literature. To semblance-based velocity analysis is performed to measure the accuracy of the rational moveout approximations to estimate parameters in VTI media. The results demonstrate the great potential of rational approximations in inverse problems. In order to adapt to VTI media, we modify two coherence measurements by semblance which are sensitive to amplitude and phase variations. The accuracy and robustness of the adapted coherence measurements are validated by estimation of in anisotropic parameters in VTI media.
Acesso aberto (Open Access)
Geotermia rasa em Belém
(Universidade Federal do Pará, 1987-12-15) ARAÚJO, Rutênio Luiz Castro de; SOUZA, José Ricardo Santos de; http://lattes.cnpq.br/2797414407717271
A detailed study of shallow geothermics was carried out at depth intervals of 0.02 to 210.0 m, in the metropolitan área of the city of Belém-Brazil. The temperature measurements were performed by using mercury and thermistor thermometers, while the thermal conductivity measurements were made on core samples obtained from boreholes, by using a needle type apparatus. The incident solar radiation flux was recorded by a Robitzech type actinography. The average geothermal gradient obtained for the metropolitan area of the city of Belém was of 0.0254 ± 0.0007 °C.m-1. The average value of the thermal conductivity of the core samples collected in this region was of 1.66 ± 0.52 W.m-1.°C-1. On the other hand the average geothermal flux observed within the studied region was of about 42.16 ± 1.14 mW.m-2. At 1.0 m depth one observes changes of the temperature values with time which can not be neglected. These changes are directly related to the incident solar radiation flux variations at the surface. The largest increment of this flux observed from one day to the other was about 30 W.m-2, which corresponded to a temperature increment of the order of 0.22 °C at 1.0 m depth. The temperature profiles obtained in this work, presented drifts in alternating directions during the one-year cycle of observations. The shallow geothermal profiles are characterized by a zone of vanishing heat flux, so called “elbow zone”, past which the temperature increases with depth. The depth of the elbow zone is mainly influenced by the heat flux generated by the incident solar radiation on the surface of the area under study. A numerical model of the thermal structure evolution in time, was developed for the 0.02 to 10.0 m depth zone. The results generated from this model of heat transfer by conduction were compared with those obtained from the field work data. One observes good agreement between the two sets of data. However the numerical adjustment representation is closer to the field data in the period of September to February. The model developed can be used for of profile drifts geothermal forecasts, as long as one knows a priori the corresponding values of the monthly averages of the soil temperatures at the surface, the regional geothermal gradient and one given geothermal profile. This work demonstrates that the incident solar radiation flux at the surface is the main source of influence on the shallow geothermal profiles. The maximum depth of such influence depends mainly on the magnitude of such flux, on the degree of protection of the surface from the direct incidence of solar radiation and the lithology of the site under study.
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)
Informação a priori na inversão de dados magnetotelúricos
(Universidade Federal do Pará, 1999-12-29) LUIZ, José Gouvêa; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This thesis includes two studies applied to the inversion of magnetotelluric data. In the first the dimensions of the subsurface parametrizing grid are determined knowing the resistivities of the heterogeneity and of its surrounding medium; and the other deals with the use of derivative operators of order greater than one to stabilize the inversion problem. In the first study the satisfactory results are obtained only if the resistivity errors are less than 20%; while in the second the results show that the operators of order greater than one can be more effective than conventional first derivative operators because they yield better resolution of the subsurface resistivity heterogeneity besides acting as stabilizers. These studies are new in the inversion of magnetotelluric data since, so far, resistivity of the grid blocks have been obtained with a fix grid, using the first derivative operator as stabilizer. In these studies, bi-dimensional models representing a subsurface with one and two prismatic heterogeneity are used. The effectiveness of the inversion techniques have been evaluated by applying them on both noisy and noise-free synthetic data, and on COPROD2 field profile. Also, a comparative study to analyze the effectiveness of the creeping and the jumping inversion techniques is included in this thesis. This study shows that a priori information on the parameters can be introduced in the creeping as easily as in the jumping contrary to what is mentioned in earlier publications.
Acesso aberto (Open Access)
Interpolação de dados de campo potencial através da camada equivalente
(Universidade Federal do Pará, 1992-09-15) MENDONÇA, Carlos Alberto; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
The equivalent layer technique is an useful tool to incorporate (in the process of interpolation of potential field data) the constraint that the anomaly is a harmonic function. However, this technique can be applied only in surveys with small number of data points because it demands the solution of a least-squares problem involving a linear system whose order is the number of data. In order to make feasible the application of the equivalent layer technique to surveys with large data sets we developed the concept of equivalent data and the EGTG method. Basically, the equivalent data principle consists in selecting a subset of the data such that the least-squares fitting obtained using only this selected subset will also fit all the remaining data within a threshold value. The selected data will be called equivalent data and the remaining data, redundant data. This is equivalent to splitting the original linear systems in two sub-systems. The first one related with the equivalent data and, the second one, with the redundant data in such way that, the least-squares solution obtained by the first one, will reproduce all the redundant data. This procedure enables fitting all the measured data using only the equivalent data (and not the entire data set) reducing, in this way, the amount of operations and the demand of computer memory. The EGTG method optimizes the evaluation of dot products in solving least-squares problems. First, the dot product is identified as being a discrete integration of a known analytic integral. Then, the evaluation of the discrete integral is approximated by the evaluation of the analytic integral. This method should be applied when the evaluation of analytic integral needs less computational efforts than the discrete integration. To determine the equivalent data we developed two algorithms namely DOE and DOEg. The first one identifies the equivalent data of the whole linear systems while the second algorithm identifies the equivalent data in sub-systems of the entire linear systems. Each DOEg's iteration consists of one application of the DOE algorithm in a given subsystem. The algorithm DOE yields an interpolating surface that fits all data points allowing a global interpolation. On the other hand, the algorithm DOEg optimizes the local interpolation because it employs only the equivalent data while the other current algorithms for local interpolation employ all data. The interpolation methods using the equivalent layer technique was comparatively tested with the minimum curvature method by using synthetic data produced by prismatic source model. The interpolated values were compared with the true values evaluated from the source model. In all tests, the equivalent layer method had a better performance than the minimum curvature method. Particularly, in the case of bad sampled anomaly, the minimum curvature method does not recover the anomalies at the points where the anomaly presents high curvature. For data acquired at different levels, the minimum curvature method presented the worse performance while the equivalent layer produced very good results. By applying the DOE algorithm, it was possible to fit, using an equivalent layer model, 3137 gravity free-air data and 4941 total field anomaly data from the marine Equant-2 Project and the aeromagnetic Carauari-Norte Project, respectively. The DOEg algorithm was also applied in the same data sets optimizing the local interpolation. It is important to stress that none of these applications would have been possible without the concept of equivalent data. The ratio between CPU times (executing the programs with the same memory allocation) required by the minimum curvature method and the equivalent layer method in global interpolation was 1:31. This ratio was 1:1 in local interpolation.
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.