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.
Navegar
Navegando Programa de Pós-Graduação em Geofísica - CPGF/IG por CNPq "CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOFISICA::GRAVIMETRIA"
Agora exibindo 1 - 13 de 13
- Resultados por página
- Opções de Ordenação
Item Acesso aberto (Open Access) Desenvolvimento de técnicas de filtragem e sua aplicação a dados aeromagnéticos da Bacia Amazônica(Universidade Federal do Pará, 1976-08) SILVA, João Batista Corrêa da; LOURENÇO, José SeixasIn this thesis two filtering methods were combined in order to enhance some features in an aeromagnetic map such as lineaments and anomalies caused by sources of large proportions. The visual filtering consisted of stereoscopic examination of pairs of maps. The numeric filtering involved the elimination of the undesirable spectral components (noise) and the enhancement of the desired spectral components (signal). Both methods were tested on aeromagnetic maps of the Middle Amazon Basin. All the numeric filtering operations were done in the space domain by convolving the aeromagnetic data with a 13x13 filter matrix. Special attention was paid to the preliminary analysis of the spectrum, as a guide in filter design. The results of the application of each filter was discussed in detail. A further topic also dealt with was basement fault models. The model analysis established a complementary interpretation method to the filtering methods. A structural interpretation of the Middle Amazon Basin's basement was provided. The dominant aeromagnetic trends were E-W, N60°W, N-S, N45°E and N60°E. Two grabens were quite evident. The younger one trends N-S, while the older one strikes N45°E. Two anomalies caused by sources of intermediate and large proportions (15-60km) exhibit remanent magnetization and high values of negative inclination, which shows that the Amazon Basin would be southernmost in the Paleozoic. In performing the integration of aeromagnetic data with other geophysical data avaiIable, the interpretation from other data proved consistent with the aeromagnetic interpretation, illustrating the reliability of the techniques employed in this thesis.Item 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/1870725463184491The 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.Item Acesso aberto (Open Access) Interpretação quantitativa de anomalias gravimétricas ar-livre da margem continental norte brasileira, setor nordeste(Universidade Federal do Pará, 1990-10-12) CASTRO, David Lopes de; BISCHOFF, Jürgen H.Free-air gravity anomalies across continental margins of passive type present a general standard form. This form can be well explained by a distribution of simple horizontal two-dimensional discontinuities. An automatic process of random search was proposed for quantitative interpretation of the data. Using the method of flexible polyhedron (Simplex), the main parameters of the model were estimated, considering a convenient ration between number of points/number of parameters to be determined. The free-air gravity anomalies over the region of the continental slope can be explained by a single horizontal discontinuity (simple step) and, as the wavenumber domain has information about this anomaly, it was also proposed an iterative graphic procedure for the analysis of the amplitude spectral response for the anomalies. Applying the Fourier transform, it would possible to determine the depth and the off-set of this discontinuity, and, once these parameters are measured, the density is calculated. The basic aim for studying these procedures was to combine the two methods of interpretation in the space and wavenumber domains, in order to obtain a more constrained solution to the geological structure of the studied area. The two interpretative procedures discussed above were applied and their respective capacity of resolution were analysed, over the free-air gravity anomalies of profiles across the continental margin of northeast Brazil, covering the states of Maranhão to Rio Grande do Norte. The survey, called Project EQUANT I, LEG 1, was carried out during the period of August 27 to October 10, 1987, by the Oregon State University (OSU) using the oceanographic ship Prof. W. Besnard of the Oceanographic Institute of São Paulo University (USP). It was demostrated that the inversion done directly in the space domain is more favorable for the interpretation, even for typical anomalies, although the treatment in the spectral domain is relatively simpler.Item Acesso aberto (Open Access) Inversão de momentos de fonte em métodos potenciais(Universidade Federal do Pará, 1993-08-16) MEDEIROS, Walter Eugênio de; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491The inversion of three-dimensional gravity source moments is analyzed in two situations. In the first one only the anomalous field is assumed to be known. In the second situation a priori information about the anomalous body is assumed to be known besides the field data. Without using a priori information, we show that it is possible to determine uniquely any moment, or linear combination of moments, whose polynomial kernel: (a) is not a function of the Cartesian coordinate which is orthogonal to the measuring plane and (b) has null Laplacian. Besides, we show that it is impossible to determine any moment whose polynomial kernel has non-null Laplacian. On the other hand, we show that a priori information is implicitly introduced if the source moment inversion method is based on the approximation of the anomalous field by the truncated series obtained from its multipole expansion. Given any center of expansion, the series truncation impores a regularization condition on the equipotential surfaces of the anomalous body that allows the moments and linear combination of moments (which are the coefficients of the multipole expansion basis function) to be uniquely estimated. So, a mass distribution equivalent to the real mass distribution is postulated, being the equivalence criterion specified by the fitting conditions between the observed anomaly and the anomaly calculated with the truncated multipole expansion series. The highest order for the retained terms in the truncated series is specified by the previously defined maximum order for the moments. The moments of the equivalent mass distribution were identified as the stationary solution of a system of first order linear differential equations, for which uniqueness and assymptotic stability are assured. For the series having moments up to 2nd order, it is implicitly assumed that the anomalous body: (1) has finite volume, (2) that it is sufficiently far from the measuring plane and (3) that its spatial naass distribution is convex and presents three orthogonal planes of symmetry. The source moment inversion method based on the approximation of the anomalous field by a truncated series (MIT) is adapted to the magnetic case. In this case, we show that in order to guarantee uniqueness and assymptotic stability it is sufficient to assume, besides the regularization condition, that the total magnetization has constant but unknown direction. The MIT method based on the 2nd order series (MIT2) is applied to three-dimensional synthetic gravity and magnetic anomalies. If the source satisfies all imposed conditions, we show that it is possible to obtain in a stable way good estimates of the total anomalous mass or dipole moment vector, of the position of center of mass or dipole moment and of the directions of all three principal axes. A partia' failure of MIT2 method may occur either if the source is dose to the measuring plane or if the anomaly presents a localized but strong effect due to a shallow and small body and an attempt is made to estimate the moments of a large and deep body. By partial failure we mean the situation when some of the estimates may be poor aproximations of the true values. In these two cases we show that the estimates of the depth and the directions of the principal axes of the (main) source may be poor but the estimates of the total anomalous mass or dipole moment vector and the projection on the measuring plane of the center of mass or dipole moment of the source are good. If the total magnetization direction is not constant, MIT2 method may produce poor estimates of the directions of the principal axes (even if the source is far from the measuring plane) but good estimates are obtained for the other parameters. A complete failure of MIT2 method may occur if the source does not have finite volume. By complete failure we mean the situation when any obtained estimate may be a poor aproximation of the true value. MIT2 method is applied to real gravity and magnetic data. In the gravimetric case we used an anomaly located in Bahia state, Brazil, which is assumed to be produced by the presence of a large granitic body. Based on the inversion results, we propose that the grafite was deformed into an oblate ellipsoid during the compressive event that generated the Middle Proterozoic Espinhaço orogeny. The center of mass estimated for this body is about 20 km. In the magnetic case, we used an anomaly produced by a seamount located in the Gulf of Guinea. Based on the inversion results, we estimate a magnetic palaeopole for the seamount at 50°48'S and 74°54'E and we suggest that no important magnetization contrast exists below the bottom of the seamount.Item Acesso aberto (Open Access) Inversão gravimétrica do relevo de bacias extencionais através da variação total(Universidade Federal do Pará, 2009) LIMA, Williams Almeida; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491We present a gravity inversion method to reconstruct the discontinuous basement relief of a sedimentary basin, whose density contrast between the basement and the sediments is constant, or decreases monotonically with depth, and known. The solution is stabilized using the total variation functional (VT), which does not impose smoothness on the solution. We compare the proposed method with the global smoothness (SG), weighted smoothness (SP) and entropic regularization (RE) methods using synthetic data produced by 2D and 3D basins presenting discontinuous relief. The solutions obtained with the proposed method were better than those obtained with SG and similar to the ones produced by SP and RE. Differently from SP, however, the proposed method does not require a priori knowledge about the basin maximum depth. As compared with RE, the present method is operationally simpler and requires the specification of just one regularization parameter. The VT, SG, and SP methods were also applied to the following areas: Ponte do Poema (UFPA), Steptoe Valley (Nevada, United States), San Jacinto Graben (California, United States) and Büyük Menderes (Turkey). Almost all those areas are characterized by the presence of highangle faults. In all cases, the VT produced basement relief estimates presenting sharp, high-angle discontinuities, in accordance with the tectonic setting of the areas.Item Acesso aberto (Open Access) Inversão gravimétrica do relevo do embasamento usando regularização entrópica(Universidade Federal do Pará, 2007) OLIVEIRA, Alexandre de Souza; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491We present a new gravity inversion method, which estimates the basement relief of a sedimentary basin, whose sediments may present a constant or a decreasing density contrast with depth relative to the basement. The method combines the minimization of the first-order entropy measure with the maximization of the zeroth-order entropy measure of the solution vector (depths to the basement). The interpretation model consists of a set of rectangular juxtaposed vertical prisms with known density contrasts and whose thicknesses represent the depths to the basement relief and are the parameters to be estimated. The minimization of the first-order entropy favors solutions presenting abrupt discontinuities, and the maximization of the zeroth-order entropy is employed just to prevent its excessive minimization. We applied our method to synthetic data simulating: (i) intracratonic basins with smooth or discontinuous relief, and (ii) faulted marginal basins. In the case of a smooth basement relief, it was well delineated by the entropic regularization, and by the global and weighted smoothness. In the case of a faulted basement, either in intracratonic or marginal basins, the entropic regularization and the weighted smoothness delineated the basement relief with good precision, differently from the global smoothness. The entropic regularization, however, did not require the knowledge of the basin’s maximum depth. We applied our method to four sets of real Bouguer anomalies. The first one comes from a profile across a bridge located in the campus of the Federal University of Pará, Belém. The solution delineated the known, discontinuous topography below the bridge. The second set comes from the northern portion of Steptoe Valley, Nevada. The solution delineated a discontinuous basement relief in accordance with the known geological setting of the area. The third set comes from the San Jacinto graben, situated in California, United States. The solution indicated an asymmetric graben, as already reported by other authors. The fourth data set comes from the Büyük Menderes graben, western Turkey. The solutions presented large vertical displacements on the northern border, which is confirmed by geological information.Item Acesso aberto (Open Access) Inversão gravimétrica estável do relevo da base e da variação da densidade com a profundidade em aterros sanitários(Universidade Federal do Pará, 2003-10-21) TEIXEIRA, Wlamir Abreu; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491The present method assumes a decrease in the density contrast of a sedimentary basin according to a hyperbolic law in a waste landfill and uses two approaches: (1) the estimation of the bottom of a waste landfill assuming that the gravity anomaly, the density contrast at the surface and the decaying factor of the density contrast with depth are known; (2) the estimation of the density contrast at the surface and of the decaying factor of the density contrast with depth, assuming that the gravity anomaly and the relief of the waste landfill bottom are known. A variation of the latter approach consists in using the known depth values to the landfill bottom to produce a fitted gravity anomaly and estimate the density contrast at the surface and the decaying factor of the density contrast with depth via minimization of the Euclidean norm of the residual vector between the observed and fitted anomalies. In both cases the interpretation model is a set of vertical rectangular 2D prisms whose thicknesses are parameters to be estimated and represent the depth to the interface separating sediments and basement. The solutions of both problems are stable because of the incorporation of additional a priori information about the smoothness of the estimated relief. The presented method was applied to synthetic data produced by simulated waste landfills having maximum depth to the bottom smaller than 15 m and presenting smooth relief. The results were consistent and demonstrated the effectiveness of the method in both approaches. The method was also applied to the residual Bouguer anomaly from the Thomas Farm, Indiana, USA, leading to the conclusion that the density contrast of the landfill is either constant or present a maximum variation of about 0.09 g/cm3.Item Acesso aberto (Open Access) Inversão gravimétrica estável do relevo do embasamento e da variação da densidade com a profundidade em bacias sedimentares(Universidade Federal do Pará, 2003-10-21) COSTA, Denis Carlos Lima; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491The present method assumes a decrease in the density contrast of a sedimentary basin according to a hyperbolic law and has two objectives: (1) to estimate the basement relief of the sedimentary basin assuming that the gravity anomaly, the density contrast at the surface and the decaying factor of the density contrast with depth are known; (2) to estimate the basement relief, the density contrast at the surface and the decaying factor of the density contrast with depth, assuming that the gravity anomaly and the depth to the basement at a few points are known. In both cases the interpretation model is a set of vertical rectangular 2D prisms whose thicknesses are parameters to be estimated and represent the depth to the interface separating sediments and basement. The solutions of both problems are stable because of the incorporation of additional a priori information about the smoothness of the estimated relief and about the depth to the basement at a few locations, presumably provided by boreholes. The method was tested with synthetic gravity anomalies produced by simulated sedimentary basins presenting smooth relief. The results showed well-resolved estimated relieves. Besides, estimated density contrasts at the surface and the decaying factors of the density contrast with depth close to the true ones were obtained, indicating the potentiality of the proposed method in gravity interpretations of sedimentary basins. The method was applied to the Bouguer anomaly from Recôncavo Basin, producing an estimated relief with a maximum depth of 6 km, a figure similar to the one obtained from seismic interpretations. The estimates of the surface density contrast and of the decaying factor of the density contrast with depth were -0.30 g/cm3 and 30 km, respectively, producing an estimate of 4% for the maximum sediment compaction.Item Acesso aberto (Open Access) Inversão gravimétrica rápida do relevo do embasamento aplicando o funcional da variação total(Universidade Federal do Pará, 2013-08-23) SANTOS, Raphael Di Carlo Silva dos; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491; SANTOS, Darcicléa Ferreira; http://lattes.cnpq.br/5813659873067927We present a new 2D linear inversion gravity method to estimate the discontinuous basement relief of a sedimentary basin. The method uses an interpretation model consisting of a set of 2D juxtaposed horizontal ribbons whose thicknesses are the parameters to be estimated. The density contrast between the basement and the sediments is assumed constant and known. The estimates of the thicknesses were stabilized with the Total Variation functional (TV) which permits solutions presenting local discontinuities in the basement relief. The estimates of the basement relief are produced by solving a single linear system of equations in the L1 norm. Because linear inversion methods underestimate the depth of the basement relief for sedimentary basins thicker than 500 m, we amplify the depth estimates through by modifying the matrix associated with the interpretation model. This procedure in general leads to overestimated depths which are corrected through the expression of the anomaly of a Bouguer slab. Tests on synthetic and real data produced similar results as compared with the nonlinear method, but required less computational time. The ratio R between the computational times required by the nonlinear and the proposed methods to produce similar solutions increases with the number of observations and parameters. For example, for 60 observations and 60 parameters R is equal to 4 whereas for 2500 observations and 2500 R increases to 16.8. The proposed method and the nonlinear inversion method were also applied to real gravity data from Steptoe Valley, Nevada, U.S.A. and from Ponte do POEMA at the Campus of Guamá, in Belém, producing solutions which are similar to the ones produced by the nonlinear method but required a smaller computer time.Item Acesso aberto (Open Access) Mapeamento do relevo do embasamento de bacias sedimentares através da inversão gravimétrica vinculada(Universidade Federal do Pará, 1998-03-02) BARBOSA, Valéria Cristina Ferreira; MEDEIROS, Walter Eugênio de; http://lattes.cnpq.br/2170299963939072; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491We present three new stable gravity inversion methods to estimate the relief of an interface separating two media. Solution stability is attained by introducing a priori information about the interface, through the minimization of one (or more) stabilizing functional. These methods are, therefore, characterized by the physical and geological information incorporated to the problem. The first method, named global smoothness, estimates the depths to the interface at discrete points by assuming that the density contrast between the media is known. To stabilize the inverse problem, we introduce two different constraints: (a) proximity between the true and estimated interface depths at a few isolated points, and (b) proximity between the estimated depths at adjacent points. The combination of these two constraints impose a uniform degree of smoothness all over the estimated interface, minimizing, simultaneously, the misfit between the known and estimated depths at a few boreholes, for example. The second method, named weighted smoothness, estimates the interface depths at discrete points, assuming that the density contrast is known a priori. In this method, it is incorporated the information that the interface is smooth almost everywhere, but at a few fault discontinuities. To incorporate this attribute to the estimated relief, we developed an iterative process where three kinds of constraints are imposed on parameters: (a) weighted smoothness between values of adjacent parameters, (b) lower and upper bounds on the estimated depths, and (c) proximity between the values of the parameters and a known numerical value. Starting with an initial solution produced by the global smoothness method, this method enhances initially estimated geometric features of the interface; that is, flat areas will tend to become flatter and steep areas will tend to become steeper. This is accomplished by weighting the constraints which require proximity between adjacent parameters. The weights are updated at each iteration so as to enhance the discontinuities detected in a subtle way by the global smoothness method. Constraints (b) and (c) are used both to compensate for the decrease in solution stability due to the introduction of small weights, and to reinforce flatness at the basin bottom. Constraint (b) imposes that any depth be nonnegative and smaller than an a priori known maximum depth value whereas constraint (c) imposes that all depths be closest to a value deliberately violating the maximum depth. The trade-off between these conflicting constraints is attained with a final relief presenting fiat bottom and steep borders. The third method, named minimum moment of inertia, estimates the density contrasts of a subsurface region discretized into elementary prismatic cells. It incorporates the geological information that the interface to be mapped encompasses an anomalous source which besides presenting horizontal extents much larger than its largest vertical extent, exhibits bordes dipping either vertically or toward the center of mass, and that most of the anomalous mass (or mass deficiency) is concentrated, in a compact way, about a reference level. Conceptually, these information are introduced through the minimization of the moment of inertia of the anomalous sources with respect to a reference level coinciding with the mean topographic surface. This minimization is performed in a subspace of parameters consisting of compact sources and presenting bordes which dip either vertically or toward the ce4ter of mass. Effectivelly, these informations are introduced by means of an iterative process starting with a tentative solution dose to the null solution, and adds, at each iteration, a contribution which has minimum moment of inertia with respect to the reference level, in such a way that the estimate of the next iteration does not violate the bounds on the density contrast and minimizes, at the same time, the misfit between the observed and the fitted data. Additionally, the iterative process "freezes" a density estimate if it becomes very dose to either bound. The final solution at the end of the iterative process is an estimated solution exhibiting a compact mass distribution concentrated about the reference level, whose density contrast distribution is dose to the upper (in absolute value) bound established a priori. All three methods were applied to synthetic and field gravity data, produced, respectively, by simulated and real sedimentary basins. The global smoothness method produced a good reconstruction of the basin structural framework even when the true basements were not globally smooth, as was the case of the Recôncavo Basin, Brazil. This method presents, however, the lowest resolution as compared with the other two methods. The weighted smoothness method improved the resolution of basements presenting disontinuities produced by gravity faults with large vertical offsets. It is, therefore, potentially useful in interpreting the structural framework of extensional basins as illustrated both with synthetic data and data from the Steptoe Valley, Nevada, USA and from Recôncavo Basin, Brazil. The minimum moment of inertia method was also applied to synthetic data and data from Recôncavo Basin and from San Jacinto Graben, California, USA. The results showed that, as compared with the other two methods, this method produces excellent estimates of a basement relief consisting of several adjacent discontinuities with small vertical offsets. This is a remarkable advantage over the weighted smoothness method which requires that the interface present few, local discontinuities with large vertical offsets.Item Acesso aberto (Open Access) Uma nova abordagem na interpretação de anomalias gravimétricas em bacias sedimentares - exemplo da Bacia do Recôncavo, Bahia, Brasil(Universidade Federal do Pará, 1990-10-12) MENEZES, Paulo de Tarso Luiz; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491Item Acesso aberto (Open Access) Uma nova abordagem para interpretação de anomalias gravimétricas regionais e residuais aplicada ao estudo da organização crustal: exemplo da Região Norte do Piauí e Noroeste do Ceará(Universidade Federal do Pará, 1989-12-18) BELTRÃO, Jacira Felipe; HASUI, Yociteru; http://lattes.cnpq.br/3392176511494801; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491Despite its great importance to the study of global geologic structures, interpreting gravity anomalies is not a trivial task because the observed gravity field is the resultant of every gravity effect produced by every elementary density contrast. Therefore, in order to isolate the effects produced by shallow sources from those produced by deep sources, I present a new method for regional-residual separation and methods for interpreting each isolated component. The regional-residual separation is perfomed by approximating the regional field by a polynomial fitted to the observed field by a robust method. This method is iterative and its starting value is the least-squares fitting. Also, the influence of observations containing substantial contributions of the residual field in the regional field fitting is minimized. The computed regional field is transformed into a map of vertical distances relative to a given datum. This transformation consists of two stages. The first one is the downward continuation of the regional field which is assumed to be produced by a smooth interface separating two homogeneous media: the crust and the mantle. The density contrast between the media is presumably known. The second stage consists in transforming the downward continued field into a map of vertical distances relative to a given datum by means of simple operations. This method presents two difficulties. The first one is related to the instability inherent to the downward continuation operation. The use of a stabilizer is therefore mandatory, leading to an inevitable loss of resolution of the features being mapped. The second difficulty, inherent to the gravity method, is the impossibility of determining the interface absolute depths. However, the knowledge of the absolute depth at one single point of the interface by independent means allows the computation of all absolute depths. The computed residual component is transformed into an apparent density map. This transformation consists in calculating the intensity of several prismatic sources by linear inversion, assuming that the real sources are confined to a horizontal slab and have density contrasts varying only along the horizontal directions. The performance of the regional-residual separation method was assessed in tests using synthetic data, always producing better results as compared either with polynomial fitting by least-squares or with the spectral analysis method. The method for interpreting the regional component was applied to synthetic data producing interfaces very close to the true ones. The limit of resolution of the features being mapped depend not only on the degree of the fitting polynomial, but also on the limitation imposed by the gravity method itself. In interpreting the residual component, a priori information is needed about the depth and thickness of the slab confining the true sources. However, results of tests using synthetic data showed that reasonable estimates for the h6rizontal limits of the sources can be obtained, even when the depth and thickness of the slab are not known. The ambiguity involving depth to the top, thickness and the apparent density can be visualized by means of curves of apparent density as a function of the presumed depth to the top of the slab, each curve corresponding to a particular assumed value for the slab thickness. An analysis of the configuration of the curves allows a semi-quantitative interpretation of the real sources depths. The sequence of all three methods described above was applied to gravity data from northern Piauí and northwestern Ceará state. As a result, a crustal organization model was obtained consisting of crustal thickenings and thinnings related to a compressive event which caused the raise of dense, lower crust rocks to shallower depths. This model is consistent with surface geological information. Also, the .gravity interpretation suggests the continuity of the Northwestern Ceará Shear Belt for more than 200 km under the Parnaíba Basin sedimentary cover. Although the sequence of methods presented here has been developed for the study of large scale crustal structures, it could also be applied to the interpretation of smaller structures, as, for example, the basement relief of a sedimentary basin where the sediments have been intruded by mafic rocks.Item Acesso aberto (Open Access) Procedimentos lineares de inversão gravimétrica para estimar o relevo 3D do embasamento de bacias sedimentares(Universidade Federal do Pará, 2013) RAMOS, Laisse Costa; MARTINS, Cristiano Mendel; http://lattes.cnpq.br/8303640454649778The technological improvement has provided the acquisition of a large volume of geophysical data, which is why the work of interpretation of gravity anomalies and pet relief sedimentary basins increasingly demand innovation in computational and mathematical formulation that allows for such offer. In order to obtain effective and computationally efficient solutions propose two new methods that incorporate linear inversion procedure for the solution of a nonlinear inversion problem. The first proposed method approximates the solution of the nonlinear inversion by linear inversion procedures and applies corrections in iterations from the initial approach, obeyed until the convergence criterion. The second method proposed solution approximates the nonlinear inversion of the linear inversion procedures only on the first iteration, the second iteration from the additional iterations are applied for correction. The investments made in the two new methods and nonlinear inversion, are stabilized by Tikhonov regularization functional first order (smoothness). The observations are modeled by a set of 3D vertical rectangular prisms whose thicknesses are parameters that define discreetly relief estimated at some points. The realization of nonlinear inversion procedure was necessary to validate the estimated basement relief of sedimentary basins produced by the new proposed methods, this validation is given by comparing the results obtained by the procedures of linear inversion with the inversion of nonlinear . We also evaluate the efficiency and effectiveness of the proposed methods and nonlinear inversion from synthetic tests. The proposed methods have proved more efficient than the nonlinear inversion method, since reduced computational time required to recover virtually identical reliefs. As regards efficiency, we conclude that the proposed methods are as effective as the non-linear inversion, since the estimates of the basement relief scale and are equivalent to each other in shape.