Navegando por Assunto "Anomalias magnéticas"

Agora exibindo 1 - 3 de 3

Acesso aberto (Open Access)
Estudo das feições lineares observadas em dados aeromagnéticos da Bacia do Solimões a partir de imagens digitais de sombreamento do relevo anômalo total
(Universidade Federal do Pará, 1991-11-25) AFFONSO, Alexandre de Oliveira; O'BRIEN, Douglas Patrick
The Solimões Basin is a magnetically complex basin characterized by a superposition of different linear magnetic anomalies. These anomalies reflect the different tectonic episodes that have occured in the Amazon Region during the Precambriam and Phanerozoic. In this work, a shaded relief method of digital image processing was applied to the aeromagnetic data from the Solimões Basin. The characteristics of this method, which allow its use as a strike enhancement filter, clarified important aspects of the relationships between magnetic lineations oriented E—W, NE—SW, e NW—SE. A magnetic lineation pattern was identified which reflects the presence of dextral strike-slip fault zones that have a preferential orientation in the E—W direction. The interaction between these systems promoted the development of a convergent stress regime which caused the formation of reversel faults associated with E—W and NE—SW magnetic lineaments; together with directional duplexes, forming positive and negative flower structures associated with N70-80E and N70-80W magnetic lineaments. A parallel quantitative analysis was also performed on the aeromagnetic data which attempts to quantify two importants aspects of these linear magnetic anomalies shown on the digital images. The first analysis was based on a forward modeling of the aeromagnetic data using a superposition of two-dimensional prismatic bodies. It is shown that these magnetic linear features can be explained by the superposition of highly magnetized basement sources and higher frequency shallow sources associated with reverse faulting of diabase sill's present in the Paleozoic sediments of the Basin. The second analysis, which was based on cross-covariance methods, calculated the offsets present in the NE—SW magnetic lineaments along dextral E—W directions. This analysis supports the explanation that the magnetic pattern from this region can be explained by means of lateral offsets presumably along pre-existing zones of weakness (Precambrian, Paleozoic), during the Mesozoic and Cenozoic.
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/1870725463184491
The 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.
Acesso aberto (Open Access)
Separação regional-residual em magnetometria através de camadas equivalentes
(Universidade Federal do Pará, 1987-12-28) COSTA, Jessé Carvalho; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491