Navegando por Assunto "Anomalias gravimétricas"

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Acesso aberto (Open Access)
Interpretação gravimétrica-magnética da zona de fratura dupla Bode Verde
(Universidade Federal do Pará, 1989-07-31) ARAÚJO, Tereza Cristina Medeiros de; BISCHOFF, Jürgen H.
In January 1980, the CENTRATLAN Project Bode Verde I was conducted by collecting geophysical and geological data at the central portion of the Middle Atlantic Ridge, south of Ascensão Island. This research was done within an agreement between the US-American and Brazilian Navy. With the objective to describe the geology of the Bode Verde fracture zone and to understand the origins of this feature on the ocean floor and continental margin, bathimetric, gravimetric and magnetic data were collected. The analysis of the bathimetric data allowed to describe this feature as a doubled fractured zone, marked by two continuous troughs, parallel to each other, separated by a high with its own rift-valley. The gravimetric anomalies confirm this double feature, with the two observed gravimetric minimum corresponding to the two troughs of the fracture zone. These troughs separate a crustal block of approximately 40 km width. Based on gravimetric data it was possible to construct nine crustal sections of the fracture zone. They permitted to observe a thinning of the crust-mantle interface beneath the fracture walls. The interface crust-mantle that usually has a depth of 8-9 km, is only 5.5-6.0 km deep beneath the fracture walls with a crust thickness of about 2.0 km. The crustal thinning is caused by the ascent of mantle material. The hidrothermal alteration in the central trough of the fracture can be responsible for the minor crustal thinning observed in this part of the fracture zone. For the interpretation of the magnetic data was used a time scale of magnetic polarity between the Lower Cretaceous and Cenozoic and a spreading rate of the ocean floor of about 2.0 cm/year. For interpretation a thickness of 0.5 km was estimated for the basalt layer, which is responsible for the observed magnetic anomalies. lts thickness decreases in the fracture walls, being interrupted at the central trough fracture.
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.
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/1870725463184491
The 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.
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/1870725463184491
The 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.
Acesso aberto (Open Access)
Mapeamento de uma interface separando dois meios homogêneos através da reconstituição gravimétrica compacta: aplicação a dados do LEPLAC-IV
(Universidade Federal do Pará, 1995-02-06) GOMES, Luzilene Regina; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
Mapping an arbitrary interface separating two homogeneous media, traditionally uses the downward continuation of the observed gravity anomaly. This procedure requires either a low-pass filtering or the introduction of a damping parameter. In both cases there is a loss in the resolution of the estimated interface relief. Moreover, the use of the downward continuation operator implies the unnecessary assumption that the interface is a harmonic surface. As a result, the use of the downward continuation operator constrains the relief mapping to interfaces which are smooth relative to its average depth. This difficulty is reduced in the inversion procedure presented in this thesis, whose aim is to map the crust-mantle interface using the minimum moment of inertia method by assuming a single horizontal axis of mass concentration, coinciding with the earth's surface. Horizontal and vertical density variations may be taken into account so that the method may be extended to complex geological settings involving the interpretation of adjacent bodies having different origins such as those present in crustal transitional zones. The perfomance of the inversion method was evaluated using sinthetic data and the results confirmed its efficiency in recovering the shape of horizontally elongated sources whose top or bottom may be represented by an arbitrary surface. The results also confirmed the method's efficiency in mapping the shape of bodies presenting lateral density variation, reflecting, therefore, a geological situation where the nature of the basement of the crustal transitional zone is being investigated. Based on these tests, the method was applied to the gravity inversion of five profiles from LEPLAC-IV, offshore the southeastern brazilian continental margin. The results permitted to infer the type of bodies making up the structural framework of the region comprising the São Paulo Ridge and the Porto Alegre Fracture Zone; in addition, it was possible to map the Moho in this region.