Navegando por Assunto "Margens continentais"

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Acesso aberto (Open Access)
Delineamento do pé do talude na margem continental do Ceará através da integração de dados geológicos e geofísicos
(Universidade Federal do Pará, 1992-12-30) CAMPOS, Luiz Gonzaga; EL-ROBRINI, Maâmar; http://lattes.cnpq.br/5707365981163429; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
The methodology of integrated interpretation applied to the geological and geophysical data observed along a profile at the continental margin of Ceará, enables the identification and integration of characteristics particular to each kind of datum. In this manner, it is possible to define the most probable location of important structural features, such as the boundary line between the continental and oceanic crust and the foot of the continental slope, which is the subject of this study. According to Article 76 (paragraph 4, item b) of the United Nations Covenant regarding the Law of the Sea, the foot of the continental slope is defined as the point of maximum variation of the gradient of the slope at its base. However this definition, despite its simplicity in a physiographic context, it is not sufficient to define the location of the foot of the slope, according to the Covenant, and for this reason geophysical methods are used. Within the geophysical-geological context, a quantitative interpretation of the free-air anomalies, can define the geophysical model, which represents the sub-surface, and can aid the integrated interpretation of the above mentioned data. An automatic procedure of curve adjustment, combining the inversion techniques of systematic search and gradients, was used to generate the geophysical model. The previous rigorous application of constraints and the constant re-evaluation of these constraints by means of an interactive process between seismics and gravimetrics, generated during the quantitative interpretation of free-air anomalies, constrained the final geophysical model to be within the geological framework of the area and within Airy's theory of isostatic equilibrium. The purpose of this research is to study the geological and geophysical characteristics observed on a profile at the continental margin of Ceará (LEPLAC III), especially at the foot of the continental slope, and to try to establish a methodology of integrated interpretation of these data, whose objective is to define in a systematic way, the most probable location for this physiographic feature. The methodology used turned out to be very efficient for the location of the foot of continental slope. In this sense, it was possible to integrate: (i) its physiographic location (distance from the coast and the water depth); (ii) a zone of tectonic instability inferred by the faulting, very common in the continental slope; (iii) the end of a disturbed magnetic anomaly zone, and that possibly delimits the beginning of magnetic quite zone, named anomaly E and (iv) a point of inflexion in the curve or free-air anomaly, associated to an gravimetric effect of contrast in the densities of the continental crusts, the sediment and the sea water. It was also possible to define the most probable location for the boundary line between the continental and oceanic crusts. Because of the rigorous application of the inversion techniques and constraints used, it is possible that the correlation of the intrinsic characteristics for each type of datum, performed at the conclusion of this research, have some foundation and can be confirmed if the methodology described is applied in a greater number of profiles.
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)
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.