Navegando por Assunto "Magnetismo"

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Acesso aberto (Open Access)
Formulação e implementação de um pacote para interpretar mapas magnéticos utilizando os polinômios de Walsh
(Universidade Federal do Pará, 1992-12-30) RAMOS, Alexandre José Santos; O'BRIEN, Douglas Patrick
In this work we develop a method for the automatic interpretation of magnetic field data. This methodology determines the boundaries and magnetizations of geologic units. The often of abrupt variations of the magnetization of geological units are modeled in this methodology. These abrupt lateral variations will be represented by discontinuous polynomials known as Walsh polynomials. Several new concepts were developed in order to apply Walsh polynomials to solve aeromagnetic data inversion problems. Among these new concepts we can list. (i) - An optimum algorithm to generate a set of "quasi-orthogonal" polynomials using a Walsh magnetization distribution. (ii) - The use of the damped least squares method to stabilize the inverse solution. (iii) An investigation of positional invariance problems, which are inherent in the Walsh polynomials. (iv) - The choice of the order of the polynomials taking into account the limitations of resolution and the behaviour of the eigenvalues. Using these features of magnetized bodies it is possible to formulate the direct problem, i.e, the bodies magnetization obeys Walsh distribution. It is also possible to formulate the inverse problem, in which the magnetization that generates the observed field obeys the Walsh series. Prior to applying the method it is necessary to estimate the depths to the magnetic sources. We have chosen a method developed by LOURES (1991), which is based on the Euler homogenous equation which requires the knowledge of the magnetic field and its derivatives. We tested the method with real data from a region in the Alto Amazonas sedimentary basin. The aeromagnetic data were provided by PETROBRÁS.
Acesso aberto (Open Access)
Geologia, geoquímica e geocronologia do magmatismo paleoproterozóico da região de Vila Riozinho, Província Aurífera do Tapajós, Cráton Amazônico
(Universidade Federal do Pará, 2001-09-27) LAMARÃO, Cláudio Nery; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
Several Paleoproteroic granitoids and two volcanic sequences were studied in the Vila Riozinho region. This region is located in the eastern area of the Tapajós Gold Province, near the border between the Tapajós and Central Amazonian tectonic provinces in the south-central part of the Amazonian craton. In the southern part of the region, it was identified the Vila Riozinho volcanic sequence composed of basaltic andesite, basaltic trachyandesite, trachyte and rhyolite, with a high-K calc-alkaline to shoshonitic geochemical signature. Pb-Pb zircon dating indicate ages of 2000 + 4 Ma and 1998 + 3 Ma for this sequence. The São Jorge granite pluton is spatially associated with this volcanic sequence. Two granitoids were distinguished in the pluton, the Old São Jorge granite, with Pb-Pb zircon ages of 1981 + 2 Ma and 1983 + 8 Ma, and the Younger São Jorge granite with an age of 1891 + 3 Ma. The Older São Jorge granite, largely dominant in the pluton, is composed of an expanded magmatic series including biotite-amphibole monzodiorite/quartz monzodiorite, amphibole-biotite monzogranite/quartz monzonite, biotite leucomonzogranite/syenogranite and granite porphyry. It has a metaluminous to mildly peraluminous character, and high-K cale-alkaline signature, similar to that of volcanic arc granitoids. The Younger São Jorge granite was initially identified in drill cores obtained in the gold mineralized area of the pluton. In that area, it corresponds to a hornblende-biotite monzogranite, but biotite leucogranites occur in the southern part of the pluton. This granite also has a high-K calc-alkaline signature, but it differs from the Older São Jorge granite in some geochemical and mineralogical aspects and it is comparatively younger. In the northern part of the Vila Riozinho region, it was identified the Moraes Almeida volcanic sequence, the Maloquinha and Jardim do Ouro granites and a granite porphyry distinct from that associated with the Older São Jorge granite. The Moraes Almeida Formation is composed of ignimbrite and rhyolite with subordinate trachyte, with Pb-Pb zircon ages of 1875 + 4 Ma, 1890 + 6 Ma and 1881 + 4 Ma, respectively. The 1880 + 9 Ma old Maloquinha granite is composed of leucosyenogranite and subordinate leucomonzogranite. This granite and the rhyolite and ignimbrite of the Moraes Almeida Formation show affinities with aluminous, A-type series. The strong petrographic and geochemical similarities between these rocks suggest that they are cogenetic. An age of 1880 + 3 Ma, similar to that of the Maloquinha grafite, was obtained for the Jardim do Ouro hornblende-biotite monzogranite. However, preliminary data indicate that it differs from the former, as well as from the Older São Jorge and Younger São Jorge granites, in petrographic and geochemical aspects. Geochemical and mineralogical data allow the distinction of two different types of grafite porphyries. The first one is spatially associated and similar to the Older São Jorge granite. The second occurs along the contact between the Maloquinha granite and the ignimbrite of the Moraes Almeida Formation and is geochemically similar to the Jardim do Ouro granite and trachyte of the Moraes Almeida Formation. The magmatic activity in the Vila Riozinho region is concentrated into two distinct periods, near the end of the Paleoproterozoic. The Vila Riozinho Formation and the Older São Jorge granite formed during the first period between 2010 and 1970 Ma. At the second period, between 1900 and 1870 Ma, the Moraes Almeida Formation, Maloquinha, Younger São Jorge and Jardim do Ouro granites were formed. The high-K calc-alkaline magmatism that was formed during the first period is probably related to subduction processes. Two hypotheses are considered to explain the diversified magmatic activity registered during the second period: (1) the different magmas could result from late tectonic activity related to the subduction processes; (2) these magmas are related to taphrogenetic processes that affected the Amazonian craton at 1.88 Ga and lasted the entire Mesoproterozoic. It implies to admit a crustal source for the magmas originated during the second period. The second hypothesis is assumed as the more plausible at this stage, but the need for additional isotopic information is emphasized.
Acesso aberto (Open Access)
Mapeamento de magnetização aparente usando regularização entrópica
(Universidade Federal do Pará, 2007) VASCONCELOS, Suzan Sousa de; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
We present a new apparent-magnetization mapping method on the horizontal plane that combines the minimization of the first-order entropy with the maximization of the zeroth-order entropy of the estimated magnetization contrasts. The interpretation model is a grid of vertical, juxtaposed prisms in both horizontal directions. We assume that the top and the bottom of the magnetic sources are flat and horizontal and estimate the prisms' magnetization contrasts. The minimization of the first-order entropy favors solutions presenting sharp borders, and the maximization of the zeroth-order entropy prevents the tendency of the source estimate to become a single prism. Thus, a judicious combination of both constraints may lead to solutions characterized by regions with virtually constant estimated magnetization contrasts separated by sharp discontinuities. We apply our method to synthetic data from simulated intrusive bodies in sediments that present flat and horizontal tops. By comparing our results with those obtained with the smoothness constraint, we show that both methods produce good and equivalent locations of the sources' central positions. However, the entropic regularization delineates the boundaries of the bodies with greater detail. Both the proposed and the smoothness constraints are applied to a real anomaly over a magnetic skarn in Butte Valley, Nevada, USA. The entropic regularization produced an estimated magnetization distribution with sharper boundaries, smaller volume, and higher apparent magnetization as compared with the results produced by the smoothness constraint.