Navegando por Assunto "Magnetismo terrestre"

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Acesso aberto (Open Access)
Aspectos fundamentais das medidas e interpretação de registros paleomagnéticos em rochas sedimentares da Formação Longá-Bacia do Parnaíba
(Universidade Federal do Pará, 1984-09-21) ALENCAR, Benaia Vieira de; PACCA, Igor Ivory Gil; http://lattes.cnpq.br/8172887185665918
Palaeomagnetic data for the upper Palaezoic and, in particular, the Devonian of South America are still very limited and therefore polar wandering curves cannot be established. We investigated the paleomagnetism of 43 stratigraphic horizons of the Upper Devonian Longá Formation. The results should make a contribution towards a better definition of that curve. Sampling was done according to the block method in profiles on highway PI-13 between Teresina, Barras and Batalha, and on highways PI-24 and BR-230 between Floriano, Nazaré do Piauí and Oeiras, all in the state of Piauí. Investigations were carried out at the "Laboratório de Paleomagnetismo" of the University of São Paulo and completed at the laboratory of the NCGG of the Federal University of Pará. In this study, we employed the technique of progressive demagnetization by alternate fields up to 700ºe and/or temperatures up to 670-700ºC. Interpretation of the data was done using vector diagrams of Zijderveld, curves type J-T/C of variation of magnetic intensity with variations of temperature or magnetic field and also by graphs of variation of the direction of the vectors of magnetization. Calculations of mean direction and poles were done following the statistical method of Fisher. Four directions of remanent magnetization were identified: 1. A secondary magnetization of chemical origin (CRM) and reverse polarity due to formation of hematite probably by deuteric alteration after magnetite. This magnetization (identified as B) shows palaeomagnetic Carboniferous - Permian age with pole coordinates: 80°S, 3°E; A95 = 13.6°. 2. A hard isotherm component (IRM) with spectre totally superimposed on the initial magnetization which was not affected by the treatment of the samples. This magnetization (identified as D) shows an almost constant direction around declination point 234.23º and inclination 41.94º. 3. A group of directions of soft magnetization of viscous origin (VRM), identified as C, with mean direction: declination= 15º and inclination = -20°. These were removed at temperatures between 300 and 600°C. 4. The principal magnetization, of normal polarity, identified as A, is probably of detrital origin (DRM). Its palaeomagnetic pole coordinates: 48°S, 331ºE; A95= 9.9° is consistent with the Upper Devonian age of the Formation. This magnetization is believed to be the original one. The palaeomagnetic poles relative to magnetizations A and B as well as other poles for South America were rotated to Africa following, the pre-drif configuration of Smith and Hallam (1970) and there is agreement when compared to the African and Australian poles of the same age. The measured polarities are consistent with the magnetostratigraphic scales of Irving and Pullaiah (1976) and Khramov and Rodionov (1981).
Acesso aberto (Open Access)
Influência de estruturas geológicas bidimensionais no campo geoeletromagnético na presença do eletrojato equatorial
(Universidade Federal do Pará, 2005) SILVA, Marcos Welby Correa; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The Earth acts as a large magnet, whose field resembles one that is generated by a magnetic dipole. This field presents intensity changes that vary with observation location and the local time. The main part of the geomagnetic field is created within the Earth by electromagnetic processes. Extensive studies showed that there are also contributions from outside the Earth, mainly from solar origin. Among these outside sources there are anomalies of the magnetic field that arise from an diurnal increase of the electric current in a narrow strip located in the ionosphere, with east-west direction, centered above the magnetic equator and denominated Equatorial Electrojet (EEJ). Occasionally these currents present flow reversions, therefore denominated Counter-Electrojet (CEJ). Several authors have been studying the effects of the EEJ on the geomagnetic observations. They are interested in the combined effect of the equatorial electrojet and the 1-D e 2-D conductive geological structure underneath. In these works the 2-D structure is always considered parallel to the electrojet, which is a quite restrictive hypotheses in view to realistic geological situation, in that two-dimensional structures can have any direction in relation to the electrojet. We present in this work the solution of this problem without this restriction. Thus, here we present the geomagnetic fields due to a two-dimensional structure that possess oblique strike in relation to Equatorial Electrojet, through profiles of the electric and magnetic fields, calculated on the surface and forming arbitrary direction to the 2-D conductive heterogeneity. Further, we also evaluate the influence of an arbitrarily oriented two-dimensional structure would cause on the Magnetotelluric data, under the quatorial Electrojet. In the development of this work, we applied the method of finite elements with the EEJ and CEJ as electromagnetic source, that was represented by a sum of gaussians distributions of current density. This source was decomposed in the parallel and the perpendicular directions to the 2-D structure, resulting in the mode TE1 and the coupled mode TE2 and TM, respectively. We solved the coupled mode applying a Fourier Transform in the Maxwell equations and one Inverse Fourier Transform in the transformed-domain solution. According to the numerical experiments on a interpretative model of Parnaíba Basin Conductivity Anomaly, formed by a large 3000 ohm-m body inside a very conductive (1ohm-m) external structure, we conclude that the presence of the CEJ causes an inversion in the anomaly. We also conclude that at high frequencies the components of the electric field present smaller influence of the internal part of the 2-D body than the external part. Otherwise, we observed this behavior in the magnetic field at low frequencies. Varying the frequencies, we saw the effects of the “skin-depth” mainly in the magnetic field. Besides, there are situations where electric field is insensitive to the internal structure of the model for all values of the frequency used. With regard to the angle θh between the strike of the conductive heterogeneity and the EEJ direction, we observe the TM mode naturally when θh is different from 0°. In this case, the TE mode is composed of two components, one decomposition of the EEJ parallel to the heterogeneity and the other perpendicular to it. As consequence, the E and B fields have all their three components. When we analyzed the influence of the angle between the direction of the profile of fields and the strike of the 2-D heterogeneity, we conclude that its variation causes an asymmetry on the anomalies, which give an idea about the profile’s direction. Finally, we conclude that one of the influences that the distance between the center of the electrojet and center of the 2-D structure causes on the fields is related to the reverse currents, because at 500 km from the EEJ’s center, these currents have their maximum intensity. In the MT soundings, we also used the EEJ and CEJ as primary source and we compared our results with the plane wave response. We noted that the components of the geomagnetic field, used to evaluate the impedance, have an influence from the coupling factor between the TE2 and TM modes. Moreover, this influence become greater with decreasing of the frequency and for resistive host. However, the coupling factor do not affects the MT response at frequencies higher than 10-2 Hz. For lower frequencies, about 10-4 Hz, we detect two kinds of pertubations on the MT data with respect to the plane-wave one: the first is due the presence of the 2-D electromagnetic source (EEJ and CEJ) as primary field, which violates the plane wave hypothesis; and the second is caused by the coupled TE and TM modes because additional electric currents arise in the heterogeneity’s direction when it is oblique to EEJ. These efects increase with the resistivity of the environment. In conclusion, the strike of a large conductive 2-D structure relative to the direction of the EEJ or CEJ do have influence on the geomagnetic field. On the other hand, for shallow magnetotelluric studies (frequencies higher than 10-3 Hz) the effect of angle between the strike of the 2-D geological structure and the direction of the EEJ is not so important. However, for litospheric studies (frequencies lower than 10-3 Hz) the coupling between the two modes can not be ignored.
Acesso aberto (Open Access)
Paleomagnetismo de rochas vulvânicas do Nordeste do Brasil e a época da abertura do Oceano Atlântico Sul
(Universidade Federal do Pará, 1983-12-28) GUERREIRO, Sonia Dias Cavalcanti; SCHULT, Axel
In the first part of this paper palaeomagnetic and rock magnetism investigations were developed in volcanic samples from the Northeast of Brasil. The age of the samples spans the Jurassic and Cretaceous periods. To accomplish this task four areas were studied and a total of 495 samples from 56 cites were analyzed. A portable drilling machine with 2.5 cm core diameter was used to collect the samples. The orientation of the samples were obtained by means of a magnetic compass, and a clinometer. The specimens were submitted to alternating field demagnetization and in, a few cases, to thermal demagnetization. Giving unit weight to each site the mean direction of the characteristic remanent magnetization of each one of the studied areas were determined. The volcanic rocks from Jurassic, lying in the western part of the Maranhão Basin (Porto Franco - Estreito) , yielded the mean direction: declination D=3.9º, inclination I=-17.9°, with the circle of 95% of confidence α95=9.3º, precision parameter k=17.9, number of sites N=15. All sites showed normal polarity. For this area was determined a palaeomagnetic pole with coordinates 85.3°N, 82.5°E (circle of 95% of confidence A95 = 6.9º) that is situated near other known palaeomagnetic poles for this period. The Lower Cretaceous rocks from the eastern part of the Maranhão Basin (Teresina-Picos-Floriano) yielded a mean direction for the characteristic remanent magnetization having D= 174.7º, I=6.0°, α95=2.8º, k=122, N=21. All sites showed reversed polarity. The calculated palaeomagnetic pole associated with these rocks has coordinates 83.6°N, 261.0ºE (A95= 1.9°) and is in agreement with other South American poles of the same age. In Rio Grande do Norte a swarm of Lower Cretaceous tholeiitic dikes was studied having a characteristic mean direction with D= 186.6º, I= 20.6º with α95= 14.0º, k= 12.9, and N= 10. The sites in this area showed mixed polarity. The computed palaeomagnetic pole is located at 80.6ºN and 94.8ºE with A95= 9.5º. The study of the volcanic rocks of the magmatic province of Cape Santo Agostinho yielded the following values for the characteristic remanent magnetization D= 0.4º, I=-20.6º with α95= 4.8º, k=114, N=9. All sites showed normal polarity and the calculated paleomagnetic pole has the coodinates: 87.6ºN 135ºE with A95= 4.5. The secular variation of the obtained directions was discussed so that each pole presented in this paper is really a palaeomagnetic pole. The analysis of the magnetic minerals of these samples was done by thermomagnetic curves and by X-ray diffraction. In most cases the magnetic phase in the rocks is mainly titanomagnetite with poor titanium content. Maghemite and sometimes hematite, usually a product of weathering, did not obscure the initial thermoremanent magnetization of these rocks. Generally the determined Curie temperature lies between 500-600º C. Frequently it was observed that the exsolved titanomagnetite has a phase near magnetite and another phase rich in titanium, near ilmenite, as a result of high temperature oxidation. The second part of this paper deals with the determination of the time of the opening of the South Atlantic ocean by means of palaeomagnetic data. In this paper, however, instead of using the polar wandering paths of the continents (the usual method) statistical tests were applied that give the probability that a certain configuration for the two continents be consistent or not with the palaeomagnetic data for a chosen period. For the Triassic, Jurassic, Lower Cretaceous and Middle-Upper Cretaceous periods the palaeomagnetic poles for Africa were compared with the respective poles of South America in pre-drift configuration by means of an F-test. Other configurations that indicate some separation between the two continents were also tested. The results of the tests showed that the pre-drift reconstruction after Martin et al (1981) is consistent with the palaeomagnetic data for Triassic, but there is a significant difference between the respective Jurassic, Lower Cretaceous and Middle-Upper Cretaceous palaeopoles for the two continents, with a probability of error of less than 5%. Other pre-drift reconstructions were tested and the results were the same. Comparing the pole positions for South America and Africa in a configuration that indicates a small separation between the two continents, as the one suggested by Sclater et al (1977) for 110 m.y.B.P. one finds a significant difference for the Triassic data. For the Jurassic and Lower Cretaceous palaeomagnetic poles, which are, however, earlier than the suggested date of the reconstruction, the results are consistent with that separation of the continents with a probability of error of less than 5%. The reconstruction for 80 m.y.B.P., after Francheteau (1973), indicanting a larger separation between the continents, is consistent with the Middle-Upper Cretaceous palaeomagnetic poles. Assuming the premise of the movements of crustal blocks relative to each other as rigid blocks, the results of the F-test indicated that South America and Africa were close together during Triassic. There was, nevertheless, a small separation between the continents in Jurassic, probably due to an earlier rifting event, and this separation was stationary until Lower Cretaceous time. This result is different from the most part of the papers that discuss the openning of the South Atlantic ocean. The Middle-Upper Cretaceous data are compatible with a fast and significant spreading of the continents in that period.