Teses em Geofísica (Doutorado) - CPGF/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/2357

O Doutorado Acadêmico pertente a o Programa de Pós-Graduação em Geofísica (CPGF) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA).

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Agora exibindo 1 - 4 de 4

Acesso aberto (Open Access)
DesG: uma nova metodologia para intervenção e coleta de dados magnetotelúricos
(Universidade Federal do Pará, 2006-08-29) SILVA, Lúcia Maria da Costa e; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
This thesis presents two contributions to magnetotelluric data interpretation. The first one is a new MT inversion method, named Geologic-Descriptive Method (DesG) because it explicitly incorporates a priori information, keeping a close correspondence with the standard description of geological features. The user defines by means of geometric elements (points and axes) the skeleton of geological features, and establishes a priori values for the resistivity of the assumed geological bodies. The method estimates the resistivity distribution in the subsurface, which are closest to the specified geometric elements and produce an acceptable fit to the observations. The analysis of an obtained solution may help in modifying the a priori outline for the sources, allowing, in this way, that successive inversions be performed until the solution fit the data and make geological sense. Among the attractive features of the presented method are: (i) the anomalous sources may present a resistivity larger or smaller than the resistivity of the host rocks, (ii) several host rocks, enclosing or not anomalous sources, may be crossed by the traverse, and (iii) the resistivity contrast between the anomalous source and the host rock may be either abrupt or gradational. Tests on synthetic data reveal, among other relevant features, excellent results in estimating the attitudes of variable-dip faults, which are of particular importance in regional tectonic studies, and in delineating diabase sills within sedimentary basins, which represent a severe obstacle in oil prospecting. The method also allows the joint interpretation of the target sources and the sources causing static shift. Tests using data from COPROD2 shows a estimative of the resistivity distribution in agreement with the geological knowledge about the area. The second contribution relates to the design of geophysical experiment. By means of several indicators, particularly the density information matrix, it is shown that one can access the theoretical resolution of the data, which guides the explorationist in carrying out a survey design. The survey design optimization allows determining the most suitable observation locations and periods to produce a more precise delineation of sources, whose locations are approximately known.
Acesso aberto (Open Access)
Informação a priori na inversão de dados magnetotelúricos
(Universidade Federal do Pará, 1999-12-29) LUIZ, José Gouvêa; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This thesis includes two studies applied to the inversion of magnetotelluric data. In the first the dimensions of the subsurface parametrizing grid are determined knowing the resistivities of the heterogeneity and of its surrounding medium; and the other deals with the use of derivative operators of order greater than one to stabilize the inversion problem. In the first study the satisfactory results are obtained only if the resistivity errors are less than 20%; while in the second the results show that the operators of order greater than one can be more effective than conventional first derivative operators because they yield better resolution of the subsurface resistivity heterogeneity besides acting as stabilizers. These studies are new in the inversion of magnetotelluric data since, so far, resistivity of the grid blocks have been obtained with a fix grid, using the first derivative operator as stabilizer. In these studies, bi-dimensional models representing a subsurface with one and two prismatic heterogeneity are used. The effectiveness of the inversion techniques have been evaluated by applying them on both noisy and noise-free synthetic data, and on COPROD2 field profile. Also, a comparative study to analyze the effectiveness of the creeping and the jumping inversion techniques is included in this thesis. This study shows that a priori information on the parameters can be introduced in the creeping as easily as in the jumping contrary to what is mentioned in earlier publications.
Acesso aberto (Open Access)
Modelagem numérica da influência do eletrojato equatorial em dados magnetotelúricos produzidos por estruturas tridimensionais
(Universidade Federal do Pará, 1993-02-05) GONZÁLEZ CARRASQUILLA, Antonio Abel; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
South America presents several unique geomagnetic features, one of which is the Equatorial Electrojet, a current system which extends east - west in Northern Brazil, for almost 3500 km. Considering the fact that the influence of the Equatorial Electrojet can be detected at great distances from its central axis, it is important to understand its effects on magnetotelluric exploration in Brazil. In magnetotelluric prospecting, the influence of the equatorial electrojet has been modelled for both one and two - dimensional geological media, employing both analytical and numerical techniques solutions such as finite element and finite difference. Three-dimensional geological media have been modeled by "thin – layers "using a "thin sheet" algorithm. Lines of current, gaussian electrojets and undulatory electrojets have been used as induction sources to simulate the equatorial electrojet in these algorithms. In this thesis the principal objective is to model the effects of the equatorial electrojet on three - dimensional structures commonly found in geophysical prospecting. To accomplish this, we have computed numerical solutions of the integral equation for three - dimensional media using the inductive sources mentioned before. As previous works have indicated, our results also show that the influence of the equatorial electrojet is prominant only for frequencies lower than 10-1 Hz. This influence decreases with distance but is detectable up to as far as 3000 km from the center of the electrojet. Thus, the presence of peaks in the apparent resistivity profiles over a homogeneous half - space indicates that the influence of the electrojet is more noticeable for this kind of medium. These peaks display different amplitudes for each type of simulated electrojet, and the peak locations also change from one electrojet to the other. However, when we use more geologically realistic one - dimensional models such as a stratified media, the effect of the electrojet source diminishes considerably and the results do not vary greatly for the different kinds of electrojet employed in the model. This effect is caused by the electromagnetic energy dissipation due to the presence of stratified conductive layers within the media. Within the 3000 km region, the three - dimensional electromagnetic response can be larger or smaller than the plane wave response, depending on location body, frequency, kind of the electrojet and geology. When the apparent resistivity is larger than the plane wave response, there is a spread between the one and the three-dimensional sounding curves caused by the electrojet, as well as a widening of the profile anomaly caused by the three - dimensional inhomogeneities relative to the one due to a plane wave. When the apparent resistivity is less the two kinds of sources yield anomalies approximately equal as well as a shortening of the profile anomaly. On the other hand, the phase usually shows an inverted way related to the apparent resistivity. This means that when one phase goes up the apparent resistivity goes down, and vice - versa. Similarily at high frequencies, the one and three - dimensional phases are offset, while at low frequencies they are the same, except for the undulating electrojet with undulation factor α = -2 and -3. Our results also show that the geometric characteristics of three-dimensional structures, such as their orientation relative to the direction of the electrojet and the dimensions of their principal direction, change the response due to the electrojet source as compared to a plane wave source. For example, when the three - dimensional structure is rotated 90 degrees, relative to the direction of the electrojet, there is a change in polarization (of the electric and magnetic fields) but there are no changes in the values of apparent resistivity at the center of symmetry of the structure. When the measurements are taken away from the center of symmetry changes in the apparent resistivity are shown when compared with the unrotated structure. This is due to the persistente of the galvanic effects at the center of the structure and the presence of inductive effects at the periphery of the three - dimensional body. When we elongate the principal direction of the three - dimensional structure, the magnetotelluric soundings start to approach those of two-dimensional structures. This is more noticiable in the XY polarization. Even so, the responses of the models tested are still quite different from the responses of the quasi two - dimensional structures. Nevertheless, the effects of the electrojet in structures with elongated principal direction are very similar to those present in smaller structures, considering the differences between the soundings of both kinds of structures. On the other hand, the apparent resistivities of this kind of elongated structures show a great increase at the edges of these structures, both for the plane wave or electrojet sources. This effect is caused by the chanelling of the current along the main direction of the structure. The modelling of gelogical structures in the Marajó Basin, confirms that the effects of the electrojet can be detected even in small horst and graben structures located at great distances from electrojet center. Electrojet effects are noticeable in both one and three - dimensional media for two frequency bands, one near 10-1 Hz and another band, between 10-3 Hz to DC. This possibly reflects the influence of a resistive geological basement and a resistive lower crust, respectively. The results of the analysis, using the electrojet as an inductive source show that at low frequencies the computed responses from the magnetotelluric soundings can be strongly distorted, both by galvanic effects in the three - dimensional structure as well as the presence of the electrojet. Therefore, if the equatorial electrojet effects are not accounted for, a misleading interpretation magnetotelluric data will result. Similarily, a three - dimensional model should be used to interpret the data, instead of one-dimensional Tikhonov-Cagniard model.
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.

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