Dissertações em Geofísica (Mestrado) - CPGF/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/4993

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

Navegar

Agora exibindo 1 - 6 de 6

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)
Condutividade elétrica complexa de rochas
(Universidade Federal do Pará, 1979-08-14) ROCHA, Brígida Ramati Pereira da; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
Laboratory measurements of complex conductivity were made on 28 drill-core samples from area MM1-Prospect 1 of the Carajás Mining District. The objective of this research was to help interpret field geophysical survey of the area using Induced Polarization and AFMAG methods. The laboratory measurements of amplitude and phase of conductance were taken in the frequency interval of 10-3Hz to 104Hz. The method used was the direct measurement of impedance using a memory osciloscope, a signal generator, and two high input impedance differential preamplifiers. The electrode system chosen for the measurements was the 2 electrode platinized-platinum because its frequency response is flat in the frequency range used. AlI the measurements were made at a constant temperature of 24°C±1°C. A petrographic study of the samples was done, using thin sections, polished sections and X-ray diffraction. Copper content, in the form of sulfides, was determined using atomic absorption. As a result of the petrographic study, the samples were classified in five distinct groups: granite, biotite schist, amphibolite and magnetite quartzite-iron formation. The grade of Cu was variable in the five groups, ranging from 50 ppm to 6000 ppm. In the conductivity measurements it was observed that, among the five groups, the samples of iron formation gave the largest variations with frequency. The granite samples had spectra flatter than those of schist or amphibole. In conclusion, these measurements show that the field Induced Polarization and AFMAG anomalies near these three drill holes (F1, F2 and F3) are due primarily to the magnetic iron formation, and secondarily due to associated low-grade chalcopyrite mineralization.
Acesso aberto (Open Access)
Um estudo da estabilidade mecânica de poços a partir de perfis geofísicos em formações pouco consolidadas
(Universidade Federal do Pará, 1992-10-05) SOARES, José Agnelo; LUTHI, Stefan Moritz
The stability of boreholes can be evaluated from the rocks' mechanical properties, which are compared to the acting stress field using an appropriate failure criterion. The elastic parameters can be measured on rock samples in the laboratory (static parameters) or can be calculated from acoustic velocities and the formation bulk density (dynamic parameters). Dynamic elastic parameters can be obtained from wireline logs, but in poorly consolidated rocks the arrival of the shear wave is often masked by the Stoneley wave, which may travel faster and has a higher amplitude. The giant Campos basin offshore Brazil contains such rocks. We performed laboratory measurements of acoustic velocities on samples from this basin and we use them to correct the shear velocities obtained from waveform processing. Based on this we calculate elastic properties (e.g. Poisson's ratio, bulk modulus) of the rocks which we assume to be homogeneous and isotropic. Mechanical properties such as the shear, compressional and tensional strength are obtained from empirical relations with mineralogy and the elastic parameters. The stress field was evaluated using an uniaxial strain model and actual formation pressure data. From this, near-wellbore stresses were calculated using the modified Kirsch equations. The stress concentrations around the wellbore are illustrated for a number of representative cases. Wellbore failure occurs when these local stresses exceed the formation strength. Three different failure criteria were tested: The Mohr-Coulomb, the Griffith and the Fairhurst criterion. They respectively represent failure due to shear, tension and a combination of compression and tension. Actual wellbore instability was determined from the differential caliper measurement. Two sequences were analyzed, both consisting of intercalations of sands and shales. We found that the Fairhurst criterion offered the best match with the actually observed wellbore failure, and we propose it as a method to predict borehole instability for this basin.
Acesso aberto (Open Access)
Estudo da hidrogeologia no município de Palestina do Pará usando o método de eletroresistividade
(Universidade Federal do Pará, 1998) MONTEIRO, Edson Cardoso; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work of geophysics applied to hydrogeology was carried out in the southeast of Pará in the Palestina district at the margin of Araguaia river. Ironically there exists problems of water supply for the city population. This is due to the fact that it is not economical to mantein a treatment station and to transport the water from the river to the city. Since is this case pumping groundwater wells are more economical than take water from the river, the Geophysics Department (UFPa) was asked to cooperate with the Health National Fundation (NSF) to carry out a electroresistivity survey to help the groundwater investigation of the area. There were made 21 vertical electrical soundings (SEV) with the Schlumberger array with 500 m and 50 m maximum AB and MN respectively. For the interpretation it was used the programs EGSLIB/SEV1D for forward modeling and EGSLIB/SEV1D1N for the inversion of homogeneous isotropic layered models. Based on the results of the interpretation of the 21 electrical soundings and the local geology, three contour maps and six geoelectrical sections were made. From the six horizons that are shown in these sections, the fourth one with thickness of 10 to 79 m at the depth of 50 m and resistivity of 150 — 850 ohm-m is the most suitable for groundwater exploration. The final analysis of the maps and geoelectrical sections show that the best place to locate the wells is near the area used today for capturing water.
Acesso aberto (Open Access)
Estudo da resposta de perfilagem de indução de camadas finas com diferentes arranjos de bobinas: modelamento analógico
(Universidade Federal do Pará, 1993-12-16) CARVALHO, Paulo Roberto de; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
Analog model studies were carried out comparing the eletromagnetic responses of various two-coil systems in a borehole, in order to improve the vertical resolution of the indution tools. For this purpose geoeletric models, simulating well-logging situations in the stratified beds of varying thicknesses, with or without fluid invasion, were constructed at a reduced scale of 20. The sensitivity of the system to measure relative fields (secondary/primary) is of the order of 0.01 %. Following conclusions were drawn after analysing the response profiles obtained for a coaxial and a coplanar coil systems: • In case of thin conducting beds placed in relatively resistive beds, such as shale beds in sandstones containing hydrocarbons, the coaxial system shows a better resolution than coplanar system both in determination and estimating the thickness of thin beds; • On the other hand, in the presence of thin resistive beds placed in relatively conductive zone, such as sandstone containing hydrocarbons lying in a shale, the coplanar coil system gives a better vertical resolution than the coaxial system; • "Shoulder effect" is much more pronounced in the coaxial system than the coplanar coils; • In case of thick beds, bed-boundaries are well defined in the coplanar coil system response. However, when the thickness is reduced the small oscillating signal indicating the interfaces disappears.
Acesso aberto (Open Access)
Estudo geofísico regional sobre águas subterrâneas na Ilha do Marajó - Pará - Brasil
(Universidade Federal do Pará, 1984-09-21) GONZÁLEZ CARRASQUILLA, Antonio Abel; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
IDESP and NCGG have conducted field work on the East half of the Marajó Island performins over 800 VES (Vertical Electrical Sounding), in the last ten years. The work was done for hidrogeological purposes. A large number of these VES, however, were not adequately interpreted at the time due to lack of application of good quantitative techniques. We have then reinterpreted all the date using recently developed interpretation techniques with the following goals: to present the regional distribution of the main aquifers in the area; to group the VES in characteristics families; to test the goodness of the interpretation; and to propose bidimensional modelling as an alternative technique for interpretation of some of the VES performed in specific locations in the area. Initially, we did interpretation on assumption of a stratified earth and could identify three types of aquifers: 1. The first, named "deep aquifer", is located at depths deeper than 50m and can be found all over the prospected area. It seems to be associated to the upper layers of the Marajo Formation or it could be related to the highly resistive material of the deep layers of Pará Group. 2. The second, named "shallow aquifer", occur in the Southern and Southeastern portions of the area at depths between 10 and 50m. It is associated to the sandy lenses of the Para Group. 3. The least is represented by paleochannels and similar structures. It occurs scattered in a aleatory way in the area, at shallow depths. We have classified the VES after a detailled study in three characteristic families with its respectives types. This study also provided the preparation of maps of location and thickness of the aquifers, and maps of longitudinal conductance and average resistivity. The last ones helped to divided the region of campos of the Marajó Island in three main zones: 1. The first one is highly resistive and located in the Southern and Southeastern portions of the area. It correlates to the outcropping ground of Para Group. 2. The second, highly conductive, is located in the Central and Northern portions of the area, where occur the scattered paleochannels. The ground in this zone presents the lowest topographic values in the area and in general in contains clays and salty water. 3. The third is fairly resistive and it is related to the ground occurring in the neighborhood of the city of Chaves (Northwest of the region of camps), which are similar to that occurring in the Sourthern and Southeastern portions of the area. At last we have tested the interpretation confidence for the horizontally stratified earth conventional model used by applying the inversion technique on one characteristic VES of each family. We found a very high statistical correlation among the model parameters, probably due to the application of a too simple irrealistic geophysical model to the complex real earth of Marajo Island. The results from digital modelling by the finite element method demonstrated the influence of the abrupt lateral variation in resistivity on VES data. The finite element method was also used for modelling a shallow structure similar to a paleochannel. This result suggests that bi-dimensional models can be very helpfull in interpreting resistivity data collected in the region of camps of Marajo.

Navegar

Navegando Dissertações em Geofísica (Mestrado) - CPGF/IG por CNPq "CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOFISICA::PROPRIEDADES FISICAS DAS ROCHAS"