Navegando por Autor "ROCHA, Brígida Ramati Pereira da"

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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)
Modelo fractal para resistividade complexa de rochas: interpretação petrofísica e aplicação à exploração geoelétrica
(Universidade Federal do Pará, 1995-12-21) ROCHA, Brígida Ramati Pereira da; HABASHY, Tarek Mohamed
Rocks containing disseminated metallics or clay particles in natural environment where electrolytic solutions fill the pore spaces, show a certain type of polarization at low frequencies known as induced electrical polarization. In this thesis, a new model to describe the electrical polarization on rocks was developed, not only for low frequencies, but spanning the entire electromagnetic spectrum used in geolectric prospection. This new model encompasses most of the other commonly used models as special cases, and overcomes some of the known limitations. The proposed circuit analog includes a non-linear impedance r(iwtf)-1 which simulates the effects of the rough surface of the interfaces between the blocking grains (metallic or clay particles) and the electrolyte. This generalized Warburg impedance is in series with the resistance of the blocking grains and both are shunted by the double layer capacitance. This combination is in series with the resistance of the electrolyte in the blocked pore passages. The unblocked pore paths are represented by a. resistance which corresponds to the normal DC resistivity of the rock. The parallel combination of this resistance with the "bulk" sample capacitance is finally connected in parallel to the rest of the above-mentioned circuit. The parameters of this model include the DC resistivity (p0), the chargeability (m), three relaxation times (T, Tf and T0), a grain resistivity factor (δr) and the frequency exponent (η). The fractal relaxation time (Tf) and the frequency exponent (η) are related to the fractal geometry of the rough pore interfaces between the conductive grains (metallic or clay minerals which are blocking the pore paths) and the electrolyte. The relaxation time T is a result of the low-frequency relaxation of the electrical double layers formed between the electrolyte and the crystals, whereas T0 is a macroscopic relaxation time of the "bulk" sample. The grain resistivity factor (δr) relates the resistivity of the conductive grains with the DC resistivity value of the rock. The DC resistivity of the rock and δr are related to the porosity, the electrolyte conductivity and the volumetric ratios between the matrix and the conductive grains. The model was tested over a wide range of frequencies against experimental data obtained for amplitude and phase of resistivity or conductivity as well as for the complex dielectric constant. The data used in this work were obtained from digitizing published experimental data, obtained by several authors from sedimentary, metamorphic and igneous rocks. The results show that the parameters of this model are related to textural and mineralogical aspects of the rocks. This model was introduced firstly as the intrinsic electric property of a homogeneous and polarizable half-space, and it was demonstrated, in this thesis, that the response observed at the surface is equivalent to the intrinsic property of the polarizable medium, been the electromagnetic coupling irrelevant to frequencies lower than 104 Hz. Next, the polarizable medium was embedded as an intermediate layer between two non-polarizable layers with the same De resistivity. The response obtained shows that the frequency exponent of the fractal medium could be determined even when the polarizable medium is at a considerable depth in relation to the dipole-dipole length. This justifies the use of simple models developed to explain the response of laboratory samples to fit field data, and that is being used without a right justification. These results shows the importance of the proposed model to the geoelectric prospection.