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).

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Acesso aberto (Open Access)
Desenvolvimento de técnicas de filtragem e sua aplicação a dados aeromagnéticos da Bacia Amazônica
(Universidade Federal do Pará, 1976-08) SILVA, João Batista Corrêa da; LOURENÇO, José Seixas
In this thesis two filtering methods were combined in order to enhance some features in an aeromagnetic map such as lineaments and anomalies caused by sources of large proportions. The visual filtering consisted of stereoscopic examination of pairs of maps. The numeric filtering involved the elimination of the undesirable spectral components (noise) and the enhancement of the desired spectral components (signal). Both methods were tested on aeromagnetic maps of the Middle Amazon Basin. All the numeric filtering operations were done in the space domain by convolving the aeromagnetic data with a 13x13 filter matrix. Special attention was paid to the preliminary analysis of the spectrum, as a guide in filter design. The results of the application of each filter was discussed in detail. A further topic also dealt with was basement fault models. The model analysis established a complementary interpretation method to the filtering methods. A structural interpretation of the Middle Amazon Basin's basement was provided. The dominant aeromagnetic trends were E-W, N60°W, N-S, N45°E and N60°E. Two grabens were quite evident. The younger one trends N-S, while the older one strikes N45°E. Two anomalies caused by sources of intermediate and large proportions (15-60km) exhibit remanent magnetization and high values of negative inclination, which shows that the Amazon Basin would be southernmost in the Paleozoic. In performing the integration of aeromagnetic data with other geophysical data avaiIable, the interpretation from other data proved consistent with the aeromagnetic interpretation, illustrating the reliability of the techniques employed in this thesis.
Acesso aberto (Open Access)
Interpretação de anomalias magnéticas usando integrais finitas dos momentos das componentes
(Universidade Federal do Pará, 1976-08) LEÃO, Jorge Wilson Delgado; LOURENÇO, José Seixas
The objective of this work is to develop a methodology to determine the parameters of a body causing a magnetic anomaly. This methodology is based on the use of analytic expressions, derived for the finite integrals of the moments of the anomaly of total intensity and of the Hx, Hy and Hz components, according to the cartesian coordinate system. By fitting these analytic expressions and the numerical integrals of the moments of the magnetic components obtained from a geophysical survey, the parameters of magnetization, position, depth and dimensions of the anomalous body can be obtained through an iterative process. When only one magnetic component is known, the method is still applicable, since the other components may be obtained by means of a mathematical filtering scheme. This methodology was succesfully tested for point dipole and line of dipoles models, which are widely used in Geophysics to interpret magnetic anomalies caused by geologic bodies which have one or two horizontal dimensions much less than the depth.
Acesso aberto (Open Access)
Tratamento quantitativo de anomalia de potencial espontâneo
(Universidade Federal do Pará, 1976-10) GUERREIRO, Sonia Dias Cavalcanti; LOURENÇO, José Seixas
Acesso aberto (Open Access)
Relaxações dielétricas do sistema carvão-pirita
(Universidade Federal do Pará, 1978-07) PAIVA, José Airton Cavalcante de; ALVAREZ BEJAR, Román
Acesso aberto (Open Access)
Estudo em laboratório de anomalias de potencial espontâneo
(Universidade Federal do Pará, 1978-11) FERREIRA, Lindalva do Carmo; ALVAREZ BEJAR, Román
This work is an application in the laboratory of the Spontaneous or Self Potential method of field geophysics. Pairs of differente rocks and residual soils were placed in contact in the laboratory to determine whether they could produce a difference in potential, as had been occasionally observed during field investigations. The samples of rock used are limestones, basalts, and rhyolite from the area of "Caldera de Los Humeros" in Mexico. The residual soils are from the area MM1 of "Serra dos Carajás", Pará, Brasil. The measurements were carried out using dry systems (room humidity) and humid systems (water added). The changes of measured potential for rocks and soils in the laboratory show differences in average values ranging from 5mv to 50 mv from one side of a contact to the other. Thus, the result of this investigation in positive and indicates that changes in Self Potential values can originate from changes in superficial soil type or by contacts between different types of rocks.
Acesso aberto (Open Access)
Métodos geofísicos aplicados a prospecção de água subterrânea no município de Ponta de Pedras - Ilha de Marajó - Pará
(Universidade Federal do Pará, 1979-02-22) KOBAYASHI, Carmelina Nobuko; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
This work was perfomed in an area of 25 km2 in Ponta de Pedras, Marajó Island. The purpose of this work was to study the geometry of the aquifers and to select favorable zones for development of ground water. Two geophysical methods were used: seismic refraction and electroresistivity. The first one was used experimentally intending to find out the possibility of its application in the area. The seismic results obtained confirm the original expectance of the existence of shallow geologic horizon at the subsurface, not able to be distinguished by the seismic method. Vertical electrical soundings and horizontal perfil resistivity were done with the electrical resistivity method. The results of the interpretation point out favorable areas in the central part of the study area and in a zone extending to the north of the prospected area. The thickness of the shallow aquifer varies between 0 and 50 m.
Acesso aberto (Open Access)
Eletroresistividade aplicada ao estudo de água subterrânea no município de Salvaterra-Marajó-Pará
(Universidade Federal do Pará, 1979-08-14) BEZERRA, Cesar Augusto Campos de Alencar; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
A pilot area of 200 Km2 in Salvaterra county along the east edge of Marajo island was selected to make a study of groundwater using electrical resistivity methods. Sixty-five Schlumberger vertical electric soundings and six horizontal Wenner resistivity profiles were made. The sounding data were first interpreted individually with the auxiliary point method, and later reinterpreted in groups using an iterative curve-calculating routine. The results show the range of resistivities in this area vary between 5 and 40000 ohm-m. The aquifer appears to be defined by a sequence of layers with resistivities between 500 and 4000 ohm-m. In this pilot area three favorable sub-areas were delimited, covering about 40% of the area, where the interpreted aquifer thickness is in excess of 20 m and sometimes as great as 80 m. The depht to the top of this aquifer zone varies between 1.3 and 6.6 m in the three sub-areas. The geophysical interpretations were verified by four shallow drill holes (to 20 m). Two of these, located within the favorable sub-areas, showed dominantly sand sediments and the other two, drilled outside the favorable areas, showed much more clay and organic material, but limited amounts of fine sand. Analyses of water samples from many existing wells and of the new drill holes show that, in general, there are no problems with the quality of these shallow groundwaters. The only exceptions are several wells withing 100 m of Marajo Bay which show considerable increases in salinity during the height of the dry season, when the Bay waters also have maximum salinity. The ages and origin of the long oval belts of sand are not known with certaint, but they could be channel sands deposited by an ancient river or distributary of this deltaic region. The results of this research have outlined three large zones of good groundwater potential which will be very important for the agricultural and industrial development of this region.
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)
Hidrogeologia da área piloto Ponta de Pedras - Ilha do Marajó
(Universidade Federal do Pará, 1979-08-14) PIUCI, Jacyro; REBOUÇAS, Aldo da Cunha; http://lattes.cnpq.br/7322958772719317; FENZL, Norbert; http://lattes.cnpq.br/6834981018643186
A shallow aquifer which is very important for the development of the Ponta de Pedras region has been found and studied. Most of this aquifer is semiconfined, excepted in some areas where he can be considered as beeing free. In various wells drilled in the western part of the area and also near Igarapé-Vilar, the aquifer could not be detected. The maximum depth of the top of the aquifer reaches 14m. The depth of the base of the aquifer and its thickness exceeds respectively 34m and 17m in the well at Mangabeira. Its recharge is principally by means of meteoric water and the infiltration rate was estimated to be 106 to 107 m3/day/km2 for the month of February 1977. The effective porosity was estimated to be between 25% and 37% permiting the calculation of a volume of stored ground-water of 250x109 liters. The average Darcy coefficient (K) of the aquifer system is approximately 200/l/day/cm2. The studied ground water has the following physical and chemical characteristics: pH always acid, between 2,4 and 6,7; electrical conductivity between 13 and 2000 μmhos/cm, the most conductive waters being from the region of the basin of the Tijucaquara River; silica with average concentration of 10,4 mg/l; total iron with very low concentration implying soft water in most of the samples; manganese with a mean concentration of 0,15 mg/l; nitrogen and phosphorous with low concentrations. Sodium and potassium showed the best correlation with seasonal precipitation. At the end of the rainy season the ground water shows extremely low concentrations of nearly all elements. During the period of infrequent rainfall and until the beginning of the rainy season, the salinity increases in some areas and this causes the water to be non-potable. The domestic use of these waters is normally limited by the following characteristics: pH too acid; total iron much over 0,3 mg/l with a maximum of 4,0 mg/l; lack of fluorides; excess of chloride in one well but generally very low contents of most chemical elements. The use of these ground waters in agriculture may be limited in some parts of the area where the pH is less than 5; the conductivity too high during the dry season and when the sodium content reaches 83% to 97% (SAR).
Acesso aberto (Open Access)
Geofísica aplicada à prospecção de água subterrânea na área do Rio Paracauari – Ilha de Marajó - Pará
(Universidade Federal do Pará, 1979-08-24) CAVALCANTI, Gerardo Majela Lima; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
The Rio Paracauari pilot study area in the east-central part of Marajo Island is typical of the low, planar, natural grasslands of this deltaic region. The cattle ranches of this area suffer considerable losses during the dry seasons because the groundwater is generally brackish. A 250 Km2 study area was chosen in wich geo-electrical methods were used to search for and outline any shallow fresh-water aquifers which might be present. Fifty-three Schlumberger vertical electric sondings were made on a grid of approximately 2 by 2.5 Km, as well as 5 horizontal Wenner resistivity profiles. The VES data were initially treated with the auxiliary point method and later, groups of models were constructed using a curve-generating routine to fit the field VES curves. The geo-electric results show two types of terrain. The first with higher resistivities, reaching to 950 ohm-m, is characterized by long (10 Km) belts up to one Km wide with dominantly sand sediments beginning near surface and extending to depths on the order of 60 m. These are saturated with potable water, with the water table typically at depths of only 2 to 5 m. These aquifer zones cover less than 30% of the area. The second type of terrain occupying the rest of the area has resistivities less than 6 ohm-m and often as low as 0.5 ohm-m, and is interpreted as silts and clays with saline water. Water samples taken from the surface and from shallow wells show a range of resistivities between 1.1 and 362 ohm-m. Seven drill holes were made, to depths as great as 54m. These verified the geophysical interpretations and clearly showed the differences between the channel-fill deposits which constitute the aquifers (channel sands or point bar deposits) and the fine, organic and clay-rich sediments of the adjacent flood-plain environment. Several of the drill holes were completed as water wells and now supply potable water on two of the ranches. Although the ages of the sediments are unknown, these large channels could have been excavated during the Pleistocene sea-level minimum and later filled with coarse, medium, and fine sand as sea level rose again, diminishing the velocities of the rivers or distributaries passing through the region. There has since been a drainage reversal, with the belts underlain by channel sands now existing as slight topographic highs, which could be explained by differential compaction.
Acesso aberto (Open Access)
Métodos geofísicos aplicados ao mapeamento geológico numa área da Região Bragantina-Pará
(Universidade Federal do Pará, 1979-12-13) FERREIRA, João Batista; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
The following research is about the aplication of geophysical methods in a pilot area in the Bragantina region, NE of Pará, as an auxiliary to geologic mapping. The area is covered with thick tertiary sediments and presents occurrences of industrial rocks such as limestone and granite, representing an outcrop surface smaller than 5% of area. Studies carried out by Goulart (1978) revealed that it is impossible to determine parent rock of the superficial material, of the A horizon. This makes difficult the surface geologic mapping and can lead to exaggerated extrapolation of the contacts, such as that which took place in the preliminary mapping. The geophysical methods applied in the area are: Electroresistivity and Magnetometry. Some mechanical sounding was carried out also. The results obtained contributed to a better understanding of the structural features mapped, and also made it possible to discover other important features. Two areas were defined for future detail study, with the objective to distinguish between the limestones of better and poorer quality. In reference to the granite region a new mapping of that area is suggested.
Acesso aberto (Open Access)
Equipamento eletromagnético para prospecção geofísica e modelamento reduzido
(Universidade Federal do Pará, 1979-12-13) FONSECA, Nélio Raimundo Medeiros da; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
In the last decade, electromagnetic methods constituted an efficient technique for prospecting of minerals. The objective of this work was to develop an equipment for quantitative electromagnetic prospecting of conducting bodies using dipole-dipole technique. The equipment can also be adopted for reduced scale model experiments. The equipment, in principle, measures the variation in the mutual electromagnetic coupling of two coils, namely the transmitting and the receiving coils. Ground will also contribute to this coupling when the coil system is used near it. This mutual impedance of the transmitter – receiver - ground system shall vary in the vicinity of an anomalous conductor which is manifested as a variation in the signal induced in the search coil. The equipment consists of two sets: a transmitter and a receiver alongwith accessories. The transmitter generates an electromagnetic field at frequencies of 520 Hz and 3090 Hz, and a reference signal for the receiver which is connected through a cable. The receiver first separates the anomalous secondary e.m. field from the uniform primary field. Then this secondary signal is decomposed into two orthogonal components, one inphase and other in quadrature with respect to the primary field. Amplitudes of these phase components can be measured directly in terms of the percentage of the primary field within a precision of 1%, using two amplitude scales. The sensibility of the receiver is 0.5 μV. Electronic circuits were rigorously tested by using precise laboratory instruments. The performance of the e.m. equipment was verified (a) by carrying out classical e.m. model experiments in the laboratory and comparing them with the published results; and (b) by taking some e. m. traverses in an area near Araci, in the State of Bahia, which has been prospected by "Rio Doce Geologia e Mineraçáo S/A-DOCEGEO". All these tests verify a highly good performance of the equipment in terms of its precision and usefulness for field work.
Acesso aberto (Open Access)
Métodos geofísicos aplicados à arqueologia no Estado do Pará
(Universidade Federal do Pará, 1979-12-13) ALVES, José Jerônimo de Alencar; LOURENÇO, José Seixas
This study is the first attempt to apply geophysical methods in research into archeological sites in Brazil. These methods were used with the objective of detecting archaeological remains in the sub-soil in two archaeological sites in the State of Pará: site PA-SA-4, from the Areão archaeological phase, and "Ilha dos Bichos" site from the Marajoara phase. In PA-SA-4, the magnetic method was employed, while at "Ilha dos Bichos" both electro-resistive and magnetic methods were used. The electro-resistive method made possible the determination of the occupied layers, since these behave like geoelectrical beds, different from those of the geological formations of the area. It also permitted the detection of compact layers of clay, planars and horizontals, of archaeological origin, located on the east side of the "Ilha dos Bichos". The magnetic method led to the detection of ancient firing places, thanks to the thermo-remnant acquired by the clay in the process of heating/cooling, and thus permitted the discovery of ancient kilns, as well as a burial urn under a thick bed of burnt clay.
Acesso aberto (Open Access)
Métodos geofísicos aplicados à prospecção de cobre na Serras de Carajás - Pará
(Universidade Federal do Pará, 1980-03-20) HOOKER, Enrique Campbell; SAUCK, William August; http://lattes.cnpq.br/6655455534234531
Through an agreement between the Federal University of Para and the Rio Doce Geologia e Mineração Company, a geophysical research was carried out in the area known as ALVO 1, MM1, Serra dos Carajás, south of Para, Brazil. The magnetic, electrical induced polarization and AFMAG methods were applied, to detect the distribution of copper in subsurface. This work was carried out both on a 9 km2 reconnaissance area (ALVO 1) and on a 0.63 km2 detail area (Corpo 2). The AFMAG method detected various conductors displaced from each other by possible faulting. The orientation of these conductors, coincides with the dominant strike of the body of schist which contains copper sulfide, associated with an iron formation. The close correlation they have with the magnetic anomaly, may imply that the AFMAG anomalies are caused, firstly by the iron formation and secondarily by the sulfide. The magnetic method detected strong anomaly over the iron formation. The displacement of the segment of the anomaly impleis faulting. This method is very important in the area due to the association of the sulfide zone with the iron formation. The electrical induced polarization/resistivity method detected anomalies with PFE, varying from 6 to 12% approximately. Several of these anomalies were confirmed by mechanical sounding and laboratory research (Rocha, 1979). It was verified that these anomalies were caused by a combined effect of the iron formation and the sulfide body. The geophysical research carried out at this pilot area turned out to be very important considering the fact that its results can be successfully extrapolated to a larger area with the same geological characteristics.
Acesso aberto (Open Access)
Métodos geofísicos aplicados ao ultramafito da Serra de Quatipuru - Estado do Pará
(Universidade Federal do Pará, 1980-05-13) BELTRÃO, Jacira Felipe; LOURENÇO, José Seixas
The objective of the present work is the application of geophysical methods for the economical evaluation of Ni, Cr and asbestos occurrences in an area of 6.25 km2 - Malha 2 - in Sierra Quatipuru 50 km SW of Conceição do Araguaia, in the South of the State of Pará. Magnetic methods were applied initially for reconnaissance, using total and vertical field magnetometers, in order to locate anomalous zones of asbestos and cromite, associated with magnetite. The electromagnetic method of dip-angle was used to define conductive zonas associated with possible mineral deposits or structural features. Based on the interpretation of the magnetic and electromagnetic data several areas defined for the application of the induced polarization-resistivity method aiming at possible deposits of dissemineted sulphides as the main targets. The possible occurrences of mineral deposits in the area were correlated with the geophysical anomalies and were investigated further by drilling. It was concluded that occurrences do not have economic interest. However, the application of these geophysical methods was successful in the investigation of this problem as they have advantages over the more conventional methods which are expensive and time consuming.
Acesso aberto (Open Access)
Efeitos do manto de intemperismo sobre anomalias eletromagnéticas provocadas por corpos tabulares inclinados: um estudo através de modelamento reduzido
(Universidade Federal do Pará, 1981-09-18) SILVA, Lúcia Maria da Costa e; VERMA, Om Prakash; http://lattes.cnpq.br/2723609019309173
Electromagnetic methods have been extensively applied in the prospecting of sulphide bodies and other conducting materials. The interpretation of e. m. data is based on the results obtained either with reduced scale or analytical modelling. In most models, the host rocks, the overburden as well as the halo of disseminated sulphides are considered highly resistive although in nature they are often weakly conductive. Presence of a well-developed conducting overburden in tropical and sub-tropical regions and a saline rich crustal layer in semi-arid regions have been found to modify significantly the e. m. anomalies in practice. Therefore, the parameters of the target, determined on the basis of simple models where the presence of the conducting environment is neglected, are found to be in considerable error. The effects of the overburden on the e. m. anomalies of a dipping tabular ore body were studied with reduced scale models for varying response parameters of both the overburden and the orebody, and also for different depths and dips of the target. The overburden and the orebody were represented by metallic sheets of varying thickness in the scale model developed in accordance with the law of electromagnetic similitude. The results of these investigations show that the overburden affects the anomaly by causing: a) phase rotation; b) amplitude reduction; c) base level displacement; d) reversion of the quadrature; and e) appearance of an extra peak in the quadrature in the case of low dipping models. The last two effects complicate the quantification of the anomalies but, on the other hand, provide a qualitative indication of the response parameters of both the ore body and the overburden. The results were assembled in the form of Argand diagrams and, finally, an interpretation scheme is suggested for the e. m. field data on the basis of these diagrams.
Acesso aberto (Open Access)
Paleocanais: uma opção para a prospecção de água subterrânea rasa na Ilha de Marajó
(Universidade Federal do Pará, 1981-09-18) PORSANI, Milton José; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
Acesso aberto (Open Access)
Métodos geofísicos aplicados a prospecção mineral na região do Igarapé Chega Tudo, Maranhão - Brasil
(Universidade Federal do Pará, 1982-12-29) ORTEGA GONZALEZ, Moisés; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work of geophysical prospecting was carried out on a NW-SE geological lineament zone from Chatão - on the right side of Gurupi river up to 5 km beyond Chega Tudo in the State of Maranhão. The field work with the application of magnetic (total field), radiometric, electroresistivity (Schlumberger, Wenner and Half-Schlumbergere) and EM (Sligram) methods was carried out in two stages: a) reconnaissance survey of a 22 km2 area with magnetic and radiometric methods, and b) detailed survey of a 3.5 km2 area with electroresistivity and EM methods. The objectives were to study the main geoelectrical structures associated with mineralized zones, and to select among the used geophysical techniques the one with greater resolution to locate sub-surface quartz veins with gold mineralization. The magnetic method shows a large anomaly in the NE region of the area of reconnaissance, whereas the radiometric method indicates only the background value everywhere. Therefore the latter will be not discussed. The electroresistivity and EM surveys indicate many important anomalies on the detailed area. The electroresistivity and magnetic interpretation were carried out by computacional techniques whereas the EM data were interpreted by phase diagrams. The results show a good correlation between the magnetic and electroresistivity anomalies and the mineralized zone. The EM method was not found effective because it could not discriminate the anomalies inside and outside the mineralized zone. All anomalies were tested with well logs. Since the studied area has the same degree of difficulties for geological prospecting as any other similar area in the Amazon region, the results presented here show that the magnetic and electroresistivity methods, which turned out to be very effective as exploration tools for prospecting gold at Chega Tudo region, could be applied to any other environment with similar geological characteristics.
Acesso aberto (Open Access)
Efeitos do strike das estruturas geológicas de duas dimensões nas pseudos-seções de IP resistividade
(Universidade Federal do Pará, 1983-12-22) GUIMARÃES, Paulo Buarque de Macedo; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The IP-resistivity surveys carried out in the Carajás mineral district were not orthogonal to the geological structures because they were executed on lines already opened by the geochemistry crews. This fact motivated us to study theoretically the influence of the direction of the IP-resistivity survey lines with respect to the strike of the structure. We used Rijo's finite element program (1977), developed for surveys perpendicular to the structures, with the necessary adaptations. The main modification was in the inverse Fourier transform routine. For the simple case of the perpendicular surveys, the inverse transform is an easy discrete integral with seven points. However, for oblique surveys the integrand is oscillatory and therefore the integral to be evaluated is more complex. We adapted a method presented by Ting and Luke (1981) using eighteen points in each integration. It was found that the effect of the direction of the line with respect to the strike is negligible for angles greater than 60 degrees. For smaller angles the effect is to "open" the anomaly with minor changes in its center. There is no obvious way to compensate this effect with changes in the model parameters.
Acesso aberto (Open Access)
Desenvolvimento de um magnetometro a precessão nuclear
(Universidade Federal do Pará, 1983-12-22) ALVES, José Geraldo das Virgens; BISCHOFF, Jürgen H.
The objetive of this thesis was to develop a proton precession magnetometer for geophysical prospecting and base stations. The proton precession magnetometer measures the total magnetic field intensity. It operates on the basis of nuclear magnetic resonance by determining the precession frequency of protons of a non vicous liquid in the terrestrial .magnetic field. The magnetometer consists of two parts: the sensor and the measuring instrument. The sensor is a solenoildal coil with the liquid as the core. Three different liquids were tested: water, n-propanol and a laboratory grade kerosene. The last one was selected because it offers more high precession signal amplitude and, consequently higer signal/noise ratio. The measuring system contains the tunning and amplifier circuits and the logical circuits for programming the measuring cycle and count on the precession frequency. Each cycle has a duration of 3 seconds; 2.3 seconds for the polarization and 0.7 seconds for the reception of the signal. The operation can be carried out manually, automatically or by remote control. The precession signal is amplified selectively in one of 14 different syntonized band-passes for measurements between 22000 and 95000 gammas. The precession frequency is multiplied by 64 and counted for a time of 0.36699 seconds, selected in consideration to the gyromagnetic ratio of the proton. The number of counted pulses is numerically equal to the value of the magnetic field in gammas. The reading is displaed digitally. For remote operation a BCD output is availabe. The precision of the measurements is 1 gamma. The instrument was tested in field to evaluate signal/noise ratio, supportable gradient and battery consuption. Application teste was carried out to take diurnal variation data and, reconnaissance and detail surveys data on an archaeological site in the Marajó Island-Pa. The test results were confronted with two commercial magnetometers-GP-70, McPhar e G-816, Geometrics - and, with data from Observatório Magnetico Ilha de Tatuoca as well. For all cases, the data comparison showed a good performance of the magnetometer tested.