Navegando por Orientadores "RIJO, Luiz"

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Acesso aberto (Open Access)
Análise da resolução do modelo de Hughes em sondagens magnetotelúricas
(Universidade Federal do Pará, 1996-02-13) SANTOS, Elinei Pinto dos; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
In most geophysical exploration methods, the interpretation is carried out assuming an uniformily stratified Earth model. Due to the lack of uniqueness in the inversion of eletromagnetic data, all of the methods require theoretical modeling techniques to aid interpretation. In the literature data has usually been interpreted in terms of a one-dimensional conductivity structure; commoly the Earth is assumed to be horizontally uniform so that the conductivity is a function of depth only. In this work a new technique was developed to model a generic two-layer Earth in which the interface separating the layers can be represented by a Fourier series. The technique involves a pertubation method to find the effects of a sinusoidal boundary with small ondulations. Because the first-order pertubation solution is a linear one, we can use the overlap principle and combine solutions for several different sinusoids to form the solution for any two-layer in which the boundary is represented by a Fourier series. Comparison with the finite elements technique, the following conclusions can be drawing: • For one model of a flat, two-layer Earth, the layers separated by an interface whose depth varies sinusoidal in one diretion, the eletromagnetic response are more strong when the first layer tickness is of the order of skin depth of the eletromagnetic wave in the medium; • On the other hand, the aparent resistivity for this model is not affected by the change in the spatial frequency (v) of the boundary; • In case of using the general solution for any two layer Earth that can be developed in Fourier series, this technique produced good results when compared to finite elements technique. The lineralization restrict the application of the technique to small structure, nevertheless, a lot of structure can be model in simple way and the computational time is very fast; • When the dimension of the first layer has the same order of magnitude of the structure, this technique is not recommended because for some particular sounding position, the apparent resistivity curves obtained show a small shift when compared to the curves obtained by finite elements.
Acesso aberto (Open Access)
Análise teórica do problema de Weaver da falha infinita, modo TE magnetotelúrico
(Universidade Federal do Pará, 2003-02-14) GUIMARÃES, Raimundo Nonato Menezes; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
In this work it is shown an analytic solution for the Magnetotelluric TE mode infinite fault, taking in consideration the presence of the air. The solution following the hybrid solution, partially analytic and partially numeric, proposed in 1985 by Sampaio. In his solution he applied eight boundary conditions. We found that four of them are mathematically inconsistent and had to be modified. The modification of them took us to the analytic solution discussed here. This solution is compared with those obtained by Weaver and by Sampaio and with finite element method, using resistivity contrast equal to 2, 10 and 50 between the two sides of the fault. As a result, the analytic solution obtained here for the normalized electric field shows a better fit with the finite element solution then those proposed by either Weaver or Sampaio. This is a very difficult problem and it is still open to a definitive analytic solution. The one shown here is just one big step toward this goal.
Acesso aberto (Open Access)
Aplicação dos métodos elétrico e eletricomagnético para o estudo hidrogeológico no município de Altamira-Pará
(Universidade Federal do Pará, 1998-11-20) BAPTISTA, João Júnior; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work due application of the electroresistivity and electromagnetic geophysical methods in the municipal district of Altamira, in the central area of the State of Pará, with the purpose of mapping the thickness of the aquifer layers and occurrence of diabase intrusions for finding a groundwater wells. In this area are many outcrops of basement crystalline rocks covered with sediments of ages from the Paleozioc to Quaternary. The objective is to obtain water from the sediments of Inferior Devonian, more exactly in the Maecuru Formation. There were made thirty four vertical electric soundings using the Shlumberger arrangement, the AB/2 distance varying from 100 to 500 meters. Also there were made 108 transient electromagnetic soundings with "coincident loop" arrangement, using an square loop of 50 meters. Due to electronic problems and supraparamagnetic noises, the results of this electromagnetic soundings became very distorted, impossible to get from them important geologic information. Then, electromagnetic synthetic models were created to represent the geology of the area. The data were obtained from the field were presented as apparent resistivity curves. They were plotted using a log-log scale and interpreted using the EGSLIB/SEV1D first and then with the EGSLIB/SEV1DIN program. The models used were horizontal, isotropic and homogeneous layers. The interpretation results allowed to determine approximate geoelectrical models which reflects the underground geological situation. The interpretation results from the electric surveys were confronted with the results of the qualitative interpretation from the isogams map of the total magnetic field, allowing to delimit with more precision the zone of incidence of diabase sills, located at the north of the area. From the four distinguished geoeletric horizons, the third is essentially constituted by sands, therefore it is best for groundwater storage.
Acesso aberto (Open Access)
Aplicação dos métodos eletromagnéticos VLF e HLEM na prospecção hidrogeológica no município de São Domingos do Araguaia-PA
(Universidade Federal do Pará, 1996-04-25) FARIAS, Valcir João da Cunha; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work deals with the geological mapping using the electromagnetic methods VLF and HLEM to hidrogeological study of the county São Domingos do Araguaia in the southest of the State of Pará. It was carried out within the cooperation agreement between the Fundação Nacional de Saúde and the Universidade Federal do Pará, through the Geophysics Department and the Curso de Pós-Graduação em Geofísica. Also, we studied the quantitative performance of the VLF method, applied to ground water prospecting. First, we analysed the VLF response, by means of the parameters (tilt angle and ellipticity) of the ellipse of polarization of various geological situations likely to be found in the local under study, and for hidrogeological studies of other areal as well. Also, we evaluated the influence of different physical parameters of selected models using finite elements method. The quantitative analyse of the field data was carried out by trial and error using theoretical curves obtained by means of model simulation suggested by the qualitative interpretation using Fraser's filter and profiles of the HLEM method. The results showed the existence of structural features with trends N-S and NE-SW which correlated with the regional geology are normal and transference faults, respectivily. Based on these results, it was selected several sites for drilling of well with maximum probability of sucess.
Acesso aberto (Open Access)
Avaliação da influência do eletrojato equatorial na interpretação de dados magnetotelúrico bidimensionais
(Universidade Federal do Pará, 1991-02-19) MOTA, Joaquim Perminio de Oliveira; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The magnetotelluric field in equatorial regions violates the plane wave condition due to a strongly concentrated electrical source of current in the ionosphere, called equatorial electrojet. In this thesis, it is analized the magnetotelluric reponses of electrical sources in order to simulates the effects of the equatorial electrojet. There were used two types of sources: an infinite current line and a gaussian distribuition of current density along one the horizontal cartisian coordenates. The apparent resistivities were computed using the Cagniard relationship and compared with the plane wave results. It was also carried out a comparison of the phase of the surface impedance for all three types of sources (plane wave, gaussian electrojet an line source). The differential equation of the models with lateral inhomogeneities were solved by the secundary field technique using the finite element method. The results show that the electrojet has minor influence in the MT responces (apparent resistivity and phase) of shallow geological structure. However, the effect can be very large in the case of deep strutures (more than 5000 m), mainly if their resistivities are larger than 100 Ωm. Therefore, the influence of the equatorial electrojet must be taken in consideration in the interpretation of the magnetotelluric data from sedimentary basins and from crustal studies.
Acesso aberto (Open Access)
Comparação do desempenho dos métodos eletroresistivo e transiente eletromagnético num caso específico de prospecção de águas subterrâneas
(Universidade Federal do Pará, 1998-11-20) CARRASCO, Alfredo Moises Vallejos; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The resistivity method is the most used geophysical prospecting technique for groundwater exploration. However, it is not easy to use it in inhabited areas due to the necessity to spread electrical wires for more than one kilometer, depending on the desired investigation depth. The time domain electromagnetic method, also used for groundwater exploration, does not have this disadvantage, because this inductive method uses a square loop whose dimension is not an obstacle to be used in inhabited places. On the other hand, the electromagnetic method is more sensible to cultural electrical noises. To compare the performance of these techniques in an urban area, we use programs for vertical electric and electromagnetic transient soundings. Both programs were developed by RIJO (1994ab, 1996). In this comparison we consider two cases: in the first, there was defined four types of models (H, K, A and Q), based on the geoelectric sections from Palestina of Pará (MONTEIRO, 1998). In the second, it was considered the 21 vertical electric soundings models carried out in this city using an arrangement type Schlumberger. These models allowed to configure apparent resistivity synthetic curves for the electromagnetic transient soundings. As a result, the apparent resistivity values given as space function (resitivity method) and given as time function (electromagnetic method), allow to obtain a lineal relationship in bilogaritmic scale between the spacing (meters) and the time (miliseconds). Thus, it is possible to estimate a desired investigation depth in urban areas using the electromagnetic method when the application of the resistivity method is limitated by the accessibility.
Acesso aberto (Open Access)
Comparação dos efeitos do eletrojato equatorial nos dados magnetotelúricos bi e tridimensionais
(Universidade Federal do Pará, 1995-11-01) PONTES, Altem Nascimento; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
This work deals with the comparison between the effects of the equatorial electrojet in two and three-dimensional magnetotelluric data. It is composed of two parts: in the first part, which consists in comparing the effects of two-dimensional with three-dimensional models (current line and gaussian electrojet), we determine the dimensions that a three-dimensional structure should have so that the MT response of that structure, in YX and XY polarizations, can be replaced by the MT response of a 2D structure in TE and TM modes, for apparent resistivity and phase. The results show that the dimension of the 3D structure in the main direction must be over 16 times its length in the initial dimension, so that the MT response of that structure in YX polarization can be replaced by the MT response at a 2D structure in TE mode. If the polarization is XY the main dimension must be 26 times the initial dimension. In the second part, which consists in determining the effects of the Onwumechillian equatorial electrojet, we placed the 3D structure at 10 km, 100 km, 300 km, 500 km, 1000 km and 1500 km, from the electrojet. In this way we determine the distance at which the effects of the Onwumechillian equatorial electrojet on the MT soundings are practically negligible. We observe that, for a conductive basement, the equatorial electrojet does not affect the MT soundings if the 3D structure is placed at 1500 km from the source. When the basement is resistive, the electrojet affects the MT soundings even if the structure is at 1500 km away from the source. We verified that the equatorial electrojet affects the MT soundings in frequencies under 10-2Hz, in both resistivity and phase and in both polarizations. We also observed that the galvanic effect is stronger on the YX than on the XY polarization.
Acesso aberto (Open Access)
Efeito da topografia em sondagens eletromagnéticas bidimensionais nos domínios da frequência e do tempo
(Universidade Federal do Pará, 1990-11-28) BARROS, Sérgio Brochado Ribeiro de; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The topographic effects of a valley, a hill, a downward slope and an upward slope, on bidimensional frequency domain eletromagnetic soundings, cause a variation in the amplitude and the phase of Hz component due to a homogeneous medium. The amplitude is less affected than the phase. The wall of the valley nearer to the current line produces a reduction in the phase values while a farther wall causes an increase. The effects of a hill are opposite to that of a valley. The effects of a downward slope and of an upward slope in the homogeneous medium correspond to that of a single wall, nearer to the current line of the valley and of the hill respectively. The response of a conductive rectangular body placed in a homogeneous medium nearer to the current line sufferc small variations due to the presence of a valley or a hill farther away from the current source. However, if these topographic features lie above the body they affect strongly the phase and amplitude of Hz component and only the amplitude in the case of Hx component. The transient response is not affected by topography at early times because only a small portion of earth is sounded; as well as at later times because the depth of investigation is very large compared to the topographic dimensions. For the models studied here, the major influence is at an intermediate time, about 7 ms, causing a retardation in cross-over positions in the sounding curves.
Acesso aberto (Open Access)
Os efeitos Delaware e Groningen: um estudo quantitativo por elementos finitos
(Universidade Federal do Pará, 1993-03-05) CHABA, Ajay; LOVELL, John Richard; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The Delaware and Groningen effects are two different kinds of anomaly affecting electrode type resistivity logging tools. Both effects take place when there is a high-resistivity bed, like halite or anhydrite, above the reservoir(s), producing a gradient on the resistivity log, which resembles an oil-water contact. The misinterpretations therefore produced have caused considerable losses to oil industry. PETROBRAS, in particular, has faced problems caused by Groningen effect on logs obtained in paleozoic basins of northern Brazil. In this work we have adapted and improved upon a methodology developed by LOVELL (1990) based on Helmholtz's equation for HΦ for modeling Delaware and Groningen effects. We solve this equation by triangular and rectangular finite elements. The finite element linear system is solved by preconditioned bi-conjugate gradient, the preconditioner being obtained by incomplete LU (Low Up) decomposition. Voltages are calculated by a recently developed, and more precise, algorithm. Logs are generated by a new algorithm involving successive switching of subdomain resistivity. This procedure allows the computation of each new stiffness matrix from the previous one by the very fast computation of matrix variation. Moreover, this method allows rapid iterative solution by using the solution from the previous tool position. Finally we compute resistivity logs subject to each effect for a model of the Dual Laterolog Tool.
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)
Eletroresistividade aplicada à caracterização hidrogeológica para o abastecimento da cidade de São Domingos do Araguaia-Pará
(Universidade Federal do Pará, 1996-02-13) SILVA, Arnaldo de Queiroz da; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
The use of geophysical methods (electrical, eletromagnetics and well logging) for groundwater exploration in many areas in the State of Pará has been possible due to a program of cooperation between the Fundação Nacional de Saúde and the Universidade Federal do Pará, in which the Department of Geophysics and the Graduate Programe in Geophysics are respossible for the research and techinical work. This thesis has been conducted under such program. Our objective in this work is to find geoelectrical criterious for caracterization of the subsurface geology for hydrogeological exploration, based on apparent resistivity, data observed in cristaline rock environment. First, we analysed the influence of some 2D structures on resistivity data using vertical electrical sounding with Schlumberger array. This study was carried out with the computer program SEV2D, developed in the Departament of Geophysics, based on the finites elements technique. The results show clearly the ambiguities which are obtained when 1D interpretation is used for two-dimentional geological environment. The pratical aspect of the thesis is concerned with the processing and interpretation of resistivity data obtained in São Domingos do Araguaia in the southeset of the State of Pará. The Schlumberger array was used in the vertical electrical sounding survey. By correlating the geophysical results with geological informations it was possible to define the best zones for groundwater exploration in the prospected area.
Acesso aberto (Open Access)
Eletroresistividade na prospecção hidrogeológica no município de Ourilândia do Norte-PA
(Universidade Federal do Pará, 1996-05-22) SOUSA, José Antonio Ferreira de; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
Geophysical prospecting for groundwater in crystalline regions is a difficult task due to the presence of faults, fractures and lateral variations of resistivity in the overburden. To analyze the influence of bidimensional (2D) variation of resistivity on Schlumberger vertical electrical soundings several layer models were studied containing 2D vertical and dipping structures. These 2D structures are located in the alteration mantle at varying distances from the sounding position. The numerical modeling was done employing the finite element method using an algorithm developed for this purpose (EGSLIB/SEV2D). The results of simulation show that the Sclumberger vertical electrical sounding technique over bidimensional media is not very effective as in sedimentry and alluvial regions due to the phenomenon of ambiguity which complicates the interpretation of the curves. Among the geophysical methods used for hydrogeological investigation, electroresistivity is simplest, economical and efficient. 32 Schlumberger vertical electrical soundings and several kms of dipole-dipole profiling were realized in the Ourilândia do Norte town in order to help locate sites for tube-wells. The apparent resistivity curves were treated and processed considering an isotropic, homogeneous, horizontal layers model, using algorithm EGSLIB/SEV1D and EGSLIB/SEV1DINV program. This permitted to establish approximate geoelectrical models for the subsurface geology of the town. Two distinct geological zones could be recognized based on interpreted geoelectric sections and available geological information. One of the zones is formed by soil/embankment followed by a soil horizon containing sandy sediments over a sandyclay layer and finally the bedrock. This geological sequence predominated in the town. The other zone is formed by soil/embankment overlying a sandyclay bed which is underlain by granitoids type Rio Maria. This second zone is limited in the central part of the town. The sand and sandyclay stratum posses good potential for water storing.
Acesso aberto (Open Access)
Ensaios sobre a contribuição do ar nos resultados da modelagem do modo transverso-elétrico do método magnetotelúrico
(Universidade Federal do Pará, 2001-09-29) HEIMER, Michael; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
We present the numerical solution of the model of an infinite fault for the TE mode using the finite elements method to investigate the effect of the air in the TE solution. The comparision of our solution for the normalized electric field with the Weaver solution showed an expected discrepancy due to the effect of the air, which was neglected intentionally by Weaver to simpify the solution of the problem. The analytic problem is substancially more difficult. A solution of the real problem, partially analytic and partially numerical, was presented by Sampaio. Due to an unexpected discrepancy between our finite elements solution and the exact Sampaio solution, we recalculated his solution. Iterating the numerical part of the Sampaio solution up to higher orders we were able to verify that it presents divergent behavior. To examine the effect of the air on apparent resistivity, we compare three series profiles of apparent resistivity obtained with our FE solution with the profiles obtained using the Weaver solution. The discrepancy between the apparent resistivities shows to be small, figuring in the range of the instrumental field error.
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 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.
Acesso aberto (Open Access)
Filtros otimizados para transformadas seno, co-seno e de Hankel j0, j1 e j2
(Universidade Federal do Pará, 2003-01-14) ALMEIDA, Frayzer Lima de; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
We present two automatic algorithms, which use the Wiener-Hopf least-square method, for the calculation of digital linear filters for sine, co-sine transforms and J0, J1 and J2 Hankel transforms. The first algorithm optimizes the following parameters: abscissa increments, initial abscissas and displacement factor for the computation of digital linear filters coefficients that are computed through sine and co-sine analytic transforms. The second algorithm optimizes the following parameters: abscissa increments and initial abscissas for the computation of digital linear filters coefficients that are computed through J0, J1 and J2 Hankel analytic transforms. This methodology led to the proposition of new digital linear filters of 19, 30 and 40 points for the co-sine and sine transforms, and to new optimized filters of 37, 27 and 19 points for the J0, J1 and J2 Hankel transforms, respectively. The comparison of the performance of these filters with respect to the others ones published in the geophysical literature is evaluated by using a analytical geophysical model composed of two half spaces. An infinite current line was used between the half spaces, giving rise to the sine and co-sine transforms. Among all simulations carried out, it was noticed that the new sine and co-sine transforms of 19 points presented better performance when compared to the existing ones in the literature. In addition, J0 and J1 transforms were obtained using a vertical magnetic dipole as source between the half spaces, noticing that better performances using the new J1 filter of 27 points were obtained among all simulations when compared to the existing 47 points J1 filters, as well as with the simulations carried out using the new 37 points J0 filter and compared with the 61 points J0 existing filters in the literature. Using a horizontal magnetic dipole between the half spaces, it was observed an equivalent performance using the 37 and 27 points new filters for the J0 and J1 transforms when compared, respectively, to the existing 61 and 47 points filters in the literature. Finally, it was observed an equivalent performance using 37 and 27 points new filters for the J0 and J1 transforms when compared, respectively, to the existing 61 and 47 points in the literature, and when applied, respectively, in Wenner and Schlumberger electrical soundings. Most of our filters contain fewer coefficients than those usually used in geophysics. This aspect is very important because these linear filter transforms are commonly used intensively in numerical massively geophysical problems.
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)
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)
Interface gráfica para pré e pós-processamentos de dados elétricos e eletromagnéticos em Geofísica
(Universidade Federal do Pará, 2001-10-09) MONTEIRO, Humberto Deodato Malcher; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
Acesso aberto (Open Access)
Interfaces gráficas de entrada e saída da eGs (electrical GEOPHYSICS suite)
(Universidade Federal do Pará, 2005-03-14) SILVA, Rodrigo Erasmo da Conceição; RIJO, Luiz; http://lattes.cnpq.br/3148365912720676
We present a graphic interface for the program eGs (electrical GEOPHYSICS suite) used for modeling and interpretation of electric and electromagnetic data in geophysical exploration. This master degree dissertation corresponds the portion of the program that treats the interfaces for bi and three-dimensional input data of geoelectrical models together with interfaces for the presentation of the modeling results and for the presentation of observed data as well. The program contains interfaces for creation, modification and visualization of bi and threedimensional geoelectrical models and for illustrations of the calculated results through graphs and images. This work gives a program with several options to alter the geometry and the physical properties of the geoelectrical models, including facilities for three dimensions visualization in order to help geophysical data interpretation. The graphic interfaces can be used to create geoelectrical models associated with the most used electric and electromagnetic geophysical methods and to analyze the answers obtained by numerical modeling associated with different types of observed data in field campaign. The program is sufficiently flexible so that new graphic interfaces of other electric and electromagnetic methods can be implemented easily.