Navegando por Orientadores "O'BRIEN, Douglas Patrick"

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Acesso aberto (Open Access)
Avaliação de dados de altimetria do satélite Geosat a partir de comparação com dados do levantamento gravimétrico marinho Equant
(Universidade Federal do Pará, 1992-02-26) MARQUES, Maria do Socorro Pereira; O'BRIEN, Douglas Patrick
The principal objective of this thesis is to evaluate the use of satellite altimetry data to map the potential surface of the earth's gravity field at sea. This evaluation is done by comparing the resolution and accuracy of the altimeter data over an equipotential surface (the sea) with that computed from conventional sea survey data. From the potential surface, quantities such as the free air anomaly or the vertical deflection can be calculated. The altimetry data utilized in this thesis were collected by the Geosat satellite and each data point was averaged over a two year (44 orbital cycles) period. This averaging was done to improve the accuracy and resolution of the sea height over ocean areas. These data are available from NOAA and were supplied to UFPa for use in this thesis. The ship survey data from EQUANT 1 and EQUANT 2 were generated from a joint research project among several institutions whose scientific objective was to study the behavior of the Brazilian Equatorial Continental Margin. In order to integrate and compare the data between these two different sources (satellite and ship measurements), either the satellite data must be reduced to a vertical acceleration on the equipotential surface or alternately the ship survey data must be transformed into an equivalent potential surface. Taking into account the wide separation of the satellite tracks relative to the ship survey track, it was decided to transform the ship survey data into a potential surface, and the correlation and comparisons were carried out along satellite tracks. In this type of transformation, various factors must be considered such as the effects of aliasing, ship survey noise levels, reduction to the geoid, computational errors in the transformation, etc. In spite of these effects (particularly the aliasing problem) the results reflect a strong correlation between the two sets of data verifying that a satisfactory level of coherence exists for wavelengths greater than 11 km. The resolution for the 2 year cycle average of the satellite is, in fact, almost as good as the ship survey data.
Acesso aberto (Open Access)
Decomposição em valores singulares aplicada a dados de campo magnético
(Universidade Federal do Pará, 1992-12-15) MOURA, Helyelson Paredes; O'BRIEN, Douglas Patrick
The singular value decomposition of a matrix A, n x m, which represents a magnetic anomaly, can be seen as a bidimensional coherence filtering method which separates the correlated information from noncorrelated information in a magnetic data matrix A. The filter is defined by expansion of matrix A into eigenimages and singular values. Each eigenimage is constructed by the scalar product of the base vectors and eigenvectors, which are associated with the eigenvectors and eigenvalues of the covariance matrices ATA and AAT. This filtering method is based on the fact that the eigenimages, which are associated with large singular values, concentrate the major part of the correlated information present in the data, while the noncorrelated part, including noise caused by external magnetic sources, compilation errors, and shallow magnetic sources comprise the remaining eigenimages. This method was employed on many examples of synthetic and real data from PETROBRÁS' Carauari-Norte project (Solimões Basin) in order to investigate the utility of the method in the identification, elimination and attenuation of noise present on magnetic data and as a possible method for enhancing certain features generated by anomalies of shallow and deep origin. This work suggests the desirability of introducing both static and dynamic shift on magnetic lines to enhance the correlation (coherence) between the magnetic lines. This shift concentrates the correlated signal in the first few eigenimages. Another important aspect of this decomposition into eigenimages and eigenvalues is the savings gained in storage of a matrix A of n x m units. Memory requerements can be diminished considerably by using p autoimages, i. e. p x (n + m + 1) units without altering the form of the anomaly. We conclude that an appropriate choice of eigenimages generated by SVD decomposition shows good promise as a processing method in magnetic data.
Acesso aberto (Open Access)
Estudo das feições lineares observadas em dados aeromagnéticos da Bacia do Solimões a partir de imagens digitais de sombreamento do relevo anômalo total
(Universidade Federal do Pará, 1991-11-25) AFFONSO, Alexandre de Oliveira; O'BRIEN, Douglas Patrick
The Solimões Basin is a magnetically complex basin characterized by a superposition of different linear magnetic anomalies. These anomalies reflect the different tectonic episodes that have occured in the Amazon Region during the Precambriam and Phanerozoic. In this work, a shaded relief method of digital image processing was applied to the aeromagnetic data from the Solimões Basin. The characteristics of this method, which allow its use as a strike enhancement filter, clarified important aspects of the relationships between magnetic lineations oriented E—W, NE—SW, e NW—SE. A magnetic lineation pattern was identified which reflects the presence of dextral strike-slip fault zones that have a preferential orientation in the E—W direction. The interaction between these systems promoted the development of a convergent stress regime which caused the formation of reversel faults associated with E—W and NE—SW magnetic lineaments; together with directional duplexes, forming positive and negative flower structures associated with N70-80E and N70-80W magnetic lineaments. A parallel quantitative analysis was also performed on the aeromagnetic data which attempts to quantify two importants aspects of these linear magnetic anomalies shown on the digital images. The first analysis was based on a forward modeling of the aeromagnetic data using a superposition of two-dimensional prismatic bodies. It is shown that these magnetic linear features can be explained by the superposition of highly magnetized basement sources and higher frequency shallow sources associated with reverse faulting of diabase sill's present in the Paleozoic sediments of the Basin. The second analysis, which was based on cross-covariance methods, calculated the offsets present in the NE—SW magnetic lineaments along dextral E—W directions. This analysis supports the explanation that the magnetic pattern from this region can be explained by means of lateral offsets presumably along pre-existing zones of weakness (Precambrian, Paleozoic), during the Mesozoic and Cenozoic.
Acesso aberto (Open Access)
Estudo do comportamento das feições lineares observado em dados aeromagnéticos da Bacia do Paraná
(Universidade Federal do Pará, 1990-12-28) MACHADO, Sérgio Augusto Moraes; O'BRIEN, Douglas Patrick
The digital image processing of aeromagnetic data has proved to be an excellent method of interpretation. This technique is referred to as the shaded relief method. The method was applied to aeromagnetic data, over part of the Parana Basin, to enhance the lineaments that are not clearly seen on the original magnetic-anomaly maps. To support our studies, we have used LANDSAT images, as many lineaments encountered in he basin are non-magnetic. This integration allowed definition of two dominant directions for the lineaments: N40° — 60°W and N40° — 65°E. With the definition of this pattern, we obtained some information about the geological trends and, hence, an idea about tectonic evolution of the basin. We also developed a procedure to compute the total magnetization vector of the principal lineament of the basin wich showed five different inclinations of the magnetization vector. These inclinations are explained by a superposition of induced magnetization effects, field reversal effects, and effects of anisotropic magnetization caused by demagnetization within the dikes.
Acesso aberto (Open Access)
Formulação e implementação de um pacote para interpretar mapas magnéticos utilizando os polinômios de Walsh
(Universidade Federal do Pará, 1992-12-30) RAMOS, Alexandre José Santos; O'BRIEN, Douglas Patrick
In this work we develop a method for the automatic interpretation of magnetic field data. This methodology determines the boundaries and magnetizations of geologic units. The often of abrupt variations of the magnetization of geological units are modeled in this methodology. These abrupt lateral variations will be represented by discontinuous polynomials known as Walsh polynomials. Several new concepts were developed in order to apply Walsh polynomials to solve aeromagnetic data inversion problems. Among these new concepts we can list. (i) - An optimum algorithm to generate a set of "quasi-orthogonal" polynomials using a Walsh magnetization distribution. (ii) - The use of the damped least squares method to stabilize the inverse solution. (iii) An investigation of positional invariance problems, which are inherent in the Walsh polynomials. (iv) - The choice of the order of the polynomials taking into account the limitations of resolution and the behaviour of the eigenvalues. Using these features of magnetized bodies it is possible to formulate the direct problem, i.e, the bodies magnetization obeys Walsh distribution. It is also possible to formulate the inverse problem, in which the magnetization that generates the observed field obeys the Walsh series. Prior to applying the method it is necessary to estimate the depths to the magnetic sources. We have chosen a method developed by LOURES (1991), which is based on the Euler homogenous equation which requires the knowledge of the magnetic field and its derivatives. We tested the method with real data from a region in the Alto Amazonas sedimentary basin. The aeromagnetic data were provided by PETROBRÁS.
Acesso aberto (Open Access)
Interpretação aeromagnética automática com uso da equação homogênea de Euler e sua aplicação na Bacia do Solimões
(Universidade Federal do Pará, 1991-09-27) LOURES, Luiz Geraldo do Carmo Lucchesi; O'BRIEN, Douglas Patrick
We present here a methodology for the rapid interpretation of aeromagnetic data in three dimensions. An estimation of the x, y and z coordinates of prismatic elements is obtained through the application of "Euler's Homogeneous equation" to the data. In this application, it is necessary to have only the total magnetic field and its derivatives. These components can be measured or calculated from the total field data. In the use of Euler's Homogeneous equation, the structural index, the coordinates of the corners of the prism and the depth to the top of the prism are unknown vectors. Inversion of the data by classical least-squares methods renders the problem ill-conditioned. However, the inverse problem can be stabilized by the introduction of both a priori information within the parameter vector together with a weighting matrix. The algorithm was tested with synthetic and real data in a low magnetic latitude region and the results were satisfactory. The applicability of the theorem and its ambiguity caused by the lack of information about the direction of total magnetization, inherent in all automatic methods, is also discussed. As an application, an area within the Solimões basin was chosen to test the method. Since 1977, the Solimões basin has become a center of exploration activity, motivated by the first discovery of gas bearing sandstones within the Monte Alegre formation. Since then, seismic investigations and drilling have been carried on in the region. A knowledge of basement structures is of great importance in the location of oil traps and understanding the tectonic history of this region. Through the application of this method a preliminary estimate of the areal distribution and depth of interbasement and sedimentary magnetic sources was obtained.
Acesso aberto (Open Access)
Modelo interpretativo para os levantamentos gravimétricos de Poço da Bacia de SE/AL
(Universidade Federal do Pará, 1991-09-27) MORAES, Fernando Sergio de; O'BRIEN, Douglas Patrick