Programa de Pós-Graduação em Geofísica - CPGF/IG

URI Permanente desta comunidadehttps://repositorio.ufpa.br/handle/2011/2355

O Programa de Pós-Graduação em Geofísica da UFPA (CPGF) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA). Foi o segundo no Brasil a formar recursos humanos em Geofísica em nível de pós-graduação stricto sensu. Criado em 1972, funcionou até 1992 junto com os Cursos de Pós-Graduação em Geoquímica e Geologia.

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Agora exibindo 1 - 5 de 5

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)
DesG: uma nova metodologia para intervenção e coleta de dados magnetotelúricos
(Universidade Federal do Pará, 2006-08-29) SILVA, Lúcia Maria da Costa e; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
This thesis presents two contributions to magnetotelluric data interpretation. The first one is a new MT inversion method, named Geologic-Descriptive Method (DesG) because it explicitly incorporates a priori information, keeping a close correspondence with the standard description of geological features. The user defines by means of geometric elements (points and axes) the skeleton of geological features, and establishes a priori values for the resistivity of the assumed geological bodies. The method estimates the resistivity distribution in the subsurface, which are closest to the specified geometric elements and produce an acceptable fit to the observations. The analysis of an obtained solution may help in modifying the a priori outline for the sources, allowing, in this way, that successive inversions be performed until the solution fit the data and make geological sense. Among the attractive features of the presented method are: (i) the anomalous sources may present a resistivity larger or smaller than the resistivity of the host rocks, (ii) several host rocks, enclosing or not anomalous sources, may be crossed by the traverse, and (iii) the resistivity contrast between the anomalous source and the host rock may be either abrupt or gradational. Tests on synthetic data reveal, among other relevant features, excellent results in estimating the attitudes of variable-dip faults, which are of particular importance in regional tectonic studies, and in delineating diabase sills within sedimentary basins, which represent a severe obstacle in oil prospecting. The method also allows the joint interpretation of the target sources and the sources causing static shift. Tests using data from COPROD2 shows a estimative of the resistivity distribution in agreement with the geological knowledge about the area. The second contribution relates to the design of geophysical experiment. By means of several indicators, particularly the density information matrix, it is shown that one can access the theoretical resolution of the data, which guides the explorationist in carrying out a survey design. The survey design optimization allows determining the most suitable observation locations and periods to produce a more precise delineation of sources, whose locations are approximately known.
Acesso aberto (Open Access)
Empilhamento sísmico pela composição de ondas planas
(Universidade Federal do Pará, 1997-04-18) CALLAPINO, German Garabito; SÖLLNER, Walter Franz
In this thesis we present a new seismic data stacking method called Plane Wave Composition (PWC). This method, applicable in a bidimensional medium with lateral velocity gradients, is developed on the basis of physical and mathematical concepts on the plane wave decomposition of spherical wave fields. In the initial part of this work, we present a review on the conventional stacking method and on plane wave decomposition of the point-source seismograms. The stacking by plane wave composition is a method which produce a normal incidence (or zero offset) section by the application of the following main processes: A double plane wave decomposition, achieved by a slant stack along the shot array and another slant stack along the receiver array, followed by a plane wave composition achieved by an inverse slant stack. The PWC stacking method is theoretically formulated here on the basis of the scattering theory applied to seismic waves, within the constraint of the Born approximation. Initially, starting with the acoustic wave equation, for a finite source-receiver configuration, a solution for the direct single scatter (Born) model is derived. That result is reduced for the coincident source-receiver (zero offset) configuration. Afterward, the mathematical expression of PWC stacking method is solved replacing the observed data function by the scattered field obtained by the Born approximation. For most clarity, the algorithm to obtain the zero offset seismic section by the PWC stacking method is described by applying it to the data corresponding to a simple model. A successful application is performed using the Marmousi seismic data set, corresponding to a geological complex model. Finally, in the same data set, a noise analysis shows that this method increase the signal-noise ratio in the seismic trace. Thus, it has been showed that the PWC stacking method is an efficient alternative to process seismic data of complex models.
Acesso aberto (Open Access)
Imageamento homeomórfico de refletores sísmicos
(Universidade Federal do Pará, 1994-10-06) CRUZ, João Carlos Ribeiro; HUBRAL, Peter; http://lattes.cnpq.br/7703430139551941
This thesis presents a new technique for seismic stacking called homeomorphic imaging, which is applicable to the imaging of seismic reflectors in a bidimensional, inhomogeneous and isotropic medium. This new technique is based on ray geometrical approximation and topological properties of reflection surfaces. For this purpose the concepts of wavefront, incidence angle, radius and caustic of wavefront and ray trajetory are used. Considering a circle as the geometrical approximation of the wavefront in propagation, it is possible to define diferent homeomorphic imaging methods, depending on processing configuration. In this way, the following methods are possible: 1) Common Source (Receiver) Element (CS(R)E), which relate to a set of seismograms with a single source (receiver) and a real reflected wavefront is considered; 2) Common-Reflecting-Element (CRE), which relate to a set of seismograms with a single reflection point and a wavefront hipotetically generated in the same reflection point is considered; 3) Common Evolute Element (CEE), which relate to a set of seismograms with each pair of source and geophone located in the same point on the seismic line and a wavefront hipothetically generated in the curvature center of the reflector is considered. In the first method is obtained a stacked seismic section using arbitrary central rays. In the last two methods the result is a zero-offset seismic section. These methods give also other two sections called radiusgram and anglegram, the latter being emergence angles and the former radii of wavefront in the moment that it reaches the observational surface. The seismic stacking is made using a local correction-time applied to the travel time of a ray that leaves the source, and after reflection, is registered as a primary reflection at a geophone, in relation to the reference time which is the travel time of the central ray. The formula used for the temporal correction depends on the radius, the emergence angle of the wavefront and the velocity which is considered constant near the seismic line. It is possible to show that in this new technique the registered signal is not submitted to stretch effects as a consequence of the temporal correction, furthermore there is no problem with reflector point dispersal as a consequence of dip reflectors, in contrast with the techniques that are based on NMO/DMO. In addition, considering that no a prori knowledge of a macromodel is necessary but the velocity near the seismic line, the homeomorphic imaging can be applied to inhomogeneous models without losing the strictness of the formulation.
Acesso aberto (Open Access)
Imageamento multifoco de refletores sísmicos
(Universidade Federal do Pará, 2000) OLIVA, Pedro Andrés Chira; CRUZ, João Carlos Ribeiro; http://lattes.cnpq.br/8498743497664023; HUBRAL, Peter; http://lattes.cnpq.br/7703430139551941
The simulation of a zero-offset section (ZO) from multi-coverage seismic reflection data for a 2-D media, through the stack, is a widely used seismic reflection imaging method, that allows to reduce the amount of data and to improve the signal-to-noise ratio. According to Berkovitch et al. (1999) the Multifocusing method is based on Theory of Homeomorphic Imaging and consists of stacking multi-coverage seismic reflection data with arbitrary distribution source-receiver according to a new local moveout correction, called Multifocusing. This moveout correction is based on a local spherical approximation of the focusing wavefront in the vicinity of the surface of the earth. This method allows to build a seismic section in the domain of the time of a zero-offset increasing the signal-to-noise ratio. The Multifocusing technique does not need any knowledge a priori of the macro-model velocity. Three parameters are used to describe the Multifocusing moveout correction, which are: 1) emergence angle of the zero-offset ray or normal reflection ray (β0), 2) the wavefront curvature at the Point of Normal Incidence (RNIP and 3) the wavefront curvature of Normal Wave (RN). Being also necessary the near-surface velocity. In this thesis work I apply this Multifocusing Stack technique for multi-coverage seismic reflection data covering the models of constant velocity and heterogeneous model, with the objective of simulating of zero-offset sections. In this case as it is the solution of forward problem, this macro velociy model is considered know apriori. In the context of the inverse problem it is had the parameters, RNIP, RN and β0 and can be determining through the analysis of applied coherence to the multi-coverage seismic reflection data. In the solution of this problem the objective function, to be optimized, is defined by the calculation of the maximum existent coherence among the data in the surface of seismic stack. In this thesis we discuss the sensibility of the travetime, used in Multifocusing Stack, as a function of the parameters RNIP, RN and β0. This sensibility analysis is done of three different manners: 1) the first derivate of the objetive function, 2) the coherence measure, denominated semblance, and 3) the sensibility in the Multifocusing Stack.

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