Navegando por Assunto "Nevada - Estado"

Agora exibindo 1 - 2 de 2

Acesso aberto (Open Access)
Inversão gravimétrica rápida do relevo do embasamento aplicando o funcional da variação total
(Universidade Federal do Pará, 2013-08-23) SANTOS, Raphael Di Carlo Silva dos; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491; SANTOS, Darcicléa Ferreira; http://lattes.cnpq.br/5813659873067927
We present a new 2D linear inversion gravity method to estimate the discontinuous basement relief of a sedimentary basin. The method uses an interpretation model consisting of a set of 2D juxtaposed horizontal ribbons whose thicknesses are the parameters to be estimated. The density contrast between the basement and the sediments is assumed constant and known. The estimates of the thicknesses were stabilized with the Total Variation functional (TV) which permits solutions presenting local discontinuities in the basement relief. The estimates of the basement relief are produced by solving a single linear system of equations in the L1 norm. Because linear inversion methods underestimate the depth of the basement relief for sedimentary basins thicker than 500 m, we amplify the depth estimates through by modifying the matrix associated with the interpretation model. This procedure in general leads to overestimated depths which are corrected through the expression of the anomaly of a Bouguer slab. Tests on synthetic and real data produced similar results as compared with the nonlinear method, but required less computational time. The ratio R between the computational times required by the nonlinear and the proposed methods to produce similar solutions increases with the number of observations and parameters. For example, for 60 observations and 60 parameters R is equal to 4 whereas for 2500 observations and 2500 R increases to 16.8. The proposed method and the nonlinear inversion method were also applied to real gravity data from Steptoe Valley, Nevada, U.S.A. and from Ponte do POEMA at the Campus of Guamá, in Belém, producing solutions which are similar to the ones produced by the nonlinear method but required a smaller computer time.
Acesso aberto (Open Access)
Mapeamento de magnetização aparente usando regularização entrópica
(Universidade Federal do Pará, 2007) VASCONCELOS, Suzan Sousa de; SILVA, João Batista Corrêa da; http://lattes.cnpq.br/1870725463184491
We present a new apparent-magnetization mapping method on the horizontal plane that combines the minimization of the first-order entropy with the maximization of the zeroth-order entropy of the estimated magnetization contrasts. The interpretation model is a grid of vertical, juxtaposed prisms in both horizontal directions. We assume that the top and the bottom of the magnetic sources are flat and horizontal and estimate the prisms' magnetization contrasts. The minimization of the first-order entropy favors solutions presenting sharp borders, and the maximization of the zeroth-order entropy prevents the tendency of the source estimate to become a single prism. Thus, a judicious combination of both constraints may lead to solutions characterized by regions with virtually constant estimated magnetization contrasts separated by sharp discontinuities. We apply our method to synthetic data from simulated intrusive bodies in sediments that present flat and horizontal tops. By comparing our results with those obtained with the smoothness constraint, we show that both methods produce good and equivalent locations of the sources' central positions. However, the entropic regularization delineates the boundaries of the bodies with greater detail. Both the proposed and the smoothness constraints are applied to a real anomaly over a magnetic skarn in Butte Valley, Nevada, USA. The entropic regularization produced an estimated magnetization distribution with sharper boundaries, smaller volume, and higher apparent magnetization as compared with the results produced by the smoothness constraint.