Maio 2014 vol. 1 num. 1 - 10th World Congress on Computational Mechanics
Full Article - Open Access.
RECONSTRUCTION OF CHLOROPHYLL-A VERTICAL PROFILES WITH ENHANCED RESOLUTION
We present a methodology to reconstruct vertical profiles of Chlorophyll-a pigment concentration in open-ocean waters based on radiance values at different depths. The inverse problem is formulated here as an optimization problem and iteratively solved by an Ant Colony System (ACS) meta-heuristic. An objective function is given by the square difference between computed and experimental radiances at each iteration. The Laplace transform discrete ordinate (LTSN) method is used to solve the radiative transfer equation (direct problem) in order to compute the radiances. As radiance intensity decays exponentially with depth, a correction factor must be employed to weight the influence of the depth-dependent radiances. We previously proposed a correction factor that is given by the ratio between the mean radiance - related to the polar angle - at each level and the mean radiance at the surface. Here we obtained results close to the exact solution even for noisy radiance data, especially for smooth profiles, where the smoothness is determined by the second order Tikhonov criterion. However, for profiles with two or more peaks of Chlorophyll-a concentration, the methodology is still not able to identify these extremes. This result can be explained partly by the relatively low number of sampling points (usually 10 points), that limits the geometric resolution of the vertical profile to be reconstructed. Alternatively, we propose in this work the reconstruction vertical profile using a higher geometric resolution. We use at least one meter between each recovered point in order to evaluate the ability of identifying any peak in Chlorophyll-a concentration. Two inversion strategies are evaluated. At first, the resolution of the vertical profile is increased with a greater number of points to be recovered. In the second strategy, a hybrid methodology is used: initially the original methodology is employed, using the ACS stochastic method to retrieve a low resolution profile (10 points), and then, this result is used as an initial guess for the deterministic method of optimization Levenberg-Marquardt to retrieve a higher resolution profile (with at least 1 point at each meter depth). A comparison between these two strategies is presented and discussed.
Palavras-chave: Inverse Problems, Hydrologic Optics, Ant Colony System, Levenberg-Marquardt,
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Souto, R. P.; Dias, P. L. S.; Velho, H. F. Campos; Stephany, S.; Kampel, M.; "RECONSTRUCTION OF CHLOROPHYLL-A VERTICAL PROFILES WITH ENHANCED RESOLUTION", p. 1486-1495 . In: In Proceedings of the 10th World Congress on Computational Mechanics [= Blucher Mechanical Engineering Proceedings, v. 1, n. 1].
São Paulo: Blucher,
ISSN 2358-0828, DOI 10.5151/meceng-wccm2012-18444
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