Abstract: Optical properties in the offshore sea water can be described by means of bio-optical models, where the vertical profiles of the absorption and scattering coefficients are related with the chlorophyll profile [2]. Therefore, the determination of some inherent optical properties can be adrressed by estimating the the ocean chlorophyll concentration. This inverse problem can be formulated as an optimization problem and iteratively solved, where the radiative transfer equation is the direct model. An objective function is given by the square difference between computed and experimental radiances at every iteration. Recently, we have used an Ant Colony System (ACS) as the optimizer, where a pre-selection scheme of the candidate solutions is performed by their smoothness, quantified by a Tikhonov norm [1,3]. In the standard ACS method, the pheromone is only reinforced on the best ant of the population (the lowest objective function value) at each iteration. The fuzzy strategy consists to put additional pheromone quantity on the best ant, but a small pheromone quantity is also spread on the other solutions close to the best one, decresing the pheromone quantity as far as the solution is from the best ant. Each candidate solution corresponds to a discrete Chlorophyll profile. The radiative transfer equation is solved using the Laplace transform discrete ordinate (LTSN) method. Test results show that the fuzzy-ACS produces better inverse solutions.

References:

[1] N.N. Arai, R.P. Souto, J.C. Becceneri, S. Stephnay, A.J. Preto, H.F. de Campos Velho (2004): A new regularization technique for an ant-colony based inverse solver applied to a crystal growth problem, 13th Inverse Problem in Engineering Seminar (IPES-2004), 14-15 June, University of Cincinnati, Ohio, USA, pp. 147-153.

[2] C.D. Mobley (1994): Light and Water: Radiative Transfer in Natural Waters, Academic Press.

[3] R.P. Souto, H.F. de Campos Velho, S. Stephany, S. Sandri (2004): Reconstruction of Chlorophyll Concentration Profile in Offshore Ocean Water using a Parallel Ant Colony Code, 16th European Conference on Artificial Intelligence (ECAI-2004), Hybrid Metaheuristics (HM-2004), 22-24 August, Valencia, Spain, pp. 19-24.