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The scattering of guided waves by a complex shaped defect in a three-dimensional (3D) waveguide is considered. For such problems, analytical solutions do not exist, and modal decomposition techniques based on the establishment of the displacement and stress fields in the vicinity of the scatterer are extremely heavy and complicated. On the other hand, finite elements (FE) based methods are known to be very memory consuming. This paper proposes a post-processing technique, based on a 3D orthogonality relation, to decompose a complex acoustic field produced by a scatterer and predicted by a 3D finite elements model, into plane waves, the amplitudes of which are quantified. This technique allows huge reductions of the (FE) model size to be obtained. An application of guided waves scattering in an aluminium plate is presented, where zero and first order Lamb and SH modes compose the scattered field. [ABSTRACT FROM AUTHOR]