Modern methods of perturbation theory for calculating hydroacoustic fields

The perturbation theory methods for approximate solution of the Helmholtz equation governing the waveguide ocean model are discussed. Nonlinear perturbation theory, or the delinearization method, is introduced in addition to the standard perturbation theory modified in accord with the specificity of the acoustic problems. The new theory does not require the knowledge of the whole eigenvalue spectrum of the unperturbed problem and, besides, all the corrections are written explicitly in quadratures. Unlike the existing theories the approach suggested allows one to avoid nonremovable singularities in basic formulae for any waveguide types by choosing appropriate wavefunction representation in the form of polynomials.