Weak shear modulus in the acoustic mode parabolic equation

By using the multiscale expansions method, we derive an elastic mode parabolic equation. As a small parameter, we choose the ratio of a typical wavelength to a typical size of horizontal inhomo-geneities. The shear modulus is of the order of the small parameter. Provided that the influence of elastic effects is essential, the derived equation can be effectively used in seismoacoustics propagation problems.