The problem of gravity wave propagation in the lower atmosphere may be formally reduced into a scattering problem with an effective 'potential'. In such a formalism, th eigenvalues (in units of inverse square horizontal phase velocity) may be discrete, corresponding to fully guided modes ('bound' states) and partially guided modes ('resonance' states), or they may be continuous, corresponding to free modes ('scattering' states). A phase shift analysis reveals two 'resonance' states corresponding to horizontal phase velocities of 309.7 and 264.2 m/s, which may be respectively identified as the Lamb mode and the s2 mode. It appears that although there are a number of means for ducting the F region gravity wve, atmospheric structure is not likely to be one of them.