Analysis of pulse propagation in the surface duct with a deep source

Pulse propagation in the surface duct with the source far below the duct is analyzed. Experimental results show that through the duct, the sound can propagate to the range much farther than that predicted by the ray theory. Besides, the propagation range is strongly related with the signal frequency. This propagation path is reproduced using the normal mode model. Furthermore, the ray representation of the normal mode is used to illustrate the mechanism of this surface-duct-related arrival. It is found that the arrival is due to the diffraction when a ray grazes the bottom of the surface duct. For lower frequency, the grazing angle is larger. Therefore, the energy diffracted back to the surface duct is weaker.