Abstract: The waveguide invariant characterizes the dispersive nature of the waveguide and relates the observed slope of the striations to range and frequency. Waveguide invariant theory is applied to underwater sound field generated by an air-borne sound source. The relation of height or depth of source or receiver and the distribution of the waveguide invariant is studied by numerical simulation. A new approach utilizing the waveguide invariant in the water near the surface can distinguish air-borne and underwater sound source. The effects of sound speed profile and wind speed profile in the air on the approach are studies by numerical simulation. In actual atmospheric environment over ocean, this approach works when there is an inversion or when the receiver is downwind of the sound source.