Green's Function Extraction from Atmospheric Acoustic Propagation

Acoustic waves propagating in the atmosphere may undergo many effects including refraction by temperature and wind velocity gradients, scattering by atmospheric turbulence, absorption by the atmosphere (fluid), diffraction by terrain features, and absorption and reflection by a porous ground. As a result, there may be insonification in acoustic shadow zones, amplitude and phase fluctuations of the propagating sound signals, loss of signal coherence, changes in the interference maxima and minima of the direct ground reflected waves, and multipath effects. Understanding these effects is important for a variety of military applications, such as acoustic source localization and classification, noise propagation in the atmosphere, and the development of new remote sensing techniques of the atmosphere.

By extracting the medium impulse response, or Green's function, one may obtain information about the medium channel in order to overcome the medium effects or deduce information about the medium. For example, in acoustic communications, information is sent through a medium from a host station to client stations. The transmitted information is subjected to a variety of signal distortions and noise caused by the medium. Using time-reversal processing, it is possible to extract the channel medium impulse response from the transmission of a known pilot signal through the channel medium. This Green's function was then used to modify the subsequent signals to overcome distortion in the channel.