Correlations Between Acoustic Travel-Time Fluctuations and Turbulence in the Atmospheric Surface Layer

The effects of atmospheric turbulence on propagating acoustic signals are studied by making spatially distributed, concurrent acoustic and turbulence measurements. The acoustic frequencies were below 600 Hz, the ranges under 200 m, and meteorological field conditions convectively neutral and windy. Phase fluctuations along the line-of-sight propagation paths are found to be dominated by a coherent travel-time delay across frequency. This coherence is not consistently observed for downwind propagation, where multiple paths may exist. Two-point frequency coherence functions are computed and consistent with frequency-independent travel-time delays across the signal. The time scales of the phase and travel-time fluctuations correspond to the large-scale (>100 m) turbulent velocity fluctuations. Time series of angles of arrival are computed and shown to correlate well with velocity fluctuations. Measured phase and intensity statistics are used to classify the propagation. Lower frequency observations are consistent with a geometrical acoustic model in the presence of strong scattering, while higher frequencies show evidence of strong scattering and partial to full saturation.