Evidence for Virtual Source‐Receiver Refractions in Cross Correlations of Infrasonic Ambient Noise

Seasonal variability in source activity and atmospheric temperature were retrieved from 11 years of infrasonic ambient noise. Variable lag times between an array and single microbarometer were obtained from envelopes of cross‐correlation functions. Beamforming and one‐bit normalization significantly enhanced the stationary phase. Both microbaroms and surf appeared abundantly present, in the 0.1–2.0 Hz frequency band. Modeling revealed both tropospheric and stratospheric propagation of the infrasound, following traditional and more unconventional propagation mechanisms. Virtual source‐receiver refractions from stratospheric altitudes appeared a plausible explanation for the unusual short lag times, which allows for new ways to passively probe the stratosphere.

Plain Language Summary
Microbarometers measure infrasound from the background noise field, that is, ambient noise. This ambient noise is continuously present and generated by oceanic waves. The sound speed is a function of wind and temperature and determines how fast the ambient noise travels from one to another microbarometer. The so‐called travel time can be obtained by comparing the recordings of ambient noise of different microbarometers. The use of advanced processing techniques with multiple microbarometers was explored and enhanced the results. The observed travel times provided evidence for propagation through the upper atmosphere, that is, the stratosphere. New ways are proposed to apply surface‐based infrasound recordings in passively probing stratospheric wind and temperature conditions.