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 to 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.