Characterisation of scattered transient infrasonic wavefields combining array and single-sensor measures

Infrasonic scattering by fine-scale atmospheric sound-speed structure causes variability in signals generated by repeated explosions in Hukkakero, Finland, recorded by the IS37 microbarometer array at 321 km distance. The sample covariance matrix and beam pattern of the second eigenmode (array measures) are sensitive to changes in wavefield structure below array resolution. The directional energy distribution is approximated by a skewed Gaussian distribution, where the inter-sensor coherence decay is inverted for the beamwidth and the skew is indicated by the second-eigenmode beam pattern. With increasing lapse time in the scattered wave train, the beamwidth increases over both apparent velocity and backazimuth, and the energy distribution is skewed with fluctuating directionality over backazimuth. Progressive over-prediction of the second-eigenmode magnitude by the best-fit Gaussian distribution throughout the wave train implies an increase in the mutual directional coherence of the wavefield—i.e., phase correlation in different look directions—relative to the beamwidth. Multi-scale entropy and spectral-amplitude correlation (single-sensor measures) indicate a time-variant mixture of ballistic and diffusive arrivals, often with similar amplitudes. These array and single-sensor measures should be considered when inverting for fine-scale sound speed structure in the middle atmosphere, extending beyond the standard analysis of infrasonic temporal and spectral content.