Site-specific variations in air-to-ground coupled seismic arrivals from the 2012 October 16 explosion at Camp Minden, Louisiana, United States

Abstract

SUMMARY Air-to-ground coupled seismic arrivals, resulting from infrasound interacting with the ground surface, can be identified on seismometers co-located with microbarometers. Dense networks of co-located sensors provide detailed spatial information about the infrasonic wavefield, allowing investigation of infrasonic propagation and an assessment of the variability in air-to-ground coupling mechanisms. Signals generated by an explosion at Camp Minden, Louisiana, United States, on 2012 October 16 were recorded across the eastern United States on the USArray Transportable Array. 233 infrasound signals were identified at distances of up to 1566 km from the source, with 105 of these sites, at distances of up to 1147 km from the source, recording above-noise air-to-ground coupled seismic arrivals in the 2–4 Hz passband (chosen to maximise seismic arrival signal-to-noise ratios). The spatially dense infrasound recordings allowed a unique investigation of the azimuthal variation in infrasound amplitudes, showing that stratospheric arrival amplitudes increased by a factor greater than 4 between the edge and centre of the stratospheric propagation duct. Air-to-ground coupling coefficients, calculated as the ratios of temporally coincident peak-to-trough seismic and infrasound time-domain amplitudes, span over two orders of magnitude ([9.0 × 10−8, 5.0 × 10−5] ms−1 Pa−1). Sites exhibiting high coupling coefficients are predominantly located on alluvial sediments that support the generation of air-to-ground coupled Rayleigh waves, resulting in poor coherence between the seismic and infrasonic waveforms. In contrast, sites exhibiting low coupling coefficients are predominantly located on chemically weathered bedrock and only support direct coupling, leading to high coherence between air-to-ground coupled seismic and infrasonic recordings. The Camp Minden explosion observations, and a consideration of expected seismic noise and air-to-ground coupled arrival amplitudes, suggest that co-located pairs of seismometers and microbarometers are likely to record signals from small (tens of tons) explosions at distances of over 1000 km under favourable propagation conditions. Co-located deployments may therefore provide a cost-effective method for augmenting current state-of-the-art infrasound array networks, by providing a higher spatial density of recordings; this may assist in correctly associating recorded signals with, and estimating the location of, source events.