Characterization of the infrasonic wavefield from repeating seismo-acoustic events

Abstract

Infrasound can provide unique data on extreme atmospheric events such as meteor impacts, severe weather systems, man-made explosions, and volcanic eruptions. Use of infrasound for remote event detection and location requires highquality temporal and spatial atmospheric models, and infrasound generated by so-called Ground Truth events (for which the time and location are known) are necessary to evaluate atmospheric models and assess network performance. Large industrial blasts and military explosions are tightly constrained in time and space using seismic data and can generate infrasound recorded both regionally and at great distances. The most useful seismo-acoustic sources are repeating sources at which explosions take place relatively frequently. Over time, these may provide records of up to many hundreds of events fromthe same location fromwhich characteristics and variability of the infrasonic wavefield and atmospheric conditions can be assessed on a broad range of timescales. Over the past 20 years or so, numerous databases of repeating explosions have been compiled in various parts of the world. Events are associated confidently with known sources, with accurately determined origin times, usually by applying waveform correlation or similar techniques to the characteristic seismic signals generated by each explosive source. The sets of sources and stations ideally result in atmospheric propagation paths covering a wide range of distances and directions, and the databases ideally include events covering all seasons. For selected repeating sources and infrasound arrays, we have assessed the variability of infrasonic observation: including the documentation of lack of observed infrasound. These observations provide empirical celerity, back azimuth deviation, and apparent velocity probability distributions. Such empirical distributions have been demonstrated in numerous recent studies to provide infrasonic event location estimates with significantly improved uncertainty estimates. Tropospheric, stratospheric, and ther-mospheric returns have been observed, even at distances below 200 km. This information is now providing essential input data for studies of the middle and upper atmosphere.