Global empirical models for infrasonic celerity and backazimuth

Abstract

SUMMARY Global empirical models for infrasound signal celerity (where celerity is defined as the epicentral distance divided by the total traveltime) and backazimuth deviation (the difference between the measured and predicted backazimuth assuming great circle propagation), can be used for the association of infrasound automatic detections, for event location and for the validation of acoustic propagation simulations. Using software developed to consistently analyse a global ground truth database, we observe 296 detections in the 0.32–1.28 Hz passband; predominantly stratospheric arrivals, with a smaller number of tropospheric and thermospheric arrivals, in agreement with previous studies. We develop an updated global celerity-range model and introduce a global backazimuth deviation model. These new models suggest that the variation in backazimuth deviation is not range-dependent; 93 per cent of the 296 detections studied have a deviation magnitude ≤5°. The maximum deviation observed is 8.9°. The variation in celerity, however, is range-dependent. An exponential range-dependent celerity model with bounds calculated using a quantile regression fit to the traveltime residuals is determined. This traveltime residual model, with uncertainties that increase with range, produces bounds on the celerity consistent with both the observations and current understanding of infrasound propagation. The traveltime residual model derived celerity bounds are not symmetric around the celerity model, which further reflects the physical processes. At long ranges (>3000 km), maximum peak-to-trough amplitude arrivals are not observed with celerities <280 ms−1, but even at long ranges, we occasionally observe celerities (>320 −1) more usually consistent with tropospheric arrivals.