Numerical Simulations of Atmospheric Infrasound Generated by Surface Vibrations (Ground Impact, Earthquake, Microbaroms), Comparison with Experimental Data

The presented work consists of full-wave numerical modelling of three cases of planetary applications. Firstly, a seismic hammer ground impact is modelled in order to study the coupling from ground to atmosphere. Secondly, earthquakes are simulated in order to show that their subsequent surface waves could be observed relatively far away. Lastly, microbaroms are modelled from ocean surface to stratosphere in order to better understand the generation and direct propagation of those signals which, prevailing in the infrasound domain, make an interesting source for atmosphere probing.

A numerical tool solving the full Navier-Stokes equations (FNS), eventually coupled to elastodynamics, is used as a starting point [3, 2]. The numerical techniques as well as the latest developments are presented. The FNS implementation takes into account horizontal wind, viscosity, and atmospheric attenuation.

In a recent experiment in a flat topography region (Pahrump, Nevada) [4], signals due to seismic hammer shots were observed both on ground and from low-altitude balloons. The presented simulations are set up matching the experiment's parameters, and corroborate experimental data well enough. Using different seismic parameters, another simulation showed a high-amplitude infrasound plane wave, highlighting the sensibility of seismically-induced atmospheric signals to underground properties.

The simulations of the two earthquakes showed that the surface wave induced infrasound's amplitude barely decreases with horizontal distance.

Microbaroms are another type of atmospheric infrasound, generated by patches of ocean standing waves. Wave spectra are extracted from ocean models' hindcasts, and used as a starting point for modelling microbarom-generating waves. Full-wave numerical simulations are conducted using a realistic atmospheric model. The data from the 2016 NASA's ULDB flight is used to compare synthetics to observed stratospheric microbaroms [1].

[1] Bowman, D. C. and Lees, J. M. (2018). DOI 10.1029/2018GL077737.

[2] Brissaud (2017). www.theses.fr/2017ESAE0016.

[3] Brissaud et al. (2017). DOI 10.1093/gji/ggx185.

[4] Krishnamoorthy et al. (2018). DOI 10.1002/2018GL077481.