Infrasound Propagation Simulation in the Upper Atmosphere

Keehoon Kim | 23-FS-012

Project Overview

Infrasound (low-frequency sound below 20 Hz) is a key technology used to monitor energetic events in the atmosphere for global or national security. Energetic man-made or natural events (e.g., chemical/nuclear explosions, rockets, and earthquakes) create large pressure disturbances which propagate as infrasound in the atmosphere. Due to the characteristic stratification of Earth's atmosphere, infrasound energy can be efficiently trapped in the stratosphere (10 - 50 km in altitude) and thermosphere (>80 km) and propagate for long distances, allowing for detection at thousands of kilometers. The detection capability for the stratospheric infrasound generally depends on the stratospheric wind conditions: infrasound energy is focused in downwind direction generating the "silence zone" of infrasound in upwind direction. Unlike the stratospheric infrasound, the thermospheric infrasound, which travels in the thermosphere, is not affected by the stratospheric wind and can be detectable on ground-based networks in any direction. While this thermospheric infrasound has the potential to greatly improve the detection coverage, its propagation characteristics are strongly affected by nonlinear effects due to the low-density of upper atmosphere. This nonlinear propagation cannot be simulated or predicted by the classical linear acoustic theory which is widely used for the infrasound traveling in the lower part of atmosphere. The nonlinearized fluid dynamics equations were suggested for a nonlinear infrasound model, but their validity for upper atmosphere has not been fully investigated.

In this study, we applied the fluid dynanmics models for nonlinear infrasound in the atmosphere and evaluate its validity based on ground-truth experiment data. We used hydrodynamic simulation codes, GEODYN-L for nonlinear infrasound propagation and identified key physics affecting atmospheric infrasound. GEODYN-L is a multimaterial Lagrangian hydrodyanmic codes with staggered finite-difference mesh. We applied GEODYN-L for infrasound traveling relative low altitudes and successfully predicted the observed infrasound waveform. Our simulation results indicated that hydrodynamic models can be used for nonlinear infrasound in the atmosphere by addressing background pressure, density, and flow conditions. However, it is challenging to simulate high-altitude atmospheres with high accuracy. Simulations for small acoustic pressure changes by hydrodynamic models require strict equilibrium conditions for atmosphere against the gravity. However, drastic changes of atmospheric densities and wind flows pose difficulties to maintaining this equilibrium. Finding accurate equilibrium conditions for hydrodynamics is the most important to simulate nonlinear infrasound accurately.

Mission Impact

This project is directly in line with  Lawrence Livermore National Laboratory's (LLNL) investment strategies for the nuclear threat reduction improving explosion monitoring capability by infrasound signals. The study of upper-atmospheric infrasound will also refine our understanding on physical conditions and structures in the upper atmosphere in support of LLNL's Earth and Atmosphere Sciences area. This project will be continued by the support of NNSA Ground-based Nuclear Detonation Detection program.

Publications, Presentations, and Patents

K. Kim, O. Vorobiev, and E. Vitali, "Nonlinear infrasound propagation simulation by hydrodynamic models" (Presentation, Seismological Society of America Annual Meeting, Anchorage, AK, April 2023).