LOW-FREQUENCY SOURCE MONITORING

Many sources of seismic activity also generates low-frequency sound waves (by definition – frequencies below 20 Hz) in time.

Sources of infrasound are in the range from very low-frequency atmospheric fluctuations up into the lower audio frequencies. These sources include natural occurrences, industrial installations, low-speed machinery, etc.

Main applications:

• Volcano monitoring (explosive eruptions, degassing, lava fountains)
• Earthquakes (secondary coupling of ground motion into atmosphere)
• Tsunamis (especially atmospheric gravity waves) – early warning
• Severe weather (thunderstorms, tornadoes, hurricanes)
• Ocean waves (microbaroms – from nonlinear interaction of ocean waves)
• Explosion detection
• Glacier movement

Seismic + infrasound methods improve detection/localisation.

Regarding glacier movement, it is possible to use an array of infrasound systems. Its back azimuth variation with time tracks the ice mass trajectory and provides a mean velocity estimate. Infrasound frequency is used to estimate a radius, that despite overestimating the volume, provides quantitative analysis in near-real time.