Scientists used a stratospheric balloon to hear the infrasound from an underground explosion

(Daniel C. Bowman et al. / Geophysical Research Letters)

Geophysicists performed an experiment, in which they made an underground explosion and recorded its sound from a stratospheric balloon.

The results confirmed that low-frequency sounds are easier to hear from the stratosphere than from the surface of the Earth.

Infrasound from various geological and anthropogenic events, for example from volcanic eruptions or from explosions, spreads over a long distance, but it is not so easy to detect it.

Natural phenomena and anthropogenic events are often accompanied by infrasound, which has a low-frequency range of sound waves - from 16 to 0.001 hertz.

Human hearing is not able to pick it up, because it is sensitive to the range of 16–20,000 hertz.

Global International Monitoring System ground microbarometers detect and locate such events, but this method has distance limitations.

Infrasound waves propagate over long distances, but when moving across the landscape, they are scattered by oncoming objects and turbulence in the lower atmosphere

Therefore, the accuracy of their monitoring strongly depends on the distance to the epicenter.

In addition, earthquakes, underground explosions and some volcanic eruptions generate highly directional vertical acoustic signals, the divergence angle of which does not exceed 45 degrees.

This means that sensors located in the same plane with the event source are unable to capture this sound.

** The experiment **
Now, geophysicist Daniel C. Bowman of Sandia National Laboratory and technologist Siddharth Krishnamoorthy of California Institute of Technology suggested that it would be easier to detect low frequency signals from the air than from the ground.

They made an underground chemical explosion in order to record this event with the help of a stratospheric balloon, and then compare the infrasound records from air and ground sensors.

The explosion was organized in a well at a depth of 51.6 meters on the territory of the Nevada Test Site in the United States and had a capacity of 10 tons of TNT.

The seismic and acoustic waves of the explosion were recorded by an extensive ground-based network of accelerometers, geophones, broadband seismometers, and infrasonic microbarometers.

From the air, the registration was carried out using a balloon to which four infrasound sensors were attached.

Both ground and airborne microbarometers were tuned to a low frequency range of 1 to 20 hertz.

Ground sensors located within 12 kilometers of the explosion recorded the event, but equipment 46 kilometers from the epicenter heard nothing.

Stratospheric balloon sensors recorded an explosion, at which time the stratosphere hovered in the lower stratosphere at an altitude of 21.8 kilometers, 56 kilometers horizontally from the well.

Comparison of the records of the stratospheric sensor with six ground-based ones located half a kilometer from the epicenter showed good convergence.

The authors explain this by the fact that the ball was in an area of very low background noise - it drifted passively with the wind and hovered much higher than localized sound sources such as buildings and vehicles.

In addition, the troposphere refracts sound upward, the authors note, so the balloon recorded the signal at a greater distance from the source than ground-based sensors.

** Importance **
The authors suggest that the stratospheric microbarometer method is suitable for tracking various natural events, as well as for monitoring underground explosions that occur during testing of nuclear weapons.

In addition, the authors note the effectiveness of the method in a geophysical mission to Venus.

There it will help track the planet's seismic activity, bypassing the unfavorable atmospheric conditions of temperature and pressure near the surface.

Source:

• Geophysical Research Letters: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL094861

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