infrasound | Acoustics.org

Listen In: Infrasonic Whispers Reveal the Hidden Structure of Planetary Interiors and Atmospheres

Quentin Brissaud – quentin@norsar.no
X (twitter): @QuentinBrissaud

Research Scientist, NORSAR, Kjeller, N/A, 2007, Norway

Sven Peter Näsholm, University of Oslo and NORSAR
Marouchka Froment, NORSAR
Antoine Turquet, NORSAR
Tina Kaschwich, NORSAR

Popular version of 1pPAb3 – Exploring a planet with infrasound: challenges in probing the subsurface and the atmosphere
Presented at the 186 ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0026837

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Low frequency sound, called infrasound, can help us better understand our atmosphere and explore distant planetary atmospheres and interiors.

Low-frequency sound waves below 20 Hz, known as infrasound, are inaudible to the human ear. They can be generated by a variety of natural phenomena, including volcanoes, ocean waves, and earthquakes. These waves travel over large distances and can be recorded by instruments such as microbarometers, which are sensitive to small pressure variations. This data can give unique insight into the source of the infrasound and the properties of the media it traveled through, whether solid, oceanic, or atmospheric. In the future, infrasound data might be key to build more robust weather prediction models and understand the evolution of our solar system.

Infrasound has been used on Earth to monitor stratospheric winds, to analyze the characteristics of man-made explosions, and even to detect earthquakes. But its potential extends beyond our home planet. Infrasound waves generated by meteor impacts on Mars have provided insight into the planet’s shallow seismic velocities, as well as near-surface winds and temperatures. On Venus, recent research considers that balloons floating in its atmosphere, and recording infrasound waves, could be one of the few alternatives to detect “venusquakes” and explore its interior, since surface pressures and temperatures are too extreme for conventional instruments.

Sonification of sound generated by the Flores Sea earthquake as recorded by a balloon flying at 19 km altitude.

Until recently, it has been challenging to map infrasound signals to various planetary phenomena, including ocean waves, atmospheric winds, and planetary interiors. However, our research team and collaborators have made significant strides in this field, developing tools to unlock the potential of infrasound-based planetary research. We retrieve the connections between source and media properties, and sound signatures through 3 different techniques: (1) training neural networks to learn the complex relationships between observed waveforms and source and media characteristics, (2) perform large-scale numerical simulations of seismic and sound waves from earthquakes and explosions, and (3) incorporate knowledge about source and seismic media from adjacent fields such as geodynamics and atmospheric chemistry to inform our modeling work. Our recent work highlights the potential of infrasound-based inversions to predict high-altitude winds from the sound of ocean waves with machine learning, to map an earthquake’s mechanism to its local sound signature, and to assess the detectability of venusquakes from high-altitude balloons.

To ensure the long-term success of infrasound research, dedicated Earth missions will be crucial to collect new data, support the development of efficient global modeling tools, and create rigorous inversion frameworks suited to various planetary environments. Nevertheless, Infrasound research shows that tuning into a planet’s whisper unlocks crucial insights into its state and evolution.

The Infrasonic Choir: Decoding Songs to Inform Decisions

Sarah McComas – sarah.mccomas@usace.army.mil

U.S. Army Engineer Research and Development Center, Vicksburg, MS, 39180, United States

Popular version of 1pPAb4 – The Infrasonic Choir: Decoding Songs to Inform Decisions
Presented at the 186th ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0026838

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Figure 1. Infrasound is a low frequency, sub-audible sound propagated over long distances (10’s to 1000’s of kilometers) and typically below the threshold of human hearing. Image courtesy of author.

The world around us is continuously evolving due to the actions of Mother Nature and man-made activities, impacting how we interact with the environment. Many of these activities generate infrasound, which is sound below the frequency threshold for human hearing (Figure 1). These signals can travel for long distances, 10s to 100s km based on source strength, while maintaining key information about what generated the signal. The multitude of signals can be thought of as an infrasonic choir with voices from a wide variety of sources which include natural signals such as surf and volcanic activity and man-made including infrastructure or industrial activities. Listening to, and deciphering, the infrasonic choir around us allows us to better understand how the world around us is evolving.

The infrasonic choir is observed by placing groupings, called arrays, of specialized sensors around the environment we wish to understand. These sensors are microphones designed to capture very low frequency sounds. An array’s geometry enables us to identify the direction the signal is observed. Using multiple arrays around a region allow for identification of the source location.

One useful application of decoding infrasonic songs is listening to infrastructure, such as a bridge. Bridges vibrate at frequencies related to the engineering characteristics of the structure, such as mass and stiffness. Bridges are surrounded by the fluid atmosphere which allow the bridge vibrations to create waves that can be measured with infrasound sensor arrays. One can visualize this as waves generated after a rock is thrown into a pond. As the bridge’s overall health degrades, whether through time or other events, its engineering characteristics change causing a change in the vibrational frequency. Being able to identify a change from a healthy, “in-tune” structure to an “out-of-tune”, unhealthy structure without having to see or inspect the bridge would enable continuous monitoring of entire regional road networks. The ability to conduct this type of monitoring after a natural disaster, such as hurricane or earthquake, would enable quick identification of damaged structures for prioritization of limited structural assessment resources.

Understanding how to decode the infrasonic choir within the symphony of the environment to better understand the world around us is the focus of ongoing research at the U.S. Army Engineer Research and Development Center. This research effort focuses on moving monitoring into source rich urban environments, the design of lightweight and low-cost sensors and mobile arrays, and the development of automated processing methods for analysis. When successful, continuous monitoring of this largely untapped source of information will provide a method for understanding the environment to better inform decisions.

Permission to publish was granted by the Director, Geotechnical and Structures Laboratory, U.S. Army Engineer Research and Development Center.

Can we detect volcanic eruptions and venusquakes from a balloon floating high above Venus?

Siddharth Krishnamoorthy – siddharth.krishnamoorthy@jpl.nasa.gov

NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, United States

Daniel C. Bowman², Emalee Hough³, Zach Yap³, John D. Wilding⁴, Jamey Jacob³, Brian Elbing³, Léo Martire¹, Attila Komjathy¹, Michael T. Pauken¹, James A. Cutts¹, Jennifer M. Jackson⁴, Raphaël F. Garcia⁵, and David Mimoun⁵

1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA
2. Sandia National Laboratories, Albuquerque, New Mexico, USA
3. Oklahoma State University, Stillwater, OK, USA
4. Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA
5. Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), Toulouse, France

Popular version of 4aPAa1 – Development of Balloon-Based Seismology for Venus through Earth-Analog Experiments and Simulations
Presented at the 184 ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0018837

Venus has often been described as a “hellscape” and deservedly so – the surface of Venus simultaneously scorches and crushes spacecraft that land on it with temperatures exceeding 460 degrees Celsius (~850 F) and atmospheric pressure exceeding 90 atmospheres. While the conditions on the surface of Venus are extreme, the temperature and pressure drop dramatically with altitude. At about 50-60 km above the surface, temperature (-10-70 C) and pressure (~0.2-1 atmosphere) resemble that on Earth. At this altitude, the challenge of surviving clouds of sulfuric acid is more manageable than that of surviving the simultaneous squeeze and scorch at the surface. This is evidenced by the fact that the two VeGa balloons floated in the atmosphere of Venus by the Soviet Union in 1985 transmitted data for approximately 48 hours (and presumably survived for much longer) compared to 2 hours and 7 minutes, which is the longest any spacecraft landed on the surface has survived. A new generation of Venus balloons is now being designed that can last over 100 days and can change their altitude to navigate different layers of Venus’ atmosphere. Our research focuses on developing technology to detect signatures of volcanic eruptions and “venusquakes” from balloons in the Venus clouds. Doing so allows us to quantify the level of ongoing activity on Venus, and associate this activity with maps of the surface, which in turn allows us to study the planet’s interior from high above the surface. Conducting this experiment from a balloon floating at an altitude of 50-60 km above the surface of Venus provides a significantly extended observation period, surpassing the lifespan of any spacecraft landed on the surface with current technology.

We propose to utilize low-frequency sound waves known as infrasound to detect and characterize Venus quakes and volcanic activity. These waves are generated due to coupling between the ground and the atmosphere of the planet – when the ground moves, it acts like a drum that produces weak infrasound waves in the atmosphere, which can then be detected by pressure sensors deployed from balloons as shown in figure 1. On Venus, the process of conversion from ground motion to infrasound is up to 60 times more efficient than Earth.

Figure 1: Infrasound is generated when the atmosphere reverberates in response to the motion of the ground and can be detected on balloons. Infrasound can travel directly from the site of the event to the balloon (epicentral) or be generated by seismic waves as they pass underneath the balloon and travel vertically upward (surface wave infrasound).

We are developing this technique by first demonstrating that earthquakes and volcanic eruptions on Earth can be detected by instruments suspended from balloons. These data also allow us to validate our simulation tools and generate estimates for what such signals may look like on Venus. In flight experiments over the last few years, not just several earthquakes of varying magnitudes and volcanic eruptions, but also other Venus-relevant phenomena such as lightning and mountain waves have been detected from balloons as shown in figure 2.

Figure 2: Venus-relevant events on Earth detected on high-altitude balloons using infrasound. Pressure waves from the originating event travel to the balloon and are recorded by barometers suspended from the balloon.

In the next phase of the project, we will generate a catalog of analogous signals on Venus and develop signal identification tools that can autonomously identify signals of interest on a Venus flight.

Copyright 2023, all rights reserved. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

Noise reduction for low frequency sound measurements from balloons on Venus

Taylor Swaim – tswaim@okstate.edu

Oklahoma State University
Stillwater, Oklahoma 74078
United States

Kate Spillman
Emalee Hough
Zach Yap
Jamey D. Jacob
Brian R. Elbing (twitter: @ElbingProf)

Popular version of 2pCA6 – Infrasound noise mitigation on high altitude balloons
Presented at the 184 ASA Meeting
Read the article in Proceedings of Meetings on Acoustics

While there is great interest in studying the structure of Venus because it is believed to be similar to Earth, there are no direct seismic measurements on Venus. This is because the Venus surface temperature is too hot for electronics, but conditions are milder in the middle of the Venus atmosphere. This has motivated interest in studying seismic activity using low frequency sound measurements on high altitude balloons. Recently, this method was demonstrated on Earth with weak earthquakes being detected from balloons flying at twice the altitude of commercial airplanes. Video 1 shows a balloon launch for these test flights. Due to the denser atmosphere on Venus, the coupling between the Venus-quake and the sound waves should be much greater, which will make the sound louder on Venus. However, the higher density atmosphere combined with vertical changes in wind speed is also likely to increase the amount of wind noise on these sensor. Thus development of a new technology to reduce wind noise on a high altitude balloon is needed.

Video 1. Video of a balloon launch during the summer of 2021. Video courtesy of Jamey Jacob.

Several different designs were proposed and ground tested to identify potential materials for compact windscreens. The testing included a long-term deployment outdoors so that the sensors would be exposed to a wide range of wind speeds and conditions. Separately, the sensors were exposed to controlled low-frequency sounds to test if the windscreens were also reducing the loudness of the signals of interest. All of the designs showed significant reduction in wind noise with minimal reduction in the controlled sounds, but one design in particular outperformed the others. This design uses a canvas fabric on the outside of a box as shown in the Figure 1 combined with a dense foam material on the inside.

Figure 1. Picture of balloon carrying the low frequency sound sensors. Compared an early design to no windscreen with this flight. Image courtesy of Brian Elbing.

The next step is to fly this windscreen on a high altitude balloon, especially on windier days and with a long flight line to increase the amount of wind that the sensors will experience. The wind direction at the float altitude of these balloons will change in May and then rapidly increase, which this will be the target window to test this new design.

4aPA4 – Acoustic multi-pole source inversions of volcano infrasound

Keehoon Kim – kkim32@alaska.edu
University of Alaska Fairbanks
Wilson Infrasound Observatory, Alaska Volcano Observatory, Geophysical Institute
903 Koyukuk Drive, Fairbanks, Alaska 99775

David Fee – dfee1@alaska.edu
University of Alaska Fairbanks
Wilson Infrasound Observatory, Alaska Volcano Observatory, Geophysical Institute
903 Koyukuk Drive, Fairbanks, Alaska 99775

Akihiko Yokoo – yokoo@aso.vgs.kyoto-u.ac.jp
Kyoto University
Institute for Geothermal Sciences
Kumamoto, Japan

Jonathan M. Lees – jonathan.lees@unc.edu
University of North Carolina Chapel Hill
Department of Geological Sciences
104 South Road, Chapel Hill, North Carolina 27599

Mario Ruiz – mruiz@igepn.edu.ec
Escuela Politecnica Nacional
Instituto Geofisico
Quito, Ecuador

Popular version of paper 4aPA4, “Acoustic multipole source inversions of volcano infrasound”
Presented Thursday morning, May 21, 2015, at 9:30 AM in room Kings 1
169th ASA Meeting, Pittsburgh
Click here to read the abstract

Volcano infrasound
Volcanoes are outstanding natural sources of infrasound (low-frequency acoustic waves below 20 Hz). In the last few decades local infrasound networks have become an essential part of geophysical monitoring systems for volcanic activity. Unlike seismic networks dedicated to monitoring subsurface activity (c.f., magma or fluid transportation) infrasound monitoring facilitates detecting and characterizing eruption activity at the earth’s surface. Figure 1a shows Sakurajima Volcano in southern Japan and an infrasound network deployed in July 2013. Figure 1b is an image of a typical explosive eruption during the field experiment, which produces loud infrasound.

Figure 1. a) A satellite image of Sakurajima Volcano, adapted from Kim and Lees (2014). Five stand-alone infrasound sensors were deployed around Showa Crater in July 2013, indicated by inverted triangles. b) An image of a typical explosive eruption observed during the field campaign.

Source of volcano infrasound
One of the major sources of volcano infrasound is a volume change in the atmosphere. Mass discharge from volcanic eruptions displaces the atmosphere near and around the vent and this displacement propagates into the atmosphere as acoustic waves. Infrasound signals can, therefore, represent a time history of the atmospheric volume change during eruptions. Volume flux inferred from infrasound data can be further converted into mass eruption rate with the density of the erupting mixture. Mass eruption rate is a critical parameter for forecasting ash-cloud dispersal during eruptions and consequently important for aviation safety. One of the problems associated with the volume flux estimation is that observed infrasound signals can be affected by propagation path effects between the source and receivers. Hence, these path effects must be appropriately accounted for and removed from the signals in order to obtain the accurate source parameter.

Infrasound propagation modeling
Figure 2. a) Sound pressure level in dB relative to the peak pressure at the source position. b) Variation of infrasound waveforms across the network caused by propagation path effects.

Figure 2 shows the results of numerical modeling of sound propagation from the vent of Sakurajima Volcano. The sound propagation is simulated by solving the acoustic wave equation using a Finite-Difference Time-Domain method taking into account volcanic topography. The synthetic wavefield is excited by a Gaussian-like source time function (with 1 Hz corner frequency) inserted at the center of Showa Crater (Figure 2a). Homogeneous atmosphere is assumed since atmospheric heterogeneity should have limited influence in this local range (< 7 km). The numerical modeling demonstrates that both amplitude and waveform of infrasound are significantly affected by the local topography. In Figure 2a, Sound Pressure Level (SPL) relative to the source amplitude is calculated at each computational grid node on the ground surface. The SPL map indicates an asymmetric radiation pattern of acoustic energy. Propagation paths to the northwest of Showa Crater are obstructed by the summit of the volcano (Minamidake), and as a result acoustic shadow zones are created northwest of the summit. Infrasound waveform also shows significant variation across the network. In Figure 2b, synthetic infrasound signals computed at the station positions (ARI – SVO) show bipolar pulses followed by oscillations in pressure while the pressure time history at the source location exhibits only a positive unipolar pulse. This result indicates that the oscillatory infrasound waveforms can be produced by not only source effects but also propagation path effects. Hence, this waveform distortion must be considered for source parameter inversion.

Volume flux estimates
Because wavelengths of volcano infrasound are usually longer than the dimension of source region, the acoustic sources are typically treated as a monopole, which is a point source approximation of volume expansion or contraction. Then, infrasound data represent the convolution of volume flux history at the source and the response of the propagation medium, called Green’s function. Volume flux history can be obtained by deconvolving the Green’s functions from the data. The Green’s functions can be obtained by two different ways: 3-D numerical modeling considering local topography (Case 1) and the analytic solution in a half-space neglecting volcanic topography (Case 2). Resultant volume histories for a selected infrasound event are compared in Figure 3. Case 1 results in gradually decreasing volume flux curve, but Case 2 shows pronounced oscillation in volume flux. In Case 2, propagation path effects are not appropriately removed from the data leading to misinterpretation of the source effect.

Summary
Proper Green’s function is critical for accurate volume flux history estimation. We obtained a reasonable volume flux history using the 3-D numerical Green’s function. In this study only simple source model (monopole) was considered for volcanic explosions. More general representation can be obtained by multipole expansion of acoustic sources. In 169th ASA Meeting presentation, we will further discuss source complexity of volcano infrasound, which requires the higher-order terms of the multipole series.

Kim3 Figure 3. Volume flux history inferred from infrasound data. In Case 1, the Green’s function is computed by 3-D numerical modeling considering volcanic topography. In Case 2, the analytic solution of the wave equation in a half-space is used, neglecting the topography.

References

Kim, K. and J. M. Lees (2014). Local Volcano Infrasound and Source Localization Investigated by 3D Simulation. Seismological Research Letters, 85, 1177-1186