Infrasound sensors as far away as Washington State detected a fireball over western Russia on 21 June 2018. Several weeks earlier scientists in Oklahoma announced they had used infrasound to track and accurately predict the timing and size of a tornado. And around the world, scientists like David Fee of the Alaska Volcano Observatory are frequently awakened by alerts that infrasound signals indicate an eruption of one of the volcanoes they monitor; if the ash could potentially interfere with aviation, they pass the information on to national weather services.
Made up of acoustic waves below the 20 Hz frequency threshold of human hearing, infrasound is increasingly used to study Earth’s crust, the atmosphere, various human activities, and more. (See the article by Alfred Bedard and Thomas Georges, Physics Today, March 2000, page 32, and the Quick Study by Curt Szuberla and Ken Arnoult, April 2011, page 74.) Potential applications of infrasound are also burgeoning—from avalanche warning systems to contributions to weather and climate change models to medical equipment. Generated when large things displace a fluid quickly, when gas is injected into air, or even when a door is opened, infrasound propagates easily through the atmosphere. The lower the frequency, the slower the attenuation and the farther waves travel. Infrasound at or below 0.1 Hz can propagate around the planet.
Noise as signal
When the Krakatoa volcano in Indonesia erupted in 1883, pressure gauges around the world recorded infrasound that circled the globe multiple times. Gauges also registered the Tunguska meteor, which flattened 2000 km2 of eastern Siberia in 1908.
Infrasound research has been driven largely by defense needs, namely to monitor nuclear and chemical explosions. Investment in the method slowed with the advent of satellites in the early 1960s and was at a near standstill by the 1980s.
In 1996 the United Nations adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which aims to prohibit nuclear explosions. To monitor compliance, the UN established the International Monitoring System (IMS; see the article by Matthias Auer and Mark Prior, Physics Today, September 2014, page 39). Run by the Vienna-based CTBT Organization, the IMS has stations around the globe. Along with forensic seismology, hydroacoustics, and radionuclide monitoring, infrasound arrays are used to detect, locate, and classify nuclear explosions. Pressure sensors can pick up infrasound signals from far away and are effective in bad weather and at night. So far, 50 out of 60 planned IMS infrasound stations have been installed; of the rest, some are in the works and others are hindered by difficult terrain or local politics.
The need to distinguish nuclear explosions from noise—and curiosity about the noise itself—has revived the dormant area of infrasound research. Besides monitoring, the IMS infrasound network has opened a new window for observing atmospheric disturbances. “We see lightning, meteorites, volcanoes, gravity waves …,” says Elisabeth Blanc, a research director at the French Alternative Energies and Atomic Energy Commission and coordinator of the Atmospheric Dynamics Research Infrastructure in Europe (ARISE) project. Infrasound and seismic waves can provide unique insights into the coupling between Earth, oceans, and atmosphere, she says.
Compact microbarometers detect with microbar or even nanobar precision anything that creates a pressure change in the atmosphere. They range in price from about $1000 to $25 000. And, says physicist Peter Brown of the University of Western Ontario, “they let you monitor lots of stuff.”
Three infrasound arrays are usually used to locate a distant, unknown source, and having more sensors improves the precision. Since infrasound monitoring has global reach, says Brown, even over the open ocean where there are no cameras, “there is a good chance that with infrasound we can pick up signals from large meteors.”
The number of infrasound researchers has swelled, with most coming from seismology or meteorology. The resurgence in interest in infrasound is aided by a growing understanding of the atmosphere through which the waves propagate. In addition, methods for blocking wind noise have improved, and there have been huge leaps in data storage, transfer, and manipulation.
Infrasonic sensors have been installed near dozens of volcanoes around the world as a complement to seismic sensors, cameras, and other tools. Infrasound arrays, Fee explains, can follow the dynamics of magma movement and pinpoint which of a volcano’s vents is the location of an eruption. “One goal is to derive quantitative information,” he says. Another is to recognize hazards and issue warnings.
A volcano vent can be compared to an organ pipe, says Boise State University volcanologist Jeffrey Johnson. “When the lava lake climbs up and falls back down, you get reverberations. You have a musical instrument that changes its frequency and its ability to project sound.” Ultimately, he says, “we want to figure out what is happening at the volcanic vent. How much stuff comes out? What is the eruption rate of gas and mass? How high is the plume? What is the geometry of the crater?”
The 3 March 2015 eruption of the Villarrica volcano in Chile entailed a mile-high lava fountain. A few months earlier, Johnson and colleagues had fortuitously placed an array of infrasound monitors nearby—on the crater rim and at sites from 4 km to 22 km from the volcano. After the eruption, the researchers combed the data from the surviving monitors for signs that might have foreshadowed the activity. He says they found an eruption precursor: In the days leading up to the eruption, infrasound revealed that the lava lake rose in the volcano’s conduit. “It is the best tool we have to remotely detect what we can’t see,” he says.
The propagation of infrasound from remote, well-characterized sources can also be used to probe the atmosphere. The effective wind speed can be derived and gives information about the atmosphere’s structure and dynamics. The method is especially useful for the stratosphere, mesosphere, and lower thermosphere, from about 30 km to 100 km in altitude, says Blanc, where “uncertainties are strong and it’s difficult to obtain measurements.” Volcanoes and quarry explosions are “validated” for such atmospheric remote sensing, she says, and scientists are also trying to use microbaroms—the ubiquitous signals created by crashing ocean waves—for that purpose. (See, for example, the article by Roel Snieder and Kees Wapenaar, Physics Today, September 2010, page 44.)
Data from the IMS network, ARISE’s 24 infrasound stations in 12 countries, other infrasound monitors, and radar and lidar are expected to improve numerical weather-prediction models, says Blanc.
Infrasound data also contribute to understanding climate change. For example, scientists can scour data accumulated over years for changes and patterns in the occurrence of thunderstorms, other severe weather events, and glacier calving in the polar regions.
In contrast to seismological measurements, which typically give the same results when repeated, the atmosphere changes quickly, says Danny Bowman of Sandia National Laboratories. “It’s as though every event is transmitted through a different medium.”
Bowman does balloon-borne infrasound exploration. (It has precedent: The US Air Force flew balloons to listen for Soviet bomb tests in the late 1940s; one landed in the desert near Roswell, New Mexico, which led to theories about a UFO there.) His first such experiment was in 2014 on NASA’s High-Altitude Student Platform. Now he and his colleagues send up balloons with microbarometers a few times a year—as often as they can muster funding. Solar balloons, which gain buoyancy as the air inside them is heated by the Sun, get to altitudes of 20–24 km, and NASA’s helium balloons rise to nearly 40 km. Balloons bearing infrasound sensors could be released elsewhere in the solar system, notes Brown. Infrasound could be used to listen for “venusquakes” and for other studies of gaseous or hot planets.
In the stratosphere, temperature rises with altitude while density falls. As a result, convection is suppressed and winds are smooth and laminar. Balloons in the stratosphere not only float for long distances, they are also free from the noise generated by turbulence. In 2016 one of Bowman’s microbarometers hitched a ride on a NASA balloon launched from New Zealand. The balloon circled Antarctica and landed in Peru 43 days later. “We’ve identified thunder, colliding ocean waves, explosions on the Earth, something we think was a bolide—or meteor—and more mysteries,” says Bowman. So far, he and his colleagues have had to retrieve their instruments to access the data, but upcoming experiments will transmit data in real time.
Infrasound can detect meteors as small as golf balls. Getting a handle on the distributions of their size and energy is interesting for many reasons, says Brown. Meteors deliver metal atoms and organic compounds, which are necessary for understanding prebiotic chemistry and the origins of life. Statistics about meteors can also yield cratering rates on airless bodies in the solar system, which can be used to estimate the surface age of the impacted body.
Brian Elbing, a fluid mechanics engineer at Oklahoma State University, began tracking tornadoes with infrasound as a side project. But his spot-on prediction of a twister on 11 May 2017 has amped his and others’ interest in the approach. Tornadoes are traditionally tracked using Doppler radar. But he says that because warnings are wrong some three-quarters of the time, people ignore them. (See also the interview with meteorologist and tornado chaser Howard Bluestein at http://physicstoday.org/bluestein.)
Elbing uses three microphones, spaced about 60 meters apart, to passively listen for tornadoes. Hoses emanating like spokes from the microphones function as a windscreen: Local pressure waves from wind enter the hoses incoherently and are canceled out, whereas signals from a tornado or other more distant sources arrive coherently.
The setup picked up signals about 10 minutes before last year’s tornado formed some 20 km away, and based on them Elbing gauged the path to be 46 meters wide—exactly the official damage path width reported afterward by NOAA.
Storms start emitting infrasound as much as an hour before a tornado is formed, which could eventually lead to more advanced warnings. One challenge is to figure out in detail how the infrasound is produced, says Kevin Knupp of the Severe Weather Institute—Radar and Lightning Laboratories at the University of Alabama in Huntsville, who is not involved in Elbing’s project. “If we can determine that one can reliably detect tornado infrasound over a broad range of scales, the next step would be to have an operational system that the weather service could use or infrasound detectors that people could have in their homes,” says Knupp.
In addition to monitoring for nuclear explosions, Omar Marcillo of Los Alamos National Laboratory explores infrasound’s potential for remotely eavesdropping on industrial areas. In combination, infrasound and seismic waves could yield information about processes occurring at or just below the surface, he says. Modern wind turbines, large fans, pumps, and ventilators in power plants and factories create a seismic–infrasonic din, he explains. “The signals could give information about activities in a place of interest. You can’t tell what type of manufacturing, but you can tell that heavy lifting is going on.”
Qamar Shams of NASA’s Langley Research Center has developed an infrasonic polyurethane foam windscreen that he claims reduces wind noise by 10–20 dB. One potential application is for shielding detectors of clear-air turbulence, which he says could be used for early identification of tornadoes and hurricanes and for locating vortices in an aircraft’s wake. Microphones installed as a test at Virginia’s Newport News/Williamsburg International Airport in 2012–13 recorded the strong vortices behind aircraft as they took off and landed. Infrasound detection could be used in a wake-avoidance system, he says.
And infrasound could have life-saving medical applications, says Shams. The human heart beats about 72 times per minute. That’s a fundamental frequency of 1.2 Hz, “so our blood circulation is in the infrasonic bandwidth.” An infrasonic stethoscope that he and colleagues developed is currently undergoing clinical tests at NASA’s Kennedy Space Center, he says. “More than 85% of the world population doesn’t have access to echocardiography, CT scans, or other advanced diagnostic tools.” The infrasonic stethoscope, he says, could provide a cheaper method for early screening of heart and lung diseases.