A new study shows that sensors on high-altitude balloons can detect pressure signals from earthquakes. Could the method be used on Venus?
By Jeng Hann Chong, University of New Mexico
Citation: Chong, J.H., 2021, Here’s what high-flying balloons can tell us about earthquakes, Temblor, http://doi.org/10.32858/temblor.189
Immediately after the large earthquakes in Ridgecrest, Calif. in early July 2019, researchers deployed numerous scientific instruments to collect data about the earthquake sequence and fault structures. One team deployed high-altitude balloons fitted with sensors that detect changes in atmospheric pressure, hoping to see evidence of aftershocks that continued to rock the region. The team found that their balloons detected pressure changes caused by a moderate-sized magnitude-4.2 earthquake south of Ridgecrest. They hope that this method could be used for a seismic investigation of Venus.
Recording pressure signals from earthquakes
Prior studies using balloons fitted with pressure sensors have successfully detected artificial seismic sources from seismic hammering and subsurface explosions, says Quentin Brissaud, a geophysicist at NORSAR (NORwegian Seismic Array), and lead author of the new study in Geophysical Research Letters. However, he says, this was the first study to successfully detect an earthquake using these pressure sensors attached to balloons.
Each balloon is about 20 or 33 feet (6 or 10 meters) wide with a small package attached underneath via a tether. Each package contained one pressure sensor and other electronic components. On July 22, 2019, the team launched the first pair of balloons, followed by another pair on August 9, 2019. The balloons flew for about 10 hours, at an altitude of more than nine miles (15 kilometers). One of the balloons, nicknamed “CrazyCat,” carried two packages and achieved an altitude of 15 miles (24 kilometers) above the ground — almost twice the height of a cruising commercial airplane.
These pressure sensors do not directly record seismic waves like ground-based seismometers. Instead, they record infrasound signatures from earthquakes — low-frequency acoustic signals generated in the air. Such signals occur as Rayleigh waves (a type of surface wave) of an earthquake roll across the surface. Brissaud likens it to the vibrations from a drum: When we hit a drum, this will excite the solid drumhead, causing the drum to vibrate. These vibrations excite the air particles around the drum and produce acoustic waves that we hear.
During the balloons’ flights, seismic stations on the ground recorded about 18 earthquakes that produced sufficiently large infrasound signals to be detected by the balloon-borne sensors. However, due to background noise, Brissaud and his colleagues could only identify and verify pressure signals from one magnitude-4.2 earthquake. Their balloons recorded the earthquake at an altitude of three miles (4.8 kilometers) and 48 miles (78 kilometers) away from the epicenter.
Wrangling a moving seismic sensor
Similar to a hot air balloon, these balloons fly because the hotter air inside the balloon is lighter than the surrounding cooler air. To heat the air within the balloon, the dark material absorbs heat from the sun, rather than using a burner like a traditional hot air balloon. As the sun sets, the temperature in the balloon decreases, causing the balloon to descend. The team tracks and recovers the balloon using onboard Global Positioning System (GPS).
However, deploying the balloons can be difficult. Specifically, Brissaud says, it is imperative to ensure that the balloons take off and fly in the desired direction to record earthquakes in the most seismically active regions. Although the team can predict a range of flight trajectories using flight models developed by co-author Daniel Bowman, there are still uncertainties due to wind variations. “If a flight is delayed by a few days, the initial predicted trajectory might become completely wrong,” Brissaud added.
Another challenge in data collection is background noise, which can be produced by a variety of sources, including infrasound-producing explosions or oscillations of the balloon itself. For example, as the balloon floats, it produces a vortex, similar to those created by flying planes. These vortices can change the surrounding air pressure and produce the same frequency ranges as pressure waves generated from earthquakes. The earthquake pressure signals can still be recovered from the noise by comparing the signals if a single balloon has two pressure sensors — like CrazyCat. Correlating balloon data with seismic signals collected on the ground provides an additional check, Brissaud says. The balloon pressure sensors are more likely to record the pressure signals for larger earthquakes because larger seismic waves tend to produce greater pressure signals, he says.
However, Voon Hui Lai, a seismologist at the Australian National University who was not involved in this study points out that the sensors that detect earthquakes are moving, whereas typical seismic sensors are immobile on the ground. The team, she says, must consider the mobility of the sensors, as well as variable atmospheric conditions as they model how the infrasound signal propagates through the atmosphere.
Applications on Venus and Earth
In the 1980s, the Russian Venera 13 and 14 probes successfully landed on Venus with onboard seismometers to search for tectonic movement. However, the probes stopped working within a matter of hours due to intense heat and pressure on the Venusian surface. By sending a balloon to the Venusian atmosphere (between 55 to 70 kilometers), which features Earth-like temperatures and atmospheric pressure at these altitudes, the balloon would have a longer lifetime, increasing the probability of detecting seismicity on Venus. The denser Venusian atmosphere near the surface, says Brissaud, would also result in a much clearer relationship between seismic and infrasound waves on Venus than on Earth.
Nevertheless, there are plenty of Earth-based applications, Brissaud says. For example, the balloons could be released over oceans to record seafloor tectonics, which would cost less than deploying ocean-bottom seismometers, he says.
Additionally, multiple balloons carrying an array of sensors could help estimate the velocity and direction of propagating waves from earthquakes, says Lai. Balloons can be used to complement the current volcanic monitoring network, which uses infrasound, she says, or even help with tracking storms.
The next step for Brissaud’s team is to head to another area where they can explore relationships between earthquake magnitude, subsurface properties and infrasound signals. Led by coauthor Siddharth Krishnamoorthy of the Jet Propulsion Laboratory (JPL), the team plans to deploy balloons in Oklahoma to analyze the pressure signals generated from earthquakes with different magnitudes and focal mechanisms, Brissaud says.
Oklahoma is ideal because of the high rate of induced earthquakes, says Lai, and the balloon deployment “will help us understand how small of a magnitude and distance is needed to detect an earthquake.”
Brissaud, Q., Krishnamoorthy, S., Jackson, J. M., Bowman, D. C., Komjathy, A., Cutts, J. A., et al. (2021). The first detection of an earthquake from a balloon using its acoustic signature. Geophysical Research Letters, 48, e2021GL093013. https://doi.org/10.1029/2021GL093013