Probing the subsurface with infrasound

Deploying seismic networks at the ground is generally used to probe the interior structure of a planet or constrain seismic sources. However, such deployment can be challenging in Earth's remote regions or on other planets such as Venus facing harsh surface conditions with a surface temperature of 464 ° C. Fortunately, a fraction of the seismic energy is transmitted to the upper atmosphere as infrasound waves, ie low-frequency pressure perturbations (<20Hz). In recent years, inversions of infrasound data have shown promising results to estimate seismic ground motions and source characteristics from ground stations. However, these operations all require ground deployments. To address this challenge, free-floating balloons equipped with pressure sensors have therefore gained interest in the seismo-acoustic community as they are inexpensive to build and deploy and have a low wind noise level. On July 22, 2019, a heliotrope balloon, equipped with pressure sensors, was launched from the Johnson Valley, CA with the objective of capturing infrasound signals from the aftershock sequence of the 2019 Ridgecrest earthquake. At 16:27:36 UTC, the sound of a natural earthquake of Mw 4.2 was detected for the first time by a balloon platform. This observation offered the opportunity to attempt the first inversion of seismic velocities from the atmosphere. Shear velocities extracted by our analytical inversion method fell within a reasonable range from the values ​​provided by regional tomographic models. While our analysis was limited by the observation's low signal-to-noise ratio, future observations of seismic events from a network of balloons carrying multiple pressure sensors could provide unique constraints on crustal properties. The current state of infrasound and balloon-based observations, the sensitivity of the acoustic wavefield on subsurface properties, and perspectives on future inversions of seismically-induced acoustic data will be discussed.