The First Detection of an Earthquake From a Balloon Using Its Acoustic Signature

Seismo-acoustic coupling in the deep atmosphere of Venus

The extreme conditions at the surface of Venus pose a challenge for monitoring the planet's seismic activity using long-duration landed probes. One alternative is using balloon-based sensors to detect venusquakes from the atmosphere. This study aims to assess the efficiency with which seismic motion is coupled as atmospheric acoustic waves across Venus's surface. It is, therefore, restricted to the immediate neighborhood of the crust-atmosphere interface. In order to account for supercritical conditions near the surface, the Peng-Robinson equation of state is used to obtain the acoustic sound speed and attenuation coefficient in the lower atmosphere. The energy transported across the surface from deep and shallow sources is shown to be a few orders of magnitude larger than on Earth, pointing to a better seismo-acoustic coupling. For a more realistic scenario, simulations were made of the acoustic field generated in the lower atmosphere by the ground motion arising from a vertical array of subsurface point-force sources. The resulting transmission loss maps show a strong epicentral cone accompanied by contributions from leaky surface waves. Results at 0.1 Hz and 1 Hz confirm that the width of the epicentral cone is larger at lower frequencies.

Infrasound direction of arrival determination using a balloon-borne aeroseismometer

Free-floating balloons are an emerging platform for infrasound recording, but they cannot host arrays sufficiently wide for multi-sensor acoustic direction finding techniques. Because infrasound waves are longitudinal, the balloon motion in response to acoustic loading can be used to determine the signal azimuth. This technique, called "aeroseismometry," permits sparse balloon-borne networks to geolocate acoustic sources. This is demonstrated by using an aeroseismometer on a stratospheric balloon to measure the direction of arrival of acoustic waves from successive ground chemical explosions. A geolocation algorithm adapted from hydroacoustics is then used to calculate the location of the explosions.

Skyfall: Signal Fusion from a Smartphone Falling from the Stratosphere

A smartphone plummeted from a stratospheric height of 36 km, providing a near-real-time record of its rapid descent and ground impact. An app recorded and streamed useful internal multi-sensor data at high sample rates. Signal fusion with external and internal sensor systems permitted a more detailed reconstruction of the Skyfall chronology, including its descent speed, rotation rate, and impact deceleration. Our results reinforce the potential of smartphones as an agile and versatile geophysical data collection system for environmental and disaster monitoring IoT applications. We discuss mobile environmental sensing capabilities and present a flexible data model to record and stream signals of interest. The Skyfall case study can be used as a guide to smartphone signal processing methods that are transportable to other hardware platforms and operating systems.