An active source seismo-acoustic experiment using tethered balloons to validate instrument concepts and modelling tools for atmospheric seismology

SUMMARY The measurements of acoustic waves created by a quake are of great interest for planets with hot and dense atmospheres, like Venus, because surface deployments of seismometers will last only a few hours, whereas freeflying balloons could fly many days. Infrasound sensors can also be used to constrain subsurface properties during active seismic experiments. This study presents a controlled source seismo-acoustic experiment using infrasonic sensors and accelerometers mounted on a tethered helium balloon. Both the acoustic waves generated below the balloon by seismic surface waves, and the ones generated by strong ground motions above the seismic source are clearly observed and separated on the records of the various instruments. This data set allows various validations and investigations. First, it validates the ground to air coupling theory and our numerical modelling tools. Then, it allows us to demonstrate that antenna processing of infrasound sensors deployed below the balloon can estimate the arrival incidence angle of the acoustic waves within 10°. Finally, a polarization analysis of the accelerometers taped on the balloon envelope is presented. It demonstrates that accelerometer records are strongly dependent on their location on the balloon due to its deformations and rotations. However, the different acoustic signals can be distinguished through their polarization, and a best sensor location is estimated at the bottom of the balloon envelope. These results are a first step towards detecting and locating seismic activity using airborne acoustic sensors on Venus and elsewhere. However, some observations of earthquake signals in a more realistic geometry are still missing.