Acoustic array

The aim of the Acoutic array is to document the range dependency of the acoustics signal emitted by lightning flashes and to improve the acoustics-based detection efficiency of lightning strikes within a range of 25 km in the monitored area. Observing strategy: We use a small array (AA) of 4 microphones and 8 supplementary stand-alone (SA) microphones. AA is composed of 4 microphones set up in a equatorial triangle of about 30 m baseline with a sensor in its barycenter. The SA microphones are deployed within a range of 10 km around AA. Microphones measure acoustic waves (atmospheric pressure change). Their bandwidth is from 0.1 to 10,000 Hz but the sampling frequency was 250 Hz. Correlation between recorded waveforms with AA sensors is performed to detect coherent acoustic wave which propagates over the network. Azimuth and horizontal velocity of the detected waves can then be calculated. From the time of occurrence of the associated flash and the time of arrival of the acoustic wave at AA, we infer the distance of the flash. Finally, from the horizontal distance, we calculate the elevation angle of the acoustic wave knowing the sound speed. With the azimuth, the elevation angle and the distance, we can then reconstruct in 3D the lightning channel. The SA microphones give the changes of the acoustic wave with the distance and the azimuth in comparison with the AA microphones. This technique has been developed and improved during the SOP1 (Gallin et al., 2016; Lacroix et al., 2018; Lacroix et al., 2019).