Azores Infrasound Network: Analysis of background noise

In 2010, the station IS42 was the first infrasound station to be installed in the Azores, located on Graciosa Island in the central group of the Azores archipelago, in the middle of the North Atlantic. This station integrates the International Monitoring System (IMS) of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). Although the mission of the CTBTO is to put an end to nuclear tests, the long-term infrasound data recorded by IMS have proven to be very valuable for monitoring and understanding natural phenomena, including seismo-volcanic activity in the Azores.

With the aim of monitoring the 2022 seismo-volcanic crisis on São Jorge Island, a first portable infrasound array was deployed to complement the data recorded from the permanent IMS station and enrich the archipelago’s monitoring network. A second portable array was subsequently deployed on Terceira Island, and an additional array is planned for deployment on Faial Island later this year, further strengthening the Azores infrasound monitoring network.

This study analyses the performance and behaviour of the portable arrays, using IS42 as a reference station. We applied a multi-channel correlation analysis in the time domain to evaluate the influence of background noise on the recorded signals and assess the impact of station location and environmental conditions on the detections. Root-mean-square (RMS) noise analyses were combined with source direction estimates based on the detections’ back azimuths. Seasonal analyses of the detections revealed a strong influence of atmospheric conditions on noise levels and, consequently, on back azimuth directions. These results highlight the importance of noise characterisation of integrated infrasound observations in oceanic islands.