International Monitoring System infrasound data products for atmospheric studies and civilian applications

Abstract. The International Monitoring System (IMS) has been established since the late 1990s for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The IMS is supposed to detect any explosion of at least 1 kt of TNT equivalent underground, underwater, and in the atmosphere. Upon completion, monitoring the Earth’s atmosphere for low-frequency pressure waves will be realized using up to 60 infrasound stations distributed over the globe. Acoustic waves in the infrasound range (between around 0.01 and 20 Hz) can efficiently propagate over long distances, subject to the winds near the stratopause at around 50 km. Therefore, infrasound observations of repeating or persistent sources have been suggested for probing the winds in the middle atmosphere, where numerical weather prediction models suffer from the lack of continuous observation technologies for data assimilation. One type of repetitive source is active volcanoes. In turn, this natural hazard for civil security can be monitored using infrasound, and first prototypes of applications for the release of early volcanic eruption warnings have been established. However, access to raw infrasound data or products of the IMS is limited to specific user groups, which might hinder the utilization of infrasound observations.

In this study, we present advanced infrasound data products for atmospheric studies and civilian applications. For this purpose, 18 years of raw infrasound data (2003–2020) were reprocessed using the Progressive Multi-Channel Correlation method. A one-third octave frequency band configuration between 0.01 and 4 Hz was chosen for running this array-processing algorithm, which detects coherent infrasound waves within the background noise. From the comprehensive detection lists, each four products for 53 IMS infrasound stations were derived. The four products cover different frequency ranges and are provided at different temporal resolutions: a very low frequency set (0.02–0.07 Hz, 30 min; https://doi.org/10.25928/bgrseis_bblf-ifsd, Hupe et al., 2021a), two so-called microbarom frequency sets – covering both the lower (0.15–0.35 Hz, 15 min; https://doi.org/10.25928/bgrseis_mblf-ifsd, Hupe et al., 2021b) and a higher (0.45–0.65 Hz, 15 min; https://doi.org/10.25928/bgrseis_mbhf-ifsd, Hupe et al., 2021c) part – named after the dominant ambient noise of interacting ocean waves that is quasi-continuously detected at IMS stations, and observations with center frequencies of 1 to 3 Hz (5 min), called the high frequency product (https://doi.org/10.25928/bgrseis_bbhf-ifsd, Hupe et al., 2021d). Within these frequency ranges and time windows, the signals from the most dominant directions in terms of number of arrivals are summarized. Along with several detection parameters, calculated quantities for assessing the relative quality of the products are provided. The validity of the data products is demonstrated by diving into examples of recent events that produced infrasound detected at IMS infrasound stations, as well as a global assessment.