NH10.15

In 2001, when the first data from an International Monitoring System infrasound station started to arrive in near real-time at the International Data Centre (IDC), its infrasound processing system was in a premature state. The IDC embarked for a multi-year design and development of its dedicated processing system, which led to operational IDC automatic processing and interactive analysis systems in 2010. In the next twelve years the IDC produced over 40,000 infrasound events reviewed by expert analysts.
In an effort to continue advancing its methods, improving its automatic system and providing software packages to Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) users, the IDC focused on several projects. First, the automatic system for the identification of valid signals was redesigned with the development of DTK-(G)PMCC (Progressive Multi-Channel Correlation), which is in IDC Operations and made available to CTBTO users within NDC-in-a-Box. And second, an infrasound model was developed for automatic waveform network processing software NET-VISA with an emphasis on the optimization of the network detection threshold by identifying ways to refine signal characterization methodology and association criteria.
Ongoing and future improvements of the IDC processing system are planned to further reduce analyst workload and improve the quality of IDC products.

How to cite: Mialle, P. and the PTS colleagues: The International Data Centre infrasound processing system, a 25 years travel, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7943, https://doi.org/10.5194/egusphere-egu22-7943, 2022.

The International Monitoring Systems (IMS) operational manuals for waveform stations require that IMS stations be calibrated regularly. Since 2012, the Provisional Technical Secretariat (PTS) had relied mostly on electrical calibration to meet that requirement. However electrical calibration has inherent challenges (no traceability, integration and sustainment issues, high operating costs…).

A part of the geophysical community, including Station Operators, has started performing regular calibrations by comparison against a co-located reference. This method allows a more systematic and centralized approach to calibration. Over the past few years, it has been increasingly used at IMS stations, particularly infrasound ones. In this context, the PTS is developing tools to support this alternative approach.

We present CalxPy, a web-application developed at the PTS for the calibration of geophysical systems by comparison. With CalxPy, one can calculate, store, and display the response of a system for a given period, or track the evolution of the response against time or environmental variables. CalxPy also allows the refinement and evaluation of the measured response against a baseline, and the reporting of calibration results.

CalxPy supports the Initial calibration and on-site yearly calibration processes, as well as data quality control. CalxPy can be deployed in the IDC pipeline and in NDC-in-a-box.

How to cite: Doury, B. and Ketata, I.: CalxPy: a software for the calibration of geophysical systems against a reference, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9074, https://doi.org/10.5194/egusphere-egu22-9074, 2022.

In the low-frequency range down to 0.1 Hz suitable and reliable calibration procedures, which include traceability to SI, for seismic and infrasonic sensors are currently missing. Although many events occur whose evaluation is of global interest, much of the low frequency range relevant to these applications is not yet covered by primary measurement standards. A laboratory calibration of sensors results in an interruption of the measurements, just as the use of built-in calibration coils disturbs the measurements. Therefore, with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS), which requires the stations to be operational 100 % of the time, on-site calibration during operation with a reference sensor previously calibrated in the laboratory is of special interest.

We have assembled sets of both natural and anthropogenic sources of seismic, infrasonic, and hydroacoustic waves with respect to their individual signal characteristics and, as part of the joint research project "Metrology for low-frequency sound and vibration - 19ENV03 Infra-AUV", evaluated their potential use as excitation signals for on-site calibration regarding aspects that include knowledge about the source characteristics, the frequency content, reproducible and stable properties as well as the applicability in terms of cost-benefit. With the aid of these sources, procedures are to be established which will allow permanent on-site calibration without any interruptions of the recordings, thereby improving data quality and consequently the identification of treaty-relevant events.

In that context, man-made controlled sources such as drop weights or loudspeakers exhibit properties that make them an interesting source signal for the calibration of seismometers and infrasound sensors. Among the natural sources, earthquake generated signals in particular stand out because of their highly suitable signal and spectral properties. In addition, microbaroms and microseisms also play an important role for calibration, since they cover the lowest frequency range of interest. In particular, we focus here on sources that may generate both seismic and infrasonic signals. By means of a joint review of the waves’ sources in the solid earth and the atmosphere, parallels and differences are highlighted. Preliminary comparisons performed with IMS stations PS19 and IS26 in Germany show that the frequency response of different excitation sources can be determined using spectral methods and correlation analyses.

How to cite: Schwardt, M., Gaebler, P., Hupe, P., and Pilger, C.: Natural and anthropogenic excitation sources for seismic and infrasonic on-site calibration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3620, https://doi.org/10.5194/egusphere-egu22-3620, 2022.

We report on a review of multiple sources and source characteristics of hydroacoustic signals recorded at the six hydrophone stations of the International Monitoring System for verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty.  We present a comprehensive list of hydroacoustic sources as well as their general waveform shape and individual spectral source characteristic, i.e. the time duration, source intensity, frequency content and signal variation.

We identify and investigate numerous natural sources like earthquakes, volcanoes, icebergs and marine mammals as well as anthropogenic sources like explosions, airgun surveys and shipping activity. We show selected example events and associated references, collected in the course of the joint research project "Metrology for low frequency sound and vibration - 19ENV03 Infra-AUV". We further use freely available recordings from e.g. seismic stations for cross-validation purposes.

This overview provides the basis for an open-access systematic source classification, where only few, fragmentary event catalogues are available up to now and in situ identification of sources and calibration of instruments are difficult and complex. This work is applicable to future activities in automatic source detectors and event catalogs, sensor calibration activities using remote excitation sources and data comparison with other hydroacoustic measurements. We invite the scientific community to discuss useful source labels for such a compilation and useful datasets for comparison and validation.

How to cite: Pilger, C., Steinberg, A., Gaebler, P., and Schwardt, M.: Characteristics of hydroacoustic sources of natural and anthropogenic origin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3710, https://doi.org/10.5194/egusphere-egu22-3710, 2022.

The Preparatory Commission for the CTBTO routinely process data from the International Monitoring System, IMS – a global network of seismic, hydro-acoustic, and infrasound stations. The data are processed to detect, locate, and screen events that may have characterization parameters similar to those from nuclear explosions. The observation and processing systems are required to be sensitive to low-magnitude events, especially in unusual locations (e.g., aseismic regions). A promising way to improve the system sensitivity is by refining the receiver velocity models underneath IMS stations by incorporating a number of ambient noise processing techniques into the International Data Center (IDC) practice. In particular, this approach should lead to reduction of the arrival time residuals between empirical and observed onset times of seismic waves. The Big Data basis for this approach is using a vast amount of seismic noise data acquired in the IDC for more than 20 years. It would also allow to shed a light on the existence of seismic velocity evolution at least for unstable crustal regions applying a time-lapse ambient noise tomography (ANT) method (4D high resolution passive seismic). A lack of reference models can be partially overcome and examining the models within the seismic array aperture can be performed by the convergence of the spatial seismic correlation methods and the local single station measurements - seismic impedance and the direct Rayleigh ellipticity estimations by the H/V ratio and random decrement techniques

We conducted a case study for ARCES IMS array-station in Northern Norway, which consists of 4 rings of all 3C broadband (120s shallow vault   seismometers. Besides building an averaged uppermost ARCES velocity model, we demonstrate the trial application of the ANT methods for the individual model retrieval at different flanks of spatially distributed sensors comprising seismic arrays as a generalized way to aggregating the block velocity models.  Modified spatial autocorrelation (MSPAC) has been applied for ARCES data both for the whole set of elements as well as for four geographically symmetrical sub-groups relative to the array center. Spatial correlation patterns demonstrate the Bessel function (relative to the ground motion frequency) behavior as predicted by Aki (1957). The cross-correlation analysis of the background noise at ARCES was carried out in the wide frequency range because of the broadband hybrid channel frequency response at each array element.  Revealed models demonstrate considerable difference and thus could be further utilized for improvement of event location and as a station specific correction instrument.

Also, we provide an example with the spiral geometry but smaller aperture seismic array in Norcia intermountain basin, Northern Italy. The model estimation based on MSPAC conducted with the medium range sensors provides the results consistent with the well and gravity study conducted in Italy (2019).

For enhancement of CTBTO OSI aftershock monitoring system, the same approach can be utilized by retrofitting velocity models produced with the noise data collected from the temporarily OSI array. The same method could be also implemented in hydrofracking and induced seismicity monitoring.

How to cite: Rozhkov, M., Starovoyt, Y., and Kitov, I.: IMS location capability improvement with the ambient noise tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2906, https://doi.org/10.5194/egusphere-egu22-2906, 2022.

Data from the stations of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) organization are being processed by automatic processing, Global Association (GA), and interactively analyzed and reviewed by analysts, resulting in the International Data Centre (IDC) bulletins. The Network Processing Vertically Integrated Seismic Analysis (NET-VISA) is a Bayesian seismic monitoring system designed to process data from the IMS to reduce the number of missed and false events in the automatic processing stage. NET-VISA has been implemented in the automatic process as an additional event scanner in operation at the IDC since January 15, 2018. In this study we assess the influence of NET-VISA automatic scanner on the number of events in the IDC bulletins, LEB (Late Event Bulletin) and REB (Reviewed Event Bulletin). In particular, the impact of NET-VISA scanner on the number of scanned events during the interactive analysis is assessed. We use three distinct time periods, each including 1200 days, two before and one after the NET-VISA implementation to evaluate the NET-VISA influence as well as the effect of the other possible factors such as global seismicity and network performance. The results show a 4.6% increase in the number of LEB events after including the NET-VISA scanner in operation, with an average of 7 events per day, and a notable increase of 17.90% in the number of scanned events. We also discuss the effect of other possible factors on such increase and conclude it can be attributed to the implementation of the NET-VISA scanner.

How to cite: Qorbani, E., Ali, S. M., Le Bras, R., and Rambolamanana, G.: Interactive analysis prospective on implementation of the NET-VISA in the IDC bulletin production, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13165, https://doi.org/10.5194/egusphere-egu22-13165, 2022.

How to cite: Gonzalez Alvarez, I., Rost, S., Nowacki, A., and Selby, N.: Lithospheric scattering and intrinsic attenuation characterization from a Bayesian energy flux model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5770, https://doi.org/10.5194/egusphere-egu22-5770, 2022.

We intercompare simulations of the dispersion of aerosol and gaseous radionuclides (¹³⁷Cs and ¹³¹I) driven by a four-member ensemble of (re-)analysis and forecast datasets to quantify statistical and systematic uncertainties. The Lagrangian particle dispersion model FLEXPART 10.4 and FLEXPART-WRF are driven by 6-hourly data from NCEP Global Forecast System (GFS) and Final Analysis (FNL), at spatial resolutions of 0.5 and 0.25 degrees. In addition, for running FLEXPART-WRF, the FNL and ECMWF Reanalysis v5 (ERA5) were first downscaled, to the finer resolutions of 10 km and 1 hour, using the Weather Research and Forecasting (WRF) model. A total of 365 experiments (each day of 2019) were conducted to produce hourly simulations at the spatial resolution of 10 km in 14 vertical levels through 96 hours after a fictitious nuclear power plant accident at Barakah, UAE, in an effort to study the potential risks to the population in the state of Qatar. The source term was scaled to the maximum estimates of the radioactive materials from the Fukushima accident in 2011 (0.042 kg of ¹³¹I and 7 kg of ¹³⁷Cs), released within 24 hours after the accident. We intercompare radionuclide age spectra, cumulative deposition, and population exposure, seasonal variance, and investigate the degree of variability and correlation between ensemble members. Results show that the computational particles corresponded to dense ¹³¹I clouds enter Qatar more frequently within 10 to 20 hours after the accident. The cumulative distribution of simulated ¹³⁷Cs depositions indicates that more than 80% of ¹³⁷Cs depositions occurs within 75 hours after the accident, with a hotspot in the southeast of Qatar. GFS and ERA-5 show a high degree of correlation, whereas FNL is different. We also observe seasonal variation due to deposition and boundary layer development.

How to cite: Nabavi, S. O., Christoudias, T., Fountoukis, C., Al-Sulaiti, H., and Lelieveld, J.: Ensemble modeling of radionuclide dispersion over the Arabian Peninsula from nuclear power plant accidents using FLEXPART, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-860, https://doi.org/10.5194/egusphere-egu22-860, 2022.

The first aftershock of the announced nuclear test conducted by the DPRK on 09.09.2016 was found by the detection method based on waveform cross correlation on September 11, 2016. This was the only aftershock which was found during the period between the first (09.10.2006) and the sixth (03.09.2017) DPRK tests, using the signals of the DPRK tests as waveform templates. The DPRK6 underground test with mb=6.1 generated a significant aftershock sequence, with some events detected at teleseismic distances. The aftershocks with the best signal quality were used as master events in the multi-master method, working as an active radar focused on the aftershock area. The multi-master method allowed to find more than 100 aftershocks, including 7 aftershocks of the DPRK3 and DPRK4. The aftershock sequence is still active, with 25 aftershocks detected between January 1 and December 10, 2021. The mutual cross correlation of the DPRK aftershocks revealed the presence of two sequences generated by the DPRK5 and DPRK6 cavity collapse. The length, intensity, and alternating character of these two sequences suggest specific mechanisms of energy release. Such a mechanism can be associated with the interaction of the damaged zones of the DPRK5 and DPRK6 and the collapse of their cavities with progressive propagation of the collapsing chimneys towards the free surface. The higher activity in 2021 indicates that the chimney collapse is not finished. We expect more aftershocks, possibly ended with the chimney reaching the free surface.

How to cite: Kitov, I.: Evolution of the DPRK5 and DPRK6 aftershock sequences: 2016 to 2022, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2230, https://doi.org/10.5194/egusphere-egu22-2230, 2022.

There was more than a dozen of aftershocks generated by the announced nuclear test conducted by the DPRK on September 3, 2017 (DPRK6) which were found in routine interactive analysis conducted by the International Data Centre. The first DPRK aftershock was found by the method of waveform cross correlation (WCC) on September 11, 2016 after the DPRK5. Dozens of aftershocks were found by cross correlation after the DPRK6 in addition to those found in the routine processing and then confirmed by IDC analysts. The set of robust aftershocks allowed to develop, test, and apply in the routine WCC processing the multi-master method. This method was consistently applied to seismic data at IMS stations KSRS and USRK collected since 2009. Many new aftershocks were found after the third (DPRK3) and the fourth (DPRK4) announced underground nuclear tests conducted by the DPRK on 12.02.2013, and 06.01.2016, respectively. The second DPRK test (25.05.2009) had no reliable aftershock hypotheses at the level of the method sensitivity and resolution. The largest aftershocks of the DPRK3 and DPRK4 could be interpreted as related to the cavity collapse process possibly followed by a chimney collapse, not reaching the free surface. The DPRK3 and DPRK4 aftershocks were confirmed by interactive analysis.

How to cite: Wang, H. and Kitov, I.: Aftershocks of the announced underground nuclear tests conducted by the DPRK on 12.02.2013 and 06.01.2016 found by waveform cross correlation and confirmed by interactive analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3164, https://doi.org/10.5194/egusphere-egu22-3164, 2022.

Almost 20 years ago, the first infrasound event built only from infrasound arrivals was reported in the Reviewed Event Bulletin (REB) of the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). Over the last 25 years, 53 infrasound stations from the International Monitoring System (IMS) have been installed and are transmitting data to the IDC for the purpose of detecting any nuclear explosions in the atmosphere. The infrasound component of the IMS daily registers infragenic signals originating from various sources such as volcanic eruptions, earthquakes, microbaroms, meteorite entering the atmosphere or explosions. The IDC routinely and automatically processes infrasound data with the objective to detect and locate events then reviewed by interactive analysis.

As the IDC advances its methods and continuously improves its automatic system for the infrasound technology, several events received global interest from the scientific community and the public. On 15 February 2013 the Chelyabinsk meteor entered the atmosphere over Ural region (Russian Federation) and generated infrasound waves that were recorded by 20 of the 42 infrasound IMS stations operating at the time. Almost 9 years later, on 15 January 2022 the Hunga Tonga–Hunga Haʻapai eruption reached a climax around 04:15 UTC, which generated acoustic waves circumnavigating the Earth for several days. In addition to seismic and hydro-acoustic recordings, all 53 IMS infrasound stations registered signals from this eruption. This event is the largest ever recorded by the infrasound component of the IMS network.

How to cite: Mialle, P., Le Bras, R., and Bittner, P. and the CTBTO Colleagues: CTBTO International Data Centre analysis of the Hunga Tonga–Hunga Haʻapai eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13421, https://doi.org/10.5194/egusphere-egu22-13421, 2022.

The Comprehensive Nuclear-Test-Ban Treaty prohibits all nuclear explosions. For detection of potential non-compliance, the International Monitoring System with 321 stations is being installed and largely completed. Seismic, hydroacoustic and infrasound stations detect, localize and characterize explosions. Highly sensitive radionuclide stations sniff for radioactive traces potentially released from nuclear explosions. The International Data Centre (IDC) in Vienna processes the IMS data and generates several standard data analysis products for distribution to the member states. However, the judgement on the character of potentially treaty relevant events it is the sole responsibility of the State Signatories. Therefore, National Data Centres (NDC) are established in many states. The German National Data Centre is hosted by BGR and supported by BfS (Federal Office for Radiation Protection) with radionuclide expertise. Furthermore NDCs can use additional observation data sources other than recorded by the IMS like national stations or remote sensing data. There have been several larger test cases for the verification system as the announced nuclear tests in the DPRK 2006-2017, the Fukushima-Daiichi radionuclide emissions 2011, the Chelyabinsk meteorite 2013 or the accidental explosion in Beirut 2020.

Recently, the very large eruption of the Hunga Tonga Hunga Ha’apai volcano occurred on January 15^th 2022 in the South Pacific Ocean and turned out to be a strong source of waveform phenomena in solid earth, water and atmosphere.

Seismic PKP phases travelling through the core of the Earth were the first seismic signal of the event registered at German IMS station PS19 and the national Gräfenberg array. A preliminary moment tensor inversion analysis for P- and S-Phases shows the mainly explosive character of the event. Sensors of the hydro-acoustic component of the IMS also recorded the main eruption as well as ancillary volcanic activity at the two hydrophone arrays in the Pacific Ocean up to nearly 10000 km distance. The eruption caused a long period atmospheric pressure wave even measurable with classical barometers and pressure sensors in smartphones around the globe. Consequently, all 53 certified IMS infrasound stations detected signals from the event. Recurrent infrasonic signatures travelled around the globe several times and were recorded by IMS stations in the following days. The eruption was presumably the strongest infrasound source since installation of the IMS started.

Finally, the atmospheric sensitivity of the IMS radionuclide stations to hypothetical releases connected with the eruption is investigated by means of Atmospheric Transport Modelling. The results show threshold values for detectable releases of radioactive fission and activation products.

Overall, the very huge volcanic eruption can serve as upper benchmark event for the CTBT compliance monitoring capability using cross-technology analysis of IMS data.

How to cite: Ross, J. O., Ceranna, L., Donner, S., Gaebler, P., Hupe, P., Plenefisch, T., Pilger, C., Schwardt, M., and Steinberg, A.: Global cross-technology analysis of the Hunga Tonga-Hunga Ha’apai explosive eruption from the perspective of CTBT monitoring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13389, https://doi.org/10.5194/egusphere-egu22-13389, 2022.