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Detection and parameter estimation of an explosion signal obscured by a coda of a strong interfering earthquake (Example of SNDA application)

Operational and functional capabilities of the SNDA as a system for SASA data analysis are illustrated below by its application to the problem of detection and parameter estimation of a so called "hidden explosion" seismic signal. The possible scenario of avoiding a Comprehensive Test Ban Treaty is to perform secret nuclear test by triggering a nuclear device with a seismic signal from a rather strong earthquake. In this case the explosion wave phases are obscured by coda waves of the earthquake. Latter typically have an intensity significantly stronger than seismic noise at observational sites of the monitoring network. Nevertheless, registering of a noise field by a SASA and processing SASA recordings with the help of special statistically optimal algorithms provides a chance of reliable CTBT monitoring even in the case of implementing of the avoiding scenario described.

Figures (shown below) illustrates results of application of the SNDA advanced processing methods to the "hidden explosion" problem. In this study we used multichannel seismograms from underground nuclear test at Novaya Zemlya site (24 Oct. 1990) and earthquake in Hindu Kush (25 Oct. 1990) registered by NORESS SASA. Simulation of NORESS recordings containing a "hidden explosion" signal obscured by an earthquake coda and seismogram processing for explosion signal detection and parameter estimation were made with the help of a special SNDA script, comprising a variety of the SNDA stack commands and SA-procedures.

Figure below shows P- wave seismogram from Novaya Zemlya explosion (NZE) (trace (1)) and P-wave with coda wave seismogram from Hindu Kush earthquake (HKE) recorded by the central NORESS sensor. The seismograms were filtered in the frequency band (0.5-5) Hz, resampled, shifted in time and scaled by SNDA stack commands.

The simulated "hidden explosion's" 25-channel NORESS data are displayed in figure below. The created by the SNDA tools mixture of the real NZE and HKE NORESS seismograms contains the NZE P-wave obscured by the HKE coda with the RMS SNR=0.5 and the onset time at 23 sec later the HKE P-wave arrival. This data is the raw material for the succeeded analysis. Note that the explosion signal is not recognizable on this seismogram mixture du to similarity of amplitude and frequency contents of the NZE P-waves and HKE coda waves.

The following figure demonstrates the results of detecting of the NZE P-wave on the HKE coda background with the help of the adaptive statistically optimal detector (ASOP) [1-3]. The detection procedure is applied to the output of the beamforming procedure with the beam steered to NZ site (trace 3). The output of the same beamforming procedure applied to "pure" HKE seismograms is shown at the trace (4) for comparison with the trace (3). One can see that conventional beamforming does not provide suppression of HKE coda waves sufficiently for reliable detecting by the standard STA/LTA detector. At the same time trace (2) containing the ASOP statistic time series demonstrates presence of the peak from NZE P-wave which significantly exceeds the ASOP statistic fluctuations due to HKE coda wave oscillations. Comparison of the trace (2) with the automatically chosen threshold (equal to the doubled root mean square value of this trace) allows to detect reliably NZE P-wave and assign the appropriate time interval containing suspected "hidden explosion" signal as the object for succeeded thorough analysis (trace 1).