Detection of the 2006 DPRK explosion by small-aperture array “Mikhnevo” using waveform cross correlationIvan Kitov and Irina Sanina

Institute of Geospheres Dynamics, Russian Academy of Sciences

Abstract. Three underground tests conducted by DPRK in 2006, 2009, and 2013 were measured by small aperture seismic array “Mikhnevo”, which has been operated by the Institute of Geospheres Dynamics since 2004. This array is designed for the purposes of regional seismic monitoring. It has aperture of approximately 1 km and includes ten vertical and two 3-C stations. Sampling rate is 200 counts per second. Automatic processing includes beamforming (azimuth and slowness values for the preset detection beams cover the range of regional and teleseismic body waves), filtering and detection by standard STA/LTA procedure. Distinct signals generated by the 2009 and 2013 events

were detected. The 2006 event was not detected by standard procedure (detection threshold was STA/LTA=3.5) and we applied waveform cross correlation (matched filter) in order to improve signal-to-noise ratio (SNR). Multichannel waveforms from the 2009 and 2013 signals filtered in various frequency bands were used as templates. The 2006 signal was detected with the cross correlation technique with SNR>4. Therefore, the matched filter technique improves detection capability of a small-aperture array even for teleseismic waves.

Corresponding author: Ivan Kitov (IDG RAS) E-mail: ikitov@mail.ru

IDG RAShttp://idg.chph.ras.ru

Disclaimer: The views expressed on this poster are those of the authorsand do not necessary reflect the views of the Institute of Geospheres Dynamics, RAS

 

 

Conclusion

Small aperture seismic array MHVAR has detected the DPRK 2006 announced underground test using the technique of waveform cross correlation (matched filter) and multichannel waveform templates recorded from the 2009 and 2013 tests. The length and frequency bands for the templates have to be selected carefully in order to maximize the performance of cross correlation.At the same time, standard method of beamforming has failed to suppress noise to the level necessary for

detection of the DPRK 2006. In both cases, detection threshold of STA/LTA=SNR>3.5 was selected as it is providing a lower rate of false alarms. The STA window has the length of 0.5 s and the LTA window was 50 s.

Modelling with changing level of microseismic noise has demonstrated that the matched filter technique allows to detect underground tests with magnitude below 4.0 from the DPRK test site while beamforming provides detection threshold of 4.1 to 4.3 for the level of microseismic noise observed on October 9, 2006.

The suppression of not correlated noise by beamforming could improve SNR by a factor of 3.5 (for 12 channels). The application of waveform cross correlation at MHVAR has reduced the threshold of detection of signals from different sources by a factor of 10 in comparison to a three component station. For underground explosions, a tenfold decrease in the amplitude threshold of signal detection means that an explosion source with seismic energy decreased 10 times can be detected. The use of various signal detection filters may improve SNR by a factor of 3.

Additional measures such as reduction of the microseismic noise level (for example, deployment of sensors in wells at the depth of a few tens of meters), increase in array aperture, and increment of the number of seismic sensors lead to further reduction in the detection threshold.

 

 

In the current study we use the records of three DPRK events at small-aperture seismic array Mikhnevo (MHVAR). The Institute of Geospheres Dynamics (IDG) of the Russian Academy of Sciences operates seismic array MHVAR (54.950N; 37.767E) since 2004. Small-aperture seismic array “Mikhnevo” includes ten vertical stations (solid triangles), with one station in the geometrical centre of the array (C00) and other nine stations distributed over three

circles with radii of 130 m, 320 m, and 600 m. The array aperture in approximately 1.1 km. Two 3-C stations (solid triangles in circles) were added to the outer circle in order to improve the overall stations sensitivity (detection threshold) and resolution. All stations are equipped with short-period seismometers SM3-KV, which are characterized by flat response between 0.8 Hz and 30 Hz and gain of 180,000 [Vs/m]. Later, a 3-C broad band station (BB) was installed in the centre of the

array for surface wave measurements. The array response function (only for 12 vertical channels) is similar to that for many small-aperture arrays. Such arrays are designed to measure high-frequency signals from regional and near-regional sources with magnitudes above 2.0.

0 1 2 3 4 5 6 7 8 9 103.0

30.0

2013 STA/LTA2013 CC 1/22013 CC 1/1Power (2013 CC 1/1)2009 STA/LTA

noise factor, C

SN

R

MHVAR design

Template 2013, Butterworth 3-d order filter 2-4 Hz

Template 2009, Butterworth 3-d order filter 3-6 Hz

Beams of MHVAR vertical channels, obtained by summation of waveforms normalized to their respective peak values. Time delays correspond to the slowness vector with the peak SNR for the explosions of 2009 and 2013. Dashed lines – for the original waveforms, solid lines – the original signals merged with preceding noise multiplied by factor (C) 3 and 6, respectively.

For signal detection, we applied the method based on the ratio of the average absolute amplitudes in the short (STA) and long (LTA) time window, STA/LTA, which is aslo used to estimate the signal to noise ratio: SNR = STA/LTA. The length of the STA window is 0.5 s, while that of the LTA window is 60 s.

To increase the SNR value, we applied filtering in seven frequency bands (Hz): 0.5–2, 1–3, 2–4, 3–6, 4–8, 6–12, and 10–20. For this purpose, we used a third order band-pass Butterworth filter.

According to the estimates by the International Data Centre of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), the magnitudes (mb)of the events in 2006, 2009, and 2013 were 4.1, 4.5, and 4.9, respectively. The estimate of

the 2006 event energy from averaged spectra of teleseismic P-waves falls within the range of 0.6–1.0 kilotons of TNT (1 kgof TNT = 4.184 × 1012 J), while it is 2.0–4.8 kilotons of TNT for the 2009 event. For the explosion of 2013, the energy estimated from the magnitudes of Lg waves is from 4.04 to 8.47 kilotons of TNT.

The problem of detecting signals from sources close in space can be solved with the help of waveform cross correlation. As a reference waveform, we chose the records of the events of 2009 and 2013, having the highest SNR value. The correlation factor was calculated for each channel of the Mikhnevo array.

Date OT Arrival time

Travel time

SNR Arrival time

Travel time

SNR_CC

09.10.2006 01:35:27 NA NA NA 01:45:24 09:57.0 3.9

25.05.2009 00:54:42 01:04:39 09:57.0 11.6 01:04:38 09:56.0 13.4

12.02.2013 02:57:49 03:07:47 09:58.3 31.2 03:07:47 09:58.0 19.0

WAVEFORM TEMPLATES

WAVEFORMS SPECTRA signal/noise

SMALL-APERTURE SEISMIC ARRAY MIKHNEVO (MHVAR) is designed for regional studies

2013

CROSS CORRELATION COEFFICIENTS, SNR, AND FK

2013/2013

2009/2013

2013/2009

2009/2009

2013/2006

Template 2013, channel C00six filters

2009

Array response

Examples of regional signals

Seismometer response

DETECTION USING WAVEFORM CROSS CORRELATION

DETECTION OF DPRK EVENTS WITH BEAMFORMING STUDYING DETECTION THRESHOLD FOR BEAMFORMING AND CROSS CORRELATION

Beamforming and detection with noise factor =С 3

SNR as a function of noise factor for the DPRK 2013 and 2009

Cross correlation and detection with noise factor =С 3

2013 2009

2009

2009

2013

2013

Cross correlation with noise factor =6, 2013С

Statistic of detections with beamforming and cross correlation

BEAMFORMING AND DETECTION

Poster No. T2.2-P7