Synthetics vs. real waveforms from underground nuclear explosions as master templates for CTBT monitoring with cross-correlation
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The document discusses the construction and performance assessment of a global grid of master events for waveform cross correlation in seismic monitoring. It outlines the significant improvements in detection and location of seismic events using this technique, emphasizing its application to underground nuclear explosions. Various design elements, testing protocols, and results from events like the DPRK 2013 are presented to support the efficacy of this method in enhancing seismic event detection capabilities.
Synthetics vs. real waveforms from underground nuclear explosions as master templates for CTBT monitoring with cross-correlation
- 1. Performance of waveform cross correlation using a global and regular grid of master events Bobrov, D., I. Kitov, and M. Rozhkov International Data Centre Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization Provisional Technical Secretariat Vienna International Centre P.O. Box 1200 A-1400 Vienna AUSTRIA Mikhail.Rozhkov@ctbto.org International Data Centre Page 1
- 2. Objectives Building a global grid of master events for waveform cross correlation. Assessing the performance of waveform cross correlation as a technique of seismic monitoring using the global grid of master events. International Data Centre Page 2
- 3. Outline 1. Motivation 2. Global seismic monitoring: IMS 3. Global seismicity: IDC view 4. Global cross correlation grid: a design 5. Cross correlation at teleseismic distances 6. Underground nuclear explosions as master events 7. Synthetic master events 8. Principal and Independent Component Analysis 9. Testing with world seismicity of February 12, 2013 10. DPRK 2013 of February 12, 2013 International Data Centre Page 3
- 4. Cross correlation as an IDC technique Motivation • Regional studies demonstrate significant improvement in detection, location, and magnitude estimation. At least an order of magnitude! • • • • • Many IMS primary stations are arrays enhancing the capability of cross correlation analysis. For arrays, correlation distance depends on phase and its slowness. At teleseismic distances, high level of cross correlation is observed for signals from events spaced by 100 km and even more. Remote events may have similar signals. Small events can be considered as point sources emitting signals identical in shape when co-located. International Data Centre Page 4
- 5. IMS, seismic network The primary network includes 25 arrays Blue circles – primary arrays, blue triangles – primary 3-C stations. Yellow circles – auxiliary arrays, yellow triangles – auxiliary 3-C stations. Red stars – underground nuclear explosions. International Data Centre Page 5
- 6. Global seismicity: the IDC view Waveform cross correlation relies on high quality master events International Data Centre Monitoring is global. How to populate the aseismic area with quality master event? Page 6
- 7. Global Cross Correlation Grid What is Grid? • Grid is a set of loci of hypothetic master events. • Master is a set of waveform templates linking array station and the locus. • Spacing between masters ~140 km. • P-wave templates from three to ten IMS primary arrays per master. • Distance for P-phase from 6 to 90 degrees. • At least three IMS stations to create an REB event. Possible templates: 1. 2. 3. Real waveforms Grand masters Synthetic waveforms International Data Centre Page 7
- 8. Global Cross Correlation Grid Segment The segment is a set of grid cells with R = 100 km International Data Centre Page 8
- 9. Multichannel waveform cross correlation 6s 6s CC essentials CC Multichannel waveform template Four frequency bands Adjusted template length Waveform quality check CC for individual channels Averaged CC trace Detection STA Detection rule: LTA SNR=STA/LTA≥3.0 CC > CCtr SNR_CC > SNRtr Multichannel CC-detector better sees signals from slave events close to the master International Data Centre Page 9
- 10. GCCG: Local Association Location mesh 1. Five circles with ~25 km increment in radius. 2. 91 nodes for origin time calculation. 3. All hypotheses at the outer circle are neglected since they have to be created by neighboring masters. International Data Centre Page 10
- 11. GCCG: resolution of conflicts between masters Detection of the same event by same stations but with different masters (case study: DPRK-2013) For given arrays the number of detecting masters depends on the distance (slowness), azimuth, and aperture. International Data Centre The number of detections by each master with nine stations (templates). Page 11
- 12. GCCG: resolution of conflicts between masters Creation of the same event by different masters # of detections relevant to the slave event # of all detections in the events with at least one detection from the slave event Only two master events have nine detections from the slave International Data Centre Page 12
- 13. Cross correlation: explosion signals Towards seismic monitoring of underground nuclear explosions • 100 waveforms • 25 underground nuclear explosions • 6.2 > mb > 4.5 • 2015 m > H > 150 m • 60 stations • 16º > Δ > 100º International Data Centre Page 13
- 14. Cross correlation of explosion signals Synthetic seismograms: Δ = 30º, 45º, 60º, 90º H = 0.1, 0.3, 0.6, 1.0, 2.0 km Fc = 0.8 Hz to 4.8 Hz International Data Centre Page 14
- 15. Cross correlation: applying PCA and ICA CC of first 5 Real Principal and Independent Components with 106 Real UNE records with cumulative ICs on right CC of first 5 Synthetic Principal and Independent Components with 106 Real UNE records with cumulative ICs on right International Data Centre Good performance in both cases Page 15
- 16. Station cases for 4 templates: AKASG DPRK-2013 source corresponds to the purple circle International Data Centre Page 16
- 17. Station case: ILAR DPRK-2013 source corresponds to the purple circle International Data Centre Page 17
- 18. Station case: WRA DPRK-2013 source corresponds to the purple circle International Data Centre Page 18
- 19. Global Cross Correlation Grid Testing, February 12, 2013 Grid: 25000 nodes Group 1 = WRA, TORD, MKAR, ILAR, GERES, PDAR, CMAR, SONM, AKASG, BRTR, GEYT Group 2 = ASAR, ZALV, YKA, ARCES, TXAR, KSRS Group 3 = USRK, FINES, NVAR, NOA, MJAR GG thresholds: Detections: SNRmin = 0.5; SNR_Ccmin = 2.5; CCmin = 0.2; FKSTATmin = 2.5; AZRESmax = 20.0º; SLORESmax = 2.0 s/º; Events: dTorigin = 6s; NSTAmin = 3; AZGAPmax = 330º xgrid1 xgrid2 xgrid3 xgrid4 arrival 22900402 24150132 21600999 34531530 origin 40396 48686 38975 95219 assoc 126595 152641 121775 297647 origin_cnf 5151 5773 5079 6542 assoc_cnf 16421 18488 16154 21488 International Data Centre Page 19
- 20. Global Cross Correlation Grid Global Grid DPRK-2013, locations for 4 cases of templates: (a) AK135 synthetics master, (b) PCA synthetic master, (c) PCA real master, and (d) DPRK-2013 master. All masters except for case 4 were produced by replicating single template at each array station element implementing predicted time delays for given master geographical position. a International Data Centre b c d Page 20
- 21. Global Cross Correlation Grid REB DPRK 2013 Cross Correlation Location International Data Centre Page 21
- 22. Global Cross Correlation Grid Global Grid Location with constructed templates Location results • • • • • REB – location by IDC, xgrid1 – AK135 synthetics as template, xgrid2 – first PC of synthetic record set, xgrid3 – first PC of UNE set, xgrid4 – DPRK-2013 genuine records at elements of all arrays is a template set. Location with synthetic PC is the same as location with genuine DPRK records. International Data Centre Page 22
- 23. Global Cross Correlation Grid • • • V0.1: All master templates are synthetics same at all stations V0.2: Master templates are station/master specific synthetics in 1D velocity model V0.3: Master templates are station/master/source (e.g. explosion) specific synthetics calculated for 2D velocity structure (e.g. ak135+CRUST 2.0) • V1.1: Real master templates are used where possible • V1.2: Replicated master templates are applied where possible • V2.0: The set of principal components are optimized where possible as obtained by the PCA or ICA applied to the complete set of actual and historical data • V3.0: Synthetic + real master templates based on principal components with classification algorithms trained on actual data International Data Centre Page 23
- 24. Discussion • IMS array stations make possible automatic processing based on waveform cross correlation. • Cross correlation is a powerful technique allowing to reduce the detection threshold and relative location accuracy by an order of magnitude, i.e. to find by 50% to 100% more (smaller) REB events. • Real and synthetic master events may reduce the magnitude threshold of seismic monitoring by 0.4 units of magnitude. • The Global Cross Correlation Grid is flexible (e.g. master density, templates, number of stations, thresholds, etc.) to fulfill various tasks including effective monitoring of UNEs. International Data Centre Page 24
- 25. Thank You! International Data Centre Page 25