Seismic monitoring of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) requires a uniform distribution of detection threshold over the earth. By design, the Primary Seismic Network of the International Monitoring System (IMS) provides an appropriate coverage. In order to effectively use the cross correlation technique and the archive waveforms from the International Data Centre (IDC) in global monitoring we have introduced a dense (spacing of ~ 140 km) and regular grid of master events with high quality templates at IMS array stations. In seismically active zones, we populated the grid with real masters. For aseismic zones, we developed an extended set of synthetic templates for virtual master events. This set includes a few simple double-couple source mechanisms in 1D and 2D velocity structures as well as explosion sources of varying size and at different depths of burial. All synthetics were first tested in seismic areas and showed the detection threshold similar to that demonstrated by real templates. We have designed three versions of the grid: V1.0 with only synthetic templates, V2.0 with real masters added where possible, and V3.0 with grand masters added. Their performance has been assessed by full processing of a few data days and a direct comparison with the Reviewed Event Bulletin issued by the IDC.

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
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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
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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
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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.
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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.
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6. Global seismicity: the IDC view
Waveform cross correlation relies on high quality master events
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Monitoring is global.
How to populate the aseismic area with quality master event?
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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
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8. Global Cross Correlation Grid
Segment
The segment is a set of grid cells with R = 100 km
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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
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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.
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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.
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The number of detections by
each master with nine stations
(templates).
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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
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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
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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
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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
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Good performance in both cases
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16. Station cases for 4 templates: AKASG
DPRK-2013 source corresponds to the purple circle
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17. Station case: ILAR
DPRK-2013 source corresponds to the purple circle
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18. Station case: WRA
DPRK-2013 source corresponds to the purple circle
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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
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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
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b
c
d
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21. Global Cross Correlation Grid
REB DPRK 2013
Cross Correlation
Location
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22. Global Cross Correlation Grid
Global Grid Location with constructed templates
Location results
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•
•
•
•
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.
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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
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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.
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25. Thank You!
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