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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 [email protected]
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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.
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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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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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IMS, seismic network
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.
The primary network includes 25 arrays
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Global seismicity: the IDC view
Waveform cross correlation relies on high quality master events
Monitoring is global.
How to populate the aseismic area with quality master event?
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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. Real waveforms
2. Grand masters
3. Synthetic waveforms
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Global Cross Correlation Grid
Segment
The segment is a set of grid cells with R = 100 km
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Multichannel waveform cross correlation 6 s 6 s
CC
STA
LTA
SNR=STA/LTA≥3.0
Detection rule:
CC > CCtr
SNR_CC > SNRtr
CC essentials
Multichannel waveform
template
Four frequency bands
Adjusted template length
Waveform quality check
CC for individual channels
Averaged CC trace
Detection
Multichannel CC-detector better sees signals from slave events close to the master
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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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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.
The number of detections by
each master with nine stations
(templates).
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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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Cross correlation: explosion signals
• 100 waveforms
• 25 underground
nuclear explosions
• 6.2 > mb > 4.5
• 2015 m > H > 150 m
• 60 stations
• 16º > Δ > 100º
Towards seismic monitoring of underground nuclear explosions
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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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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 Good performance in both cases
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Station cases for 4 templates: AKASG DPRK-2013 source corresponds to the purple circle
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Station case: ILAR DPRK-2013 source corresponds to the purple circle
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Station case: WRA DPRK-2013 source corresponds to the purple circle
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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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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 b c d
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Global Cross Correlation Grid
REB DPRK 2013
Cross Correlation
Location
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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.
Global Cross Correlation Grid
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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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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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