synthetics vs. real waveforms from underground nuclear explosions as master templates for ctbt...

International Data Centre

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]


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.


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


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.


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


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?


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



Segment

The segment is a set of grid cells with R = 100 km


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


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.


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).


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


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


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


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


Station cases for 4 templates: AKASG DPRK-2013 source corresponds to the purple circle


Station case: ILAR DPRK-2013 source corresponds to the purple circle


Station case: WRA DPRK-2013 source corresponds to the purple circle



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



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



REB DPRK 2013

Cross Correlation

Location


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.




• 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


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.


Thank You!

synthetics vs. real waveforms from underground nuclear explosions as master templates for ctbt...

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