Detection of weak earthquakes by small seismic

array using DP/EPin the Czech Republic

Jaroslav Strunc

November 4, 2009 NORSAR 2

Outline

• Shortly about Academy of Sciences

• Areas of interest

• Experimental arrays

• Detections – classic & correlation

• Implementation model for WEBNET

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Academy of Sciences

• Institute of Rock Structure &

Mechanics−dept. of Seismology

• Czech Technical University−PhD student – measurement tech.

• What should be done−evaluation of experimental data−implementation in the Czech Rep.

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maps.google.com

Areas of Interest (1)

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Areas of Interest (Natural Seismicity)

• Western Part−swarm area−Eger Rift & Marianske Lazne Fault

−MLmax = 5

• Eastern Part−rare events but „strong“−Hronov-Porici Fault

−MLmax = 3.5

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East Bohemia

• Mount Ostas, 2005

• Aperture 40m

• 1 broadband Guralp (30s)

• 3-D geophones SM6 (4.5Hz)

• 10 km NE far from Hronov-Porici Fault

• Near-surface events

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West Bohemia

• WEBNET (West Bohemia Network)

• 13 permanent stations−Guralp (30s) & SM3 (1s)−Lennartz, Nanometrics

• 2 experimental arrays – NKCA, KVCA−triangular shape−aperture 90m−SM6 geophones (4.5Hz)

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NKCA & KVCA

• Novy Kostel (2007), Kvetna (2008)

• Aperture 90m

• 3-D geophones SM6 (4.5Hz)

• Hypocentre b. Novy Kostel (5~15 km)

• KVCA placed about 5 km SE

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KVCA (Kvetna)

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www.mapy.cz

www.ig.cas.cz

KVCA

NKCA

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Swarm Activities (1)

www.ig.cas.cz

Last Swarm Activity :

October/November 2008

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Swarm Activities (2)

www.ig.cas.cz

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What with the data?

• 3 arrays−several years of continuous data

• No software for data processing−go through data manually?

• lack of time

• Is the shape and place of array

suitable?

That‘s the purpose of my visit.

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Objectives (1)

• Get familiar with array processing

tech.

• Evaluate−array shape−array location−used type of sensors

• Find out the optimal method of

detection for current configuration

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Objectives (2)

• Select on array−KVCA (West Bohemia)

• Perform all experiments with the data−detections−locations

• Prepare automated process for the

other arrays

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Array Equipment

• Sensors−geophones – velocity (SM6b, Sensor Netherland)

−theoretical flat resp. in range 4.5~1000Hz−sensitivity 2600 V/(m/s)

• Digitizers−RUP2004 (our product)

−21-bit A/D Converter−sampling 100 Hz

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But...?

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Period of Study, KVCA

• April 17 ~ August 12, 2008−117 days

• 131 local detections by WEBNET−MLmin = -1

• Array sensors „best“ features at−freq. band 8 ~ 16 Hz−spurious resonances present

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Array Transfer Function

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Classic DP

• 4 freq. bands (8-10, 10-12, 12-14, 14-16)

• used 227 beams (horizontal, vertical)

• threshold 12.5 ~ 6.5

• lower SNR−increased false detections rapidly

• caused by spurious resonances of geophones

−many „visible“ events stayed undetected

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Event Example - Resonances

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Event Example - Detail

!

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DP Results

• Best Results for−vertical SNR ≥ 6.5−horizontal SNR ≥ 10−backazimuth 270° ~ 360°

• April 19, 2008−Σ 135 events

• 54 false• at least 15 missed (visible without beamforming)

Data are too bad for classic DP

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What about Cross-Correlation?• 131 detected events by WEBNET

− from 22 near locations

• 11 suitable templates− distinct phases

− „weak“ enough (ML < 0)

− only 7 of 9 array traces used (malfunction)

Finally: it takes time bud it works!

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CC Results – Unique Detections• April 19, 2008

−Σ 103 events (including template)• for CC_SNR ≥ 9.5

−9 false−2 missing?

−Σ 90 events (including template)• for CC_SNR ≥ 10.5

−3 false−7 (+2?) missing!

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Whole Period of Time (117 days)

SNR ≥ Detections Reduction6.5 60627.5 2305 62%8.5 991 57%9.5 540 46%

10.5 391 28%11.5 302 23%12.5 251 17%13.5 217 14%14.5 191 12%15.5 167 13%

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7.6 19.8 9.3 11.8 16.8

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Detections per Day

0

5

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110

113

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128

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134

137

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Day

Det

ecti

on

s

61103

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Templates & Detections

Letter Time PlaceBackazimuth

[°] ML

WEBNET detections

≥ 9.5 CC detections

A 2008-110:11.28.16 Pocatky 340 -0.3 26 162B 2008-111:18.55.52 Mlynek 250 -0.6 1 12C 2008-114:19.04.40 Novy Kostel 310 0.1 11 159G 2008-145:18.54.06 Luby - Zalubi 315 0.7 51 54H 2008-147:16.32.44 Luby 300 0.5 5 6L 2008-154:00.27.02 Plesna - Vackov 295 -0.3 1 3M 2008-156:05.44.33 Lazy 135 0.5 2 4O 2008-168:23.28.38 Tachov 140 0.3 2 4S 2008-183:23.17.06 Lesna 270 0.3 13 116T 2008-192:13.19.08 Kostelni 315 -0.1 1 15V 2008-220:10.38.01 Milhostov 200 0.3 1 5

114 540

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Conclusions

• Geometry−suitable for detections in Fault Zone−but: none redundancy

• Features of the Sensors disable to use DP−they should be replaced

• CC-DP increases no. of detections−template necessary−cannot fish out new events

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Future Work

• Implementation of on-line DP/EP in

Academy of Science−at the beginning as template based CC-

DP

• Process data from the other arrays

• Prepare a short paper

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• Thank you for attention

• Many thank to Johannes & Steven

• and to everybody for the nice

atmosphere at Norsar