Small aperture seismic arrays instruments and detectability
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Small-aperture seismic arrays: instruments and detectability. Jiří Málek, Milan Brož and Jaroslav Štrunc Institute of Rock Structure and Mechanics AS CR, Prague, Czech Republic, [email protected] Kick-off meeting Advanced Industrial Microseismic Monitoring AIM Prague 25-26.11.2009.

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Small aperture seismic arrays instruments and detectability l.jpg

Small-aperture seismic arrays: instruments and detectability

Jiří Málek, Milan Brož

and Jaroslav Štrunc

Institute of Rock Structure and Mechanics AS CR,

Prague, Czech Republic, [email protected]

Kick-off meeting

Advanced Industrial Microseismic Monitoring AIM

Prague 25-26.11.2009


Contents l.jpg
Contents

  • Small-aperture seismic array (SSA) for microseismic monitoring

  • SSA’s built by our department ( IRSM AS CR)

  • Optimal aperture of SSA

  • Linear and non-linear stacking of coherent signals

    Examples of studies using SSA

  • Converted SP waves detected at Nový Kostel array

  • Non-volcanic tremors in Provadia (Bulgaria)


Small aperture seismic array ssa for microseismic monitoring l.jpg
Small-aperture seismic array (SSA) for microseismic monitoring

  • Detection of very weak earthquakes: local shallow earthquakes M>-1.5 can be detected

  • Detection of (non-volcanic) tremors, recognition from industrial noise

  • Recognition of new phases on the seismograms of local earthquakes

  • Measurements of rotational components of seismic waves


Small aperture seismic array ssa for microseismic monitoring special features l.jpg
Small-aperture seismic array (SSA) for microseismic monitoringSpecial features:

  • Higher frequencies (10 – 30 Hz)

  • Smaller aperture

  • Less stations


Ssa s built by irsm as cr l.jpg
SSA monitoring’s built by IRSM AS CR

  • OSTA – Eastern Bohemia – Hronov-Poříčí Fault (2005)

  • NKCA – Western Bohemia – Seismic swarm epicentral zone (2007)

  • KVCA – Western Bohemia – Seismic swarm epicentral zone (2008) (now out of operation)

  • PROA – Provadia (Bulgaria) – Induced seismicity (2009)


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  • Nový Kostel Array (NK monitoringCA) was built around the NKC station at the main epicentral zone of the West Bohemia swarms

  • NKCA consists of 3 short-period three-component stations: NK1, NK2, NK3

  • Distance between stations is 100 m

  • Registration started on Feb 22, 2007


Optimal dimensions l.jpg
Optimal dimensions monitoring

  • Signals have to be coherent – small aperture

  • Time differences between stations should be significant – bigger aperture

  • Our recommendation:

    aperture is equal to wave-length / 4

    of the typical signal

    V=8 km/s, F= 20 Hz => A=100 m


Local event re corded at nk c a august 8 2007 19 26 00 8 m 0 7 h 1 0 9 km r 3 km l.jpg
Local event re monitoringcorded at NKCA, August 8, 2007, 19:26:00.8, M = 0.7 h = 10.9 km r = 3 km

P

S


P wave group nkca shifted seismograms optimal azimuth 130 and apparent velocity 15 km s l.jpg
P-wave group, NKCA, shifted seismograms monitoringOptimal azimuth (130°) and apparent velocity (15 km/s)


P wave group nkca shifted seismograms wrong azimuth 310 and correct apparent velocity 15 km s l.jpg
P-wave group, NKCA, shifted seismograms monitoringWrong azimuth (310°) and correct apparent velocity (15 km/s)


P wave group nkca shifted seismograms correct azimuth 130 and wrong apparent velocity 6 km s l.jpg
P-wave group, NKCA, shifted seismograms monitoringCorrect azimuth (130°) and wrong apparent velocity (6 km/s)


Linear and non linear stacking of coherent signals l.jpg
Linear and non-linear stacking of coherent signals monitoring

  • To determine azimuth and phase velocity f-k analysis is normally applied

  • Alternatively, we use optimization of stacked seismograms in time domain.

  • Linear or non-linear filters can be used

  • A new method GAS (Generalized Average Method) was developed and applied



Generalized average of complex numbers x i crosses y p dots l.jpg
Generalized average of complex numbers monitoringxi = crosses, yp = dots




Example 1 identification of converted sp waves l.jpg
Example 1 monitoringIdentification of converted SP waves

  • NKCA array

  • Earthquake at the depth of 9.5 km

  • Velocity interface at the depth of 4.5 km

  • Intensive S-waves are generated

  • SP conversion is expected on Z-component between P and S onsets


Slide18 l.jpg

Searching for discontinuities monitoring

above hypocenters:

Converted SP wave

mainly on Z component

Double reflected S wave mainly on transversal component



Slide20 l.jpg
Non-linear stacking of seismograms (white line), Z component velocity, GAS algorithm, Malek et al., 2007 (Acta Geod. et Geom., Vol. 4, No.3)

SP


Example 2 identification of tremors l.jpg
Example 2 velocity, Identification of tremors

  • PROA array (Bulgaria)

  • Induced seismicity from salt mines

  • 3 broadband sensors – GURALP CMG-40T

  • Local induced earthquakes

  • Harmonic tremors were detected (fr = 4,5 Hz)




Conclusions l.jpg
Conclusions velocity,

  • Small-aperture seismic arrays (SSA) can be applied for microseismic monitoring of natural and induced seismicity

  • Non-linear filtering is a reasonable alternative to f-k analysis

  • Special phases on seismograms can be revealed with help of SSA

  • Some unusual seismic events can be recognized using SSA

  • We recommend to built SSA at Dobrá Voda (Little Carpathians, Slovakia) for microseismic monitoring of the region of Jaslovské Bohunice nuclear powerplant


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