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Small-aperture seismic arrays: instruments and detectability

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, malek@irsm.cas.cz. Kick-off meeting Advanced Industrial Microseismic Monitoring AIM Prague 25-26.11.2009.

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Small-aperture seismic arrays: instruments and detectability

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  1. 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, malek@irsm.cas.cz Kick-off meeting Advanced Industrial Microseismic Monitoring AIM Prague 25-26.11.2009

  2. 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)

  3. 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

  4. Small-aperture seismic array (SSA) for microseismic monitoringSpecial features: • Higher frequencies (10 – 30 Hz) • Smaller aperture • Less stations

  5. SSA’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)

  6. Nový Kostel Array (NKCA) 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

  7. Optimal dimensions • 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

  8. Local event recorded at NKCA, August 8, 2007, 19:26:00.8, M = 0.7 h = 10.9 km r = 3 km P S

  9. P-wave group, NKCA, shifted seismogramsOptimal azimuth (130°) and apparent velocity (15 km/s)

  10. P-wave group, NKCA, shifted seismogramsWrong azimuth (310°) and correct apparent velocity (15 km/s)

  11. P-wave group, NKCA, shifted seismogramsCorrect azimuth (130°) and wrong apparent velocity (6 km/s)

  12. Linear and non-linear stacking of coherent signals • 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

  13. Generalized average of complex numbers p>0

  14. Generalized average of complex numbersxi = crosses, yp = dots

  15. Generalized average of signals 3

  16. Phase-weighted stack (Schimmel and Paulssen, 1997)

  17. Example 1 Identification 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

  18. Searching for discontinuities above hypocenters: Converted SP wave mainly on Z component Double reflected S wave mainly on transversal component

  19. Linear stacking of seismograms (white line) for P-wave velocity, Z component SP

  20. Non-linear stacking of seismograms (white line), Z componentGAS algorithm, Malek et al., 2007 (Acta Geod. et Geom., Vol. 4, No.3) SP

  21. Example 2 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)

  22. PROA array (3 stations), Z component, local induced event

  23. PROA array (3 stations), Z component, harmonic tremor

  24. Conclusions • 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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