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 monitoring Special 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 seismograms Optimal azimuth (130°) and apparent velocity (15 km/s)

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

11. P-wave group, NKCA, shifted seismograms Correct 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 numbers x i = crosses, y p = 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 component GAS 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 power plant