Small-aperture seismic arrays: instruments and detectability Ji Mlek, Milan Bro and Jaroslav trunc Institute of Rock Structure and Mechanics AS CR, Prague, Czech Republic, Kick-off meeting Advanced Industrial Microseismic Monitoring AIM Prague Contents Small-aperture seismic array (SSA) for microseismic monitoring SSAs 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 Nov Kostel array Non-volcanic tremors in Provadia (Bulgaria) 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: Higher frequencies (10 30 Hz) Smaller aperture Less stations SSAs built by IRSM AS CR OSTA Eastern Bohemia Hronov-Po 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) Nov 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 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 Local event recorded 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) P-wave group, NKCA, shifted seismograms Wrong 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) 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 Generalized average of complex numbers p>0 Generalized average of complex numbers x i = crosses, y p = dots Generalized average of signals 3 Phase-weighted stack (Schimmel and Paulssen, 1997) 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 Searching for discontinuities above hypocenters: Converted SP wave mainly on Z component Double reflected S wave mainly on transversal component Linear stacking of seismograms (white line) for P-wave velocity, Z component SP Non-linear stacking of seismograms (white line), Z component GAS algorithm, Malek et al., 2007 (Acta Geod. et Geom., Vol. 4, No.3) SP 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) PROA array (3 stations), Z component, local induced event PROA array (3 stations), Z component, harmonic tremor 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 Dobr Voda (Little Carpathians, Slovakia) for microseismic monitoring of the region of Jaslovsk Bohunice nuclear power plant