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, [email protected]

Kick-off meeting Advanced Industrial

Microseismic Monitoring AIM

Prague 25-26.11.2009

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

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

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)

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

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 numbersxi = crosses, yp = 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

44000 45000 46000 47000 48000 49000 50000 51000

-1000

0

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NKCNS

Direct S

Converted P

Reflected S

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 Dobrá Voda (Little Carpathians, Slovakia) for microseismic monitoring of the region of Jaslovské Bohunice nuclear power plant