1. Infrasound detector for Apatity group Asming V.E., Kola Regional Seismological Center, Apatity, Russia

2. Layout of Apatity seismic and infrasound groups

3. Let F si - i-th band-pass filtered sample of sensor s i - index of a sample, i=1,N samples s - index of a sensor, s=1,N sensors Consider a plain wave arriving from backazimuth  and and with an angle  to the day surface (tangage angle). The time delays of the wave arrivals to the array’s sensors are  t s ( ,  ) = (  X s cos  +  Y s sin  ) cos  / v sound Expressed in numbers of samples, the delays are  i s ( ,  ) =Round(  t s /h) Beamforming-style detector of acoustic signals

4. Consider two kinds of beams for each ( ,  ) a) Average beam b) Maximal beam In idealized case when ( ,  ) are true parameters of an incident plain wave, all sensors have the same response and there are no noise and signal attenuation due to relief A i =M i, otherwise A i < M i

5. Basically we may use the ratio A i /M i for detection but there may be uncertainties when M are small. To avoid this we use window averaging of A i and M i : - has a meaning of coherency between recordings at different sensors and can be used as a detector

6. BUT:amplitudes of a signal can be different at different sensors (up to 1.5 times). The differences depend on a direction of a wave propagation. Probably due to relief. This can diminish the ratio R j. If we take smaller values for threshold we significantly increase a number of false alarms. To avoid this we use the following “trick” : we calculate “normalized” recordings where N norm >>N aver And by we compute as described above

7. If to use as a detector a new source of false alarms appears when pieces of recordings of very different amplitudes have high coherency Finally, we decided to use ratios R calculated by both normalized and non-normalized data. And the detector appeared to be: where typically 0.6-0.65 versus 0.75-0.8 and

8. The detector described above does not use amplitudes Indeed, weak but obviously true events do exist

9. To separate events by signal-to-noise ratio we implement a statistical algorithm to noise level estimation by average amplitude Estimated noise level

10. Implementation for Apatity array A PC program which can process CSS 3.0 data or a data stream from Apatity array

11. Band-pass filter:1-5 Hz Length of averagingN aver :20 samples (0.5 sec) Length of normalizing N norm :100 samples (2.5 sec) Tangage angles (  ):0, 20, 40, 60 degrees Azimuth angles (  ):0-359 degrees, step 1 Threshold for initial ratio:0.6 Threshold for norm ratio:0.75 The detector parameters

12. Results for 01.12.2005-22.12.2005 2263 events

13. 464 events with SNR>15

14. Explosions detected by Apatity system

15. Typical wave forms of infrasound events Event from South, band-pass filtered 1-5 Hz

16. One of the strongest events (SNR=216)

17. Weak but obviously true event, filtered 1-5 Hz (SNR=10)

18. How to locate infrasound events ? Error of backazimuth estimation = 1°

19. Error of backazimuth estimation = 2°