Array Geometry and Signal Observations

1. Array Geometry and Signal Observations David A McCormack Head of Monitoring and Analysis Geological Survey of Canada, Ottawa, Canada Infrasound Workshop, Kona

2. Topics • IMS design array response for ‘realistic’ signals – how bad is bad? • Initial studies of signal coherence as an indicator of maximum source distance Infrasound Workshop, Kona

3. Array Response • Brown(2000) and Christie(2001) have illustrated problem of significant sidelobes in response pattern of 4-element infrasound array. Christie described response of various geometries with more elements and demonstrated superior characteristics of 7 and 8 element arrays. Infrasound Workshop, Kona

4. Brown(2000) - 1 0.125 Hz 0.5 Hz Spatial Aliasing: DLIAR Array Infrasound Workshop, Kona

5. Brown (2000) – 2 1.0 Hz Spatial Aliasing: DLIAR Array 2.0 Hz Infrasound Workshop, Kona

6. Array Geometry Infrasound Workshop, Kona

7. SHUTTLE - time Infrasound Workshop, Kona

8. Shuttle Infrasound Workshop, Kona

9. Brown(2000) - 3 N IS10 Winnipeg, Canada December 16, 1999 to September 1, 2000 Approximately 4200 detections microbaroms, Hudson Bay ? Local blasting, Too small to register on local seismic ? W E Local blasting Thunder Bay area, too small to register on local seismic mining activity, Minnesota, registers strongly on local seismic microbaroms, Pacific Ocean (?) S Infrasound Workshop, Kona

10. Sine – 4 elements Infrasound Workshop, Kona

11. Sine – 7 elements Infrasound Workshop, Kona

12. Synthetic Signals • Difficult to find narrowband real signals • Synthetic Signals • Bandlimited white noise – identical on each channel – perfectly coherent • Superimposed on a bandlimited white background incoherent between channels • S/N adjustable, duration adjustable Infrasound Workshop, Kona

13. Synth 0.25-1,4 Infrasound Workshop, Kona

14. Synth 0.25-1,7 Infrasound Workshop, Kona

15. Synth 0.4-0.6,4 Infrasound Workshop, Kona

16. Synth 0.4-0.6,7 Infrasound Workshop, Kona

17. Summary I • Even for narrow-band signals, side lobes are not large • Suggests more analysis of real events is required before final decisions on cost/benefit of densification should be made Infrasound Workshop, Kona

18. Signal Coherence • Grew out of studies of optimal array aperture for detection/location • Coherence as a routinely calculated intrinsic parameter of an observed signal Infrasound Workshop, Kona

19. Local – time series Infrasound Workshop, Kona

20. Local – freq content Infrasound Workshop, Kona

21. Local - coherence Infrasound Workshop, Kona

22. MINNTAC - time Infrasound Workshop, Kona

23. MINNTAC – freq content Infrasound Workshop, Kona

24. MINNTAC - coherence Infrasound Workshop, Kona

25. SHUTTLE - time Infrasound Workshop, Kona

26. Shuttle – freq content Infrasound Workshop, Kona

27. Shuttle - coherence Infrasound Workshop, Kona

28. Pacific Freq Infrasound Workshop, Kona

29. Pacific Bolide Coherence Infrasound Workshop, Kona

30. Alberta Freq Infrasound Workshop, Kona

31. Alberta Bolide Coherence Infrasound Workshop, Kona

32. Recorded sources Infrasound Workshop, Kona

33. Processing • Noise corrected under assumption of incoherent noise • Interpolate to 1.5 km station spacing • Ignore differences between radial and transverse coherence (this is wrong) Infrasound Workshop, Kona

34. Coherence vs distance Infrasound Workshop, Kona

35. Ms 7.4 Hector Mine, California Earthquake - Acoustic Arrival Infrasound Workshop, Kona

36. Summary II • Contributions to coherence from source and path • General trend of maximum coherence vs. distance • May be useful for to establish maximum distance to observed source Infrasound Workshop, Kona