1. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office Geological Survey of Canada

2. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Systematic study of signal coherence with range and source type - outgrowth of studies of signal coherence for array design Motivated by considerations of screening requirements - relatively simple, parameter based characterization - automated after choice of window - suitable for ‘operationalization’ for testing Introduction

3. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Supplemented CMR ground truth dataset: 20 known events recorded with good SNR at 1or more of 6 arrays at ranges of 200-10000 km 117 independent coherence measurements in up to 6 passbands source types: -Bolides -Rocket launches -Chemical explosions -Gas pipeline explosions -Earthquakes Dataset

4. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Arrays Sources

5. Raypaths sampled

6. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

7. Data Processing - 1 Calculate coherence after Blandford (1998): Assume noise uncorrelated between channels, stationary and uncorrelated with signal  = C ij / (A s -A n ) Choose standard window length and equivalent length of pre-event noise window for calculation. Default = 100 secs

8. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

9. Data Processing - 2 Need to normalize coherence measurements to a standard sensor spacing: We know that different inter-element spacing will give different coherence measurements for the same source. Unfortunately, we know that coherence for a given inter- element spacing is a function of orientation relative to the direction of propagation (Mack & Flynn, 1971).

10. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

11. Parallel Perpendicular

12. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Data Processing - 3 Calculate ‘effective distance’ d eff between elements as: d 2 eff = d 2 par +  2 d 2 perp Where  is derived empirically from the observations in the appropriate frequency band, and is typically around 2.

13. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Data Processing - 4 Interpolate between the various coherence – effective distance pairs for a given event at a given array to derive a single ‘average’ coherence estimate for the array at some nominal station spacing (2 km) Method used for interpolation permits e.g. outlier eliminations, at least for arrays with larger numbers of elements.

14. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Results 3 bands: 0.4-0.6 Hz 0.8-1.2 Hz 1.6-2.4 Hz

15. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

16. Gas Pipeline Explosion (12 Dec 1998) at DLIAR

17. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

19. Summary General degradation of coherence with source-receiver distance. Systematic variations over several frequency bands of coherence with source type.

20. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Conclusion Demonstration of simple, comprehensible infrasound signal parameter suitable for automated postprocessing. Potentially useful for checking associations. May contain information on source type.

21. D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002