D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of...

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office Geological Survey of Canada

Transcript of D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of...

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

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Towards Characterization of Infrasound Signals

David A McCormack

CTBT Verification Office

Geological Survey of Canada

Page 2: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

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

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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

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

ArraysArraysSourcesSources

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Raypaths sampledRaypaths sampled

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Data Processing - 1Data Processing - 1

Calculate coherence after Blandford (1998):

Assume noise uncorrelated between channels, stationary and uncorrelated with signal

= Cij / (As-An)

Choose standard window length and equivalent length of pre-event noise window for calculation.

Default = 100 secs

Page 8: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Page 9: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Data Processing - 2Data 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).

Page 10: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Page 11: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Parallel

Perpendicular

Page 12: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Data Processing - 3Data Processing - 3

Calculate ‘effective distance’ deff between elements as:

d2eff = d2

par + 2d2

perp

Where is derived empirically from the observations in the appropriate frequency band, and is typically around 2.

Page 13: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Data Processing - 4Data 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.

Page 14: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

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ResultsResults

3 bands:

0.4-0.6 Hz 0.8-1.2 Hz1.6-2.4 Hz

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Page 16: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

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

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

Page 18: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

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Page 19: D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002 Towards Characterization of Infrasound Signals David A McCormack CTBT Verification Office.

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SummarySummary

General degradation of coherence with source-receiver distance.

Systematic variations over several frequency bands of coherence with source type.

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002

ConclusionConclusion

Demonstration of simple, comprehensible infrasound signal parameter suitable for automated postprocessing.

Potentially useful for checking associations.

May contain information on source type.

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D McCormack, CTBT Infvrasound, KNMI Netherlands 29 October 2002