13 nov 2001infrasound workshop, kona array geometry and signal observations david a mccormack head...
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13 Nov 2001 Infrasound Workshop, Kona
Array Geometry and Signal Observations
David A McCormackHead of Monitoring and Analysis
Geological Survey of Canada, Ottawa, Canada
13 Nov 2001 Infrasound Workshop, Kona
Topics
• IMS design array response for ‘realistic’ signals – how bad is bad?
• Initial studies of signal coherence as an indicator of maximum source distance
13 Nov 2001 Infrasound Workshop, Kona
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.
13 Nov 2001 Infrasound Workshop, Kona
Brown(2000) - 1Brown(2000) - 10.125 Hz 0.5 Hz
Spatial Aliasing: DLIAR Array
13 Nov 2001 Infrasound Workshop, Kona
Brown (2000) – 2Brown (2000) – 21.0 Hz 2.0 HzSpatial Aliasing: DLIAR Array
13 Nov 2001 Infrasound Workshop, Kona
Array Geometry
13 Nov 2001 Infrasound Workshop, Kona
SHUTTLE - time
13 Nov 2001 Infrasound Workshop, Kona
Shuttle
13 Nov 2001 Infrasound Workshop, Kona
Brown(2000) - 3IS10 Winnipeg, Canada
December 16, 1999 to
September 1, 2000 Approximately 4200 detections
N
E
S
W
microbaroms,Pacific Ocean (?)
microbaroms, Hudson Bay ?
mining activity, Minnesota, registersstrongly on local seismic
Local blasting Thunder Bay area,too small to register on local seismic
?
Local blasting,Too small to registeron local seismic
13 Nov 2001 Infrasound Workshop, Kona
Sine – 4 elements
13 Nov 2001 Infrasound Workshop, Kona
Sine – 7 elements
13 Nov 2001 Infrasound Workshop, Kona
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
13 Nov 2001 Infrasound Workshop, Kona
Synth 0.25-1,4
13 Nov 2001 Infrasound Workshop, Kona
Synth 0.25-1,7
13 Nov 2001 Infrasound Workshop, Kona
Synth 0.4-0.6,4
13 Nov 2001 Infrasound Workshop, Kona
Synth 0.4-0.6,7
13 Nov 2001 Infrasound Workshop, Kona
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
13 Nov 2001 Infrasound Workshop, Kona
Signal Coherence
• Grew out of studies of optimal array aperture for detection/location
• Coherence as a routinely calculated intrinsic parameter of an observed signal
13 Nov 2001 Infrasound Workshop, Kona
Local – time series
13 Nov 2001 Infrasound Workshop, Kona
Local – freq content
13 Nov 2001 Infrasound Workshop, Kona
Local - coherence
13 Nov 2001 Infrasound Workshop, Kona
MINNTAC - time
13 Nov 2001 Infrasound Workshop, Kona
MINNTAC – freq content
13 Nov 2001 Infrasound Workshop, Kona
MINNTAC - coherence
13 Nov 2001 Infrasound Workshop, Kona
SHUTTLE - time
13 Nov 2001 Infrasound Workshop, Kona
Shuttle – freq content
13 Nov 2001 Infrasound Workshop, Kona
Shuttle - coherence
13 Nov 2001 Infrasound Workshop, Kona
Pacific Freq
13 Nov 2001 Infrasound Workshop, Kona
Pacific Bolide Coherence
13 Nov 2001 Infrasound Workshop, Kona
Alberta Freq
13 Nov 2001 Infrasound Workshop, Kona
Alberta Bolide Coherence
13 Nov 2001 Infrasound Workshop, Kona
Recorded sources
13 Nov 2001 Infrasound Workshop, Kona
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)
13 Nov 2001 Infrasound Workshop, Kona
Coherence vs distance
13 Nov 2001 Infrasound Workshop, Kona
Ms 7.4 Hector Mine, California Earthquake - Acoustic Arrival
13 Nov 2001 Infrasound Workshop, Kona
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