Changes in the Performance of the IMS Infrasound Network due to Seasonal Propagation Effects David Norris and Robert Gibson BBN Technologies 1300 N. 17 th Street Arlington, VA 22209 Infrasound Technology Workshop La Jolla, California 27-30 Oct 2003
Dec 14, 2015
Changes in the Performance of the IMS Infrasound Network due to Seasonal
Propagation Effects
David Norris and Robert Gibson
BBN Technologies 1300 N. 17th Street
Arlington, VA 22209
Infrasound Technology WorkshopLa Jolla, California
27-30 Oct 2003
Network Performance Factors
• Network coverage
• Array performance and signal-to-noise ratio
• Propagation effects and uncertainties
Network Coverage
• Lack of azimuthal coverage can lead to elongated error ellipses
• Example: Pacific bolide 23 Apr 01:
IS57
NVIAR
DLIAR
IS59
IS10
IS26
Source
Signal-to-Noise Ratio
• Some stations are inherently noisier than others– Full exposure vs. cover (e.g. tree canopy)– Island vs. mainland– Regional wind conditions (e.g. Windless Bight vs. Palmer)
• Signal gain– Nominal array beamforming gain: 10log(N)– Nominal Bandwidth gain: 5log(W)
• To improve SNR– Increase number of sensors– Improve wind filter– Advanced signal processing
Propagation factors
• Stratospheric arrival– Shorter path– Less absorption– Duct presence depends
on stratospheric winds
With wind Counter wind
• Thermospheric arrival– Longer path– More absorption– Duct always present
Stratosphericduct
thermosphericduct
Propagation Parameterizations
• Signal Strength– Empirical equations that
account for effect of stratospheric winds
– Received pressure (P) function of range (R), yield (W) and winds at 50 km (Vs)
– 38 dB difference between 50 m/s upwind and downwind propagation
)(019.0)log(36.1)log(68.037.3)log( sVRWP +−+=
•Mutschlecner, J. et al., “An Empirical Study of Infrasonic Propagation,” Los Alamos National Laboratory report LA-13620-MS, 1999.
•Stevens, J. et al., “Infrasound Scaling and Attenuation Relations from Soviet Explosion Data and Instrument Design Criteria from Experiments and Simulations,” Proceedings of the 21st Seismic Research Symposium, Las Vegas, NV, 1999.
Propagation Parameterizations
• Azimuthal uncertainty– Theoretical formulations,
based on• Signal-to-noise ratio• Signal and noise
coherence• Array geometry
– Empirical formulations
•R. Shumway and S. Kim, “Signal Detection and Estimation of Directional Parameters for Multiple Arrays,” Defense Threat Reduction Agency Technical Report DSWA-TR-99-50, 2001.
•Blandford, R., “Detection and Azimuth Estimation by Infrasonic Arrays as a Function of Array Aperture and Signal Coherence,” AFTAC report, 1998.
•C. Szuberla, “Array Geometry and the Determination of Uncertainty,” Infrasound Technology Workshop, Kailua-Kona, HI, 2001.
•Clauter, D. and R. Blandford, “Capability Modeling of the Proposed International Monitoring System 60-Station Infrasonic Network,” Proceedings of the Infrasound Workshop for CTBT Monitoring, Santa Fe, NM. Los Alamos National Laboratory report LA-UR-98-56, 1997.
InfraMAP
• InfraMAP is a software tool kit– Infrasonic Modeling of Atmospheric Propagation
• Designed for infrasound researchers and analysts
• Supports infrasonic-relevant R&D– Sensitivity studies– Network performance– Modeling specific sources of interest
IMS Coverage Simulation
• Goal: – Characterize seasonally-dependent effects of stratospheric
ducting on localization accuracy (AOU).
• Previous studies– Clauter, D. and R. Blandford, “Capability Modeling of the
Proposed International Monitoring System 60-Station Infrasonic Network,” Proceedings of the Infrasound Workshop for CTBT Monitoring, Santa Fe, NM. Los Alamos National Laboratory report LA-UR-98-56, 1997.
– E. Blanc and J. L. Plantet, “Detection Capability of the IMS Infrasound Network: A More Realistic Approach,” Proceedings of the Informal Workshop on Infrasounds, Bruyeres-Le-Chatel, France, 1998.
• Simulation parameters:
IMS Coverage Simulation
Variable Value/Description Comments
Background Station Noise 0.5 PaLow wind noise condition. Assume uniform across network
Array configuration 4 element, 1 km baseline Standard IMS array configuration
Array Gain, 10log(N) 6.0 dB Assume correlated signal across array
Bandwidth Gain, 5log(W)Strato (W=4 Hz): 3 dB
Thermo (W= 2 Hz): 1.5 dBProcessing over 1 sec window
Signal velocity UncertaintyStrato: 0.01 km/s
Thermo: 0.02 km/sOn order of that assumed in Blandford, 1998
Azimuthal Uncertainty Fit to data in Clauter and Blandford, 1997
SNR Detection Threshold 2
Source Yield 10kT
Received Signal Strength LANL wind-corrected eqn.Stratospheric winds at 50 km found from HWM averaged along propagation path
Conclusions• Performance of IMS network strongly dependent on seasonally
varying flow of stratospheric winds– Winter
• Northern Hemisphere: East flow• Southern Hemisphere: West flow
– Summer• Northern Hemisphere: West flow• Southern Hemisphere: East flow
• Localization capabilities of a given station improve in direction of stratospheric headwinds
• Recognized area of poor coverage: Southern Ocean• Shift in “Hole” in coverage:
– January: Off West coast of South America – July: Off of East coast of New Zealand
• AOU Radius southeast of Easter Island– January: > 400 km– July: < 100 km
Future Research
• Include station configuration– Number and location of elements– Wind filter properties
• Characterize local station background noise
• Improved characterization/modeling of propagation effects
– Signal strength– Azimuthal bias and uncertainty– Signal velocity and associated uncertainty