INFRAMAP PROPAGATION MODELING ENHANCEMENTS AND THE STUDY OF RECENT BOLIDE EVENTS

 David Norris and Robert Gibson

BBN TechnologiesArlington, Virginia, USA

Infrasound Technology WorkshopKailua-Kona, Hawaii

12-15 November 2001 

Sponsored by the U.S. Defense Threat Reduction Agency Contract No. DTRA01-00-C-0063

• Integrates the software tools needed to perform infrasound propagation modeling over global domain

• Used to support nuclear explosion monitoring R&D and resolve operational issues

Infrasonic Monitoring

System Analysis Tool InfraMAP

(Infrasonic Modeling of Atmospheric Propagation)

InfraMAP…

…defines state of atmosphere for use in propagation analysis

– Empirical Wind model, HWM-93– Empirical Temperature Model, MSIS-90

…predicts phases (travel times, bearings, amplitudes) using acoustic propagation models

– Geometric ray tracing

– Normal mode analysis

– Parabolic equation solution

InfraMAP…

• Exploding meteor observed 09 Oct 1997 at DLIAR (445 km) and TXIAR (359 km)

• Source height approximately 29 km

• Measured data at TXIAR studied to evaluate ability to verify/refine source height estimate

– Eigenrays found over range of source heights

– Synthetic waveforms cross-correlated with measured waveform

El Paso Bolide 1997

Propagation Amplitude

• Amplitude Important in the study of network performance– Defines the station coverage for a given event scenario– Determines the observability of thermospheric paths

• Amplitude predictions based upon– Spreading loss (20 log R)– Atmospheric absorption

• Classical: translation and diffusion• Relaxation: rotation and vibration

• Sutherland and Bass Absorption model– “Atmospheric Absorption in the Atmosphere at High Altitude”, L.C.

Sutherland and H.E. Bass, 7th Long Range Sound Propagation Symposium, Lyon, France, 1996.

– Extension/modifications of the ISO/ANSI standard

– Domain up to 160 km altitude

– High altitude: classical and rotational absorption dominate

Absorption Coefficient

F = 0.2 Hz F = 2.0 Hz

0.1 dB/km 10 dB/km

Frequency = 0.5 HzAbsorption at 500 km = 32 dB

Thermospheric Absorption

Waveform Synthesis

Spreading Loss 55 dB

Eigenrays

Absorption

Source Waveform

Synthetic Waveform

F = 0.5 Hz

• Provides time series or envelope predictions based on Eigenray and attenuation calculations.

• Source waveform is scaled by propagation loss and convolved with Eigenray arrival times.

Eigenray arrivals

SS

S

T

T

TSynthetic waveforms over range of source heights

range (km)

bolide TXIAR

Eigenray solution at 30 km height

Height estimate

• Measured waveform filtered over 2-4 Hz

• 2 strong arrivals, 1 weak arrival

• Match with synthetic waveform data

– 1st arrival: z = 33 km– 2nd arrival: z = 26 km– 3rd arrival: z = 23 km

Synthetic waveform

(dB)

Measured waveform

“Automated” height estimate

• Cross-correlation between measured intensity and synthetic waveform (on linear scale)

• Correlation peak at 30 km, in close agreement with ground truth height of 29 km

Peak at 30 km

Confession– Measured waveform shifted

by time delay to account for modeling shortfall (t = 18 sec)

• Synthetic waveform centered on arrival time

• Detections centered at beginning of waveforms

– Technique promising– Robustness issues need

further evaluation

Spreading Loss 55 dB

Eigenrays

Absorption

Synthetic Waveform

Waveform Synthesis

Dispersion effects

Full wave absorption

Received waveforms

Source Waveform

Tagish Lake Bolide and Infrasound StationsOn 18 January 2000, sensors aboard

DOD satellites detected the impact of a meteoroid at:

– 16:43:43 UTC near Whitehorse in the Yukon territory, Canada.

– The object detonated at an altitude of 25 km at 60.25 degrees North latitude, 134.65 degrees West Longitude.

• Study frequency dependence of propagation over typical path

• Potential impact on• Travel time predictions

• Amplitude predictions

• Scenario• Bolide - Lac du Bonnet

• PE model computed at 1 Hz

• Fan of rays computed over +/- 50 deg launch angle at 1 deg steps

PE-Ray Comparison

• Monotonically increasing difference between ray paths and PE energy bands.

– At 200 km: 10% difference in bounce range

• Source of mismatch– Frequency dependent

propagation effects

– PE model: phase error term

– Time evolution of environment (not fully modeled in PE)

PE-predicted Attenuation vs. Range

• General trend of increasing frequency absorption with range

• Evolution of sharp shadow zone boundaries as frequency increases

• Shadow zone formation over limited frequency bands

• Note: Atmospheric turbulence will tend to smear energy path boundaries and fill in shadow zones

Observations

Pacific Event

• Several infrasound arrays observed an event over the Pacific, 23-Apr-01

• Five arrays used for localization study:

– IS57: 255.5o +/- 3o

– IS59: 61o +/- 3o

– SGAR: 251o +/- 5o

– DLIAR: 261o +/- 2o

– IS10: 246o +/- 3o

Pacific Event

• Observations: consistent with stratospheric paths• Predictions: Stratospheric rays did not reach ground but were

trapped in elevated duct• Energy “Leaks” out of elevated duct through diffraction/scattering

Sound speed profile

Elevated Duct

hei

gh

t

*source

Sound speed profile

Ground Duct

hei

gh

t

*source

N

E

S

N S E

Global Duct Heights

40 km 38 km

120 km

Mid latitudes•Strong Stratospheric ducts

Equator•Thermospheric duct

Duct height (km)• 01 Jan 2001, 12 UT• With wind• Lower boundary: 0 km

N

C

S

N S C

Global Duct Heights

Mid latitudes•Thermospheric ducts

Equator•Thermospheric duct

Duct height (km)•01 Jan 2001, 12 UT•Counter wind•Lower boundary: 0 km

120 km 110 km 125 km

N

C

S

N S C

Global Duct HeightsDuct height (km)•01 Jan 2001, 12 UT

•Counter wind•Lower boundary: 5 km

Mid latitudes•Thermospheric ducts

Equator•Thermospheric duct, patched of stratospheric ducts

115 km 110 km 125 km

N

C

S

N S C

Global Duct HeightsDuct height (km)•01 Jan 2001, 12 UT

•Counter wind•Lower boundary: 10 km

South latitudes•Thermospheric ducts

North latitudes•Weak stratospheric duct

Equator•Stratospheric duct

20 km

105 km

40 km

• Multipath arrivals can be used to estimate source height and support model calibration

• Frequency dependence of propagation observed with PE - ray comparisons

• Elevated stratospheric ducts above 10 km can form over large regions of the globe

– These ducts can be excited from either elevated sources or ground source energy that leaks into duct

– Scattering/diffraction must be incorporated into propagation predictions to model ground reception of elevated duct energy

Conclusions