1. INFRAMAP PROPAGATION MODELING ENHANCEMENTS AND THE STUDY OF RECENT BOLIDE EVENTS David Norris and Robert Gibson BBN Technologies Arlington, Virginia, USA Infrasound Technology Workshop Kailua-Kona, Hawaii 12-15 November 2001 Sponsored by the U.S. Defense Threat Reduction Agency Contract No. DTRA01-00-C-0063

2. 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)

3. InfraMAP… …defines state of atmosphere for use in propagation analysis –Empirical Wind model, HWM-93 –Empirical Temperature Model, MSIS-90

4. …predicts phases (travel times, bearings, amplitudes) using acoustic propagation models –Geometric ray tracing –Normal mode analysis –Parabolic equation solution InfraMAP…

5. 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

6. 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, 7 th 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

7. Absorption Coefficient F = 0.2 Hz F = 2.0 Hz 0.1 dB/km10 dB/km

8. Frequency = 0.5 Hz Absorption at 500 km = 32 dB Thermospheric Absorption

9. 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.

10. Eigenray arrivals S S S T T T Synthetic waveforms over range of source heights range (km) bolide TXIAR Eigenray solution at 30 km height

11. Height estimate Measured waveform filtered over 2-4 Hz 2 strong arrivals, 1 weak arrival Match with synthetic waveform data –1 st arrival: z = 33 km –2 nd arrival: z = 26 km –3 rd arrival: z = 23 km Synthetic waveform (dB) Measured waveform

12. 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

13. Spreading Loss 55 dB Eigenrays Absorption Synthetic Waveform Waveform Synthesis Dispersion effects Full wave absorption Received waveforms Source Waveform

14. Tagish Lake Bolide and Infrasound Stations On 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

15. 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)

16. PE-predicted Attenuation vs. Range

17. 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

18. Pacific Event Several infrasound arrays observed an event over the Pacific, 23-Apr-01 Five arrays used for localization study: –IS57: 255.5 o +/- 3 o –IS59: 61 o +/- 3 o –SGAR: 251 o +/- 5 o –DLIAR: 261 o +/- 2 o –IS10: 246 o +/- 3 o

19. 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 height * source Sound speed profile Ground Duct height * source

20. N E S NSE 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

21. N C S NSC 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

22. N C S NSC Global Duct Heights Duct 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

23. N C S NSC Global Duct Heights Duct 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

24. 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