Locating Bolide Fragmentations and Terminal Explosions using Arrival times of Acoustic Waves

1. Locating Bolide Fragmentations and Terminal Explosions using Arrival times of Acoustic Waves Wayne N. Edwards and Alan R. Hildebrand Department of Geology & Geophysics, University of Calgary, Alberta, Canada 2003 AGU Infrasound Technology Workshop October 29th, 2003

2. What is a Supracenter? • Analogous to earthquake location in the solid Earth • Complications • P wave velocities slower • Winds vary in magnitude & direction with altitude • Fireball explodes – point source of finite duration • Wavefront propagates to ground where seismometers, microphones & infrasound arrays record its arrival Photo by: Brad Gledhill

3. Propagation in the Atmosphere • Potential Arrivals depend on distance of receiver station • Direct arrivals • Ducted waves • Thermospheric returns • Stratospheric returns • Skips (Brown et al. 2003) Red Box shows region of direct arrivals

4. Recognizing the Seismic Signal • A – Slow initial rise • ground roll arrivals • B – Prominent peak • direct atmospheric • Terminal Burst or Sonic Boom? • C – Long drawn out tail • higher altitude sources • e.g. Early fragmentation Duration: order of minutes long Propagation: low trace velocities across arrays A C B

5. Finding a Solution 1 – Identify & pick station arrival times 2 – Construct model atmosphere  Acoustic velocity 3 – Assume that all arrivals from the same event: TA = TB = TC = TD = … = Tb = Initial time of burst

6. 6a – Find the mean time of occurrence (Nelson & Vidale 1990) 6b – Use a known, observed occurrence time • Earth-observing satellites • Recorded video Finding a Solution 5 – Choose a position: ray trace to receivers 7 – Calculate station traveltime residuals 8 – Vary position to minimize the mean residual

7. Previous Supracenter Locations • Have only treated atmosphere as an isotropic velocity medium. • Johnston 1987: Missile silo explosion & supersonic aircraft • Qamar 1995: Fireball terminal bursts • Borovička and Kalenda 2003: Fireball fragmentation • Assumed atmosphere is static in most cases Result: Solutions may mis-locate an event by several kilometers depending upon wind conditions in the atmosphere at the time of the event.

8. Ray Tracing Complications • Winds  Ray propagation becomes direction dependant • Winds perpendicular to azimuth add motion outside of azimuth plane RESULT: rays bend! • Azimuth & Elevation angle UNKNOWN • Solution: • Modified Tau-p Equations (Garcés et al. 1998) • Iteratively refining “Ray Net” to identify ray orientation angles connecting source to receiver

9. Analytic Model 30 km Source in a windy, (45 m/s from the North) Isotropic (300 m/s) Atmosphere (15% of Local Sound Speed or L.S.S.) Maximum Error: ~0.0048% of Traveltime Structure of Traveltime Error(Ray tracing vs. Analytic)

10. The SUPRACENTER Program • Uses a stratified model of the atmosphere • Local or nearby radiosonde soundings • Atmospheric models (e.g. MSIS-E, HWM) • 1978 U.S. Standard Atmosphere (as option) • Includes the effects of winds as it traces rays! • NOT a correction for wind applied after locating an otherwise static solution.

11. Simplifications & Assumptions • Flat Earth Approximation • Geometrical rays • - diffraction is minimal over the travel time of a ray • Atmospheric motions are predominantly horizontal - (i.e. vertical motions are negligible) • Horizontal variations in temperature and wind are negligible. • Atmosphere is approximated by discrete layers, each with its own characteristic temperature and wind vector. • Use only direct air arrivals. (i.e. receivers ≤ 100 km to the event epicenter)

12. Testing SUPRACENTER … Three seismically detected fireball events were chosen where independent solutions existed. Two Historical: • El Paso Superbolide, October 9th 1997. • Mt. Adams Fireball, January 25th 1989. • Movávka meteorite fall, May 6th 2000. One Recent:

13. Case Study #1: El Paso Superbolide October 9th, 1997 • Daytime fireball at local noon hour ~18:47:15 UT • Many eyewitnesses • 19 photographs of the dust cloud • 6 video recordings • 8 seismic detections & 2 infrasonic • Terminal burst of fireball produced a circular dust cloud ~1 km in diameter  supersonic shock • Photographic observations produced an accurate determination of the position for the terminal explosion. (Hildebrand et al. 1999)

14. Distribution of Stations • Non-ideal linear orientation (NW-SE) • Long distances between stations • Limited # of potential stations with direct arrivals

15. Atmospheric Sounding • Radiosonde Data • largest winds at ~15 km where local sound speed is lowest • winds predominantly from WSW below 20 km • prominent wind shearing at ~30 km

16. Comparison of Solutions • Hildebrand et al. (1999) • Observed Event time ~18:47:15 UT • 31.80oN, 106.06oW at ~28.5 km altitude • Derived from eyewitness reports, photographic and video records • SUPRACENTER • 31.790oN, 106.080oW at 27.6 km a.s.l. + 0.5 km shock • Occurrence time constrained to 18:47:15 UT • Avg. residual of 0.240 seconds • ~2.1 km WSW from Hildebrand et al. solution found through independent methods

17. N

18. Case Study #2: Mt. Adams Fireball January 25th, 1989. • Bright Daytime fireball at local noon hour. 12:51 pm, Pacific Standard Time • NW to SE track over Puget Sound, Washington ending near the NW flank of Mt. Adams (Pugh 1990). • During decent fireball split in two with each fragment producing its own terminal burst. • Both bursts were recorded by 26 seismic stations (Qamar 1995) of the Pacific Northwest Seismic Network.

19. January 25th, 1989 Model Atmosphere • Radiosonde Data + 1978 U.S. Std Atmosphere + HWM • dual temperature inversions • increased winds correlate to region of lowest temperature • Predominantly NNW winds

20. Comparison of Solutions • Qamar (1995) • Burst A: 46.435oN, 122.094oW at 35.1 ± 1.0 km Height @ 20:51:10.1 UT • Burst B: 46.396oN, 122.062oW at 30.4 ± 0.7 km Height @ 20:51:10.9 UT • SUPRACENTER • Burst A: 46.460oN, 122.096oW at 34.62 km a.s.l @ 20:51:14.5 UT • Avg. residual: 0.925 sec. Stations Untimed: 5 • ~2.7 km NNW of Qamar’s solution • Burst B: 46.418oN, 122.065oW at 29.82 km a.s.l @ 20:51:15.1 UT • Avg. residual: 0.903 sec. Stations Untimed: 5 • ~2.5 km NNW of Qamar’s solution

21. Differences? • Low winter atmospheric temperatures • lower sound speeds • bursts at lower heights  later times • Without independent measure of fireball’s time of occurrence, determination of which is correct event time is unlikely to be resolved

23. Mt. Adams Fireball Trajectory Trajectory Parameters: Azimuth: 152o Elevation: 43o Velocity: 11.7 km/s Consistent with investigation of Pugh (1990): “Entered atmosphere over Puget Sound … disruption over northwest flank of Mt. Adams”

24. Conclusions • Using arrivals of acoustic waves at the surface and realistic ray tracing it is possible to locate atmospheric explosions. • Significant position “drift” does occur when strong unidirectional winds are present. • Position “drift” can be on the order of several kilometres  width’s of meteorite strewn fields • Method is independent of the time of the fireball • SUPRACENTER demonstrates both consistency with and improvement over the simple isotropic (average velocity) atmosphere treatments of the past.

25. Implications • Potential for 24 hr monitoring for fireballs • More monitoring stations needed • Simple as installing a microphone + recorder on current & future fireball camera networks • How does this help meteorite recovery efforts? • Better estimates for locations of potential strewn fields • Potential recovery of more freshly fallen meteorites • Another tool for fireball trajectory tracking • Accurate location  Constrain energy calibrations

26. Future Work • Extension of supracenter location method to stratospheric and thermospheric returns • Allow distant stations to be used in solution • Provide more constraint to poorly sampled events • Requirements: • Choice between multiple arrivals • Path that minimizes the station residual

27. References Garcés, M.A., Hansen, R.A. and Lindquist, K., G. (1998) Traveltimes for infrasonic waves propagating in a stratified atmosphere, Geophysical Journal International, 135, pp. 255-263. HildebrandA., Brown P., Crawford D., Boslough M., Chael E., Revelle D., Doser D., Tagliaferri E., Rathbun D., Cooke D., Adcock C. and Karner J. (1999) The El Paso Superbolide of October 9, 1997, In Lunar and Planetary Science XXX, Abstract #1525, Lunar and Planetary Institute, Houston (CD-ROM). Johnston C. (1987) Air blast recognition and location using regional seismographic networks, Bulletin of the Seismological Society of America, 77, no.4, pp. 1446-1456. Nelson G. and Vidale J. (1990) Earthquake locations by 3D finite-difference traveltimes, Bulletin of the Seismological Society of America, 80, no.2, pp. 395-410.  Pugh R. (1990) The Mt. Adams, Washington Fireball of January 25, 1989, Meteoritics, 25, p. 400. Qamar A. (1995) Space Shuttle and Meteoroid – Tracking Supersonic Objects in the Atmosphere with Seismographs, Seismological Research Letters, 66, no.5, pp. 6-12.

28. Case Study #3: Morávka Meteorite Fall May 6th, 2000 • Bright daytime fireball observed by 1000’s of eyewitnesses and 3 amateur video’s (Borovička et al. 2003). • Fireball produced a cascade of individual fragmentations while passing directly over a seismic network. • Arrivals for 12 fragmentation events were identified from complex amplitudes and located using an isotropic method by Borovička and Kalenda (2003). • Both the fireball’s trajectory & pre-fall orbit were well determined through video analysis (Borovička et al. 2003).

29. Stations & Arrival times • 6 of 12 Fragmentation acoustic arrivals identified by Borovička & Kalenda from 11 station records • Atmospheric model of Brown et al. (2003) constructed from a nearby radiosonde release • (Poprad, Slovakia) (Borovička and Kalenda 2003)

30. Model Atmosphere to 50 km(Brown et al. 2003) Winds are relatively light. Peak @ 13.2 m/s (4.4% of L.S.S.) Wind direction is not unidirectional – generally from the South Result: “Wind drift” should be minimal for supracenters

31. Comparison of Solutions Borovička & Kalenda (2003) SUPRACENTER • Fit to satellite observed time of 11:51:52.5 UT • Very little wind “drift”: ~0.1 – 1 km • Difference between Borovička & Kalenda & SUPRACENTER solutions: 0.4 – 1.5 km • Event K: repositioned ~1.5 km to the Southwest

32. ~1.5 km (2003) C E F G K L

33. Morávka Fireball Trajectory Trajectory Parameters via SUPRACENTER Azimuth: 171.8o Elevation: 18.9o Determined through Video Analysis (Borovička et al. 2003) Azimuth: 175.5o Elevation: 20.4o Difference? Fragments travelling along slightly different trajectories. or Mis-identification of acoustic arrivals?

34. -300m Comparison to Kunovice Video • Fragmentations show alignment improvement • New K position at start of 1st stream of fragments • L – misalignment likely due to later occurrence time • Fit L time to ~13o elevation • 300 m lower • Occ. time: +0.91 sec. • Fireball Velocity: 22.1 km/s From video analysis: 22.5 km/s (Borovička et al. 2003) NOTE: Small squares: positions of individual fragments mapped from the Kunovice video

35. References Borovička J., Spurny P., Kalenda P., and Tagliaferri E. (2003) The Morávka Meteorite Fall I: Description of the events and determination of the fireball trajectory and orbit from video records, Meteoritics & Planetary Science, In Press. Borovička, J. and Kalenda, P. (2003) Meteoroid dynamics and fragmentation in the atmosphere, Meteoritics and Planetary Science, In Press. Brown P., Kalenda P., ReVelle D., and Borovička J. (2003) The Morávka Meteorite Fall II: Interpretation of Infrasonic and Seismic Data, Meteoritics & Planetary Science, In Press.