Il-young CHE, Hee-il LEE* Earthquake Research Center

1. A Study on Characteristics of Seasonally Dependent Infrasound Propagation Based on the Ground-Truth Events from a Long-Term Experiment at a Quarry mine Il-young CHE, Hee-il LEE* Earthquake Research Center Korea Institute of Geoscience & Mineral Resources(KIGAM), Daejeon, Korea With the contribution of Dr. Alexis Le Pichon Presented at 2008 Infrasound Technology Workshop Bermuda, November 3 – November 7, 2008

2. 516 SA events in 2006 649 SA events in 2007 169 Events Seismic magnitude (M ) of SA in 2007 L 350 300 250 200 341 Numbers 150 100 194 50 79 10 Seismic Data 25 0 ~< 0.5 ~<1.0 ~<1.5 ~<2.0 ~<2.5 Infrasound Local magnitude (M ) L Motivation Total number of 649 seismic events, i.e., 10% of total seismic events, were discriminated as surface explosion by the seismo-acoustic analysis in 2007 Most seismo-acoustic events are closely gathered in several regions and show similar distribution pattern to previous years Increase accuracy of localization of the seismo-acoustic events

3. Developing accurate locator for small seismo-acoustic events Assessing detectability of the stations Characterizing infrasound propagation according to season – arrival time, back-azimuth, attenuation, etc Deriving experimental relationship between charge size and amplitude from source time function Objectives

4. (37.5478N, 128.9614E) Experiment at a Quarry Mine (2007-2008) • Deployment of 2 Stations - CMT40T-1, Chaparral M2, Q330 • Operation : - 1.5 Month Test in 2006 - from Apr. 2007 - • Blasting Information - Everyday at 3h (UTC) - 0.1~10 tons ANFO

5. Experiment at a Quarry Mine (2007-2008) • Station information • KSGAR (2003) - 11 elements (4) - Distance : 126.9km - Back-azimuth : 155.2° • CHNAR (1999) - 11 elements (4) - Distance : 180.4km - Back-azimuth : 115.8° • BRDAR (2004) - 13 elements (5) - Distance : 383.0km - Back-azimuth : 95.6° • TJIAR (2002) - 4 elements (1) - Distance : 192.7km - Back-azimuth : 47.2°

6. Infrasound Seismic Air wave Origin Time : 02:55:45.4 Source Time Function (1) • Source signals measured near the blasting point (2007/06/01) - Charge Size : 0.1 t - Dominant Frequency ~ 2Hz - P-to-P Amplitude : 39 μbar - Empirical expression relating acoustic pressure to yield - Calculate source energy - Amplitude attenuation - etc.

7. Infrasound Seismic Air wave Source Time Function (2) • Source signals measured near the blasting point (2007/06/15) - Charge Size : 8.0 t - Dominant Frequency ~ 1.8Hz - P-to-P Amplitude : Clipped Aseismic↔ Ainfrasound

8. Data Analysis • Detection & Parameter Estimation • Phase Identification - KSGAR: mainly Iw, (Is) - CHNAR: Is - BRDAR: Is, IsIs, It - TJIAR: Is Based on our previous experience gained in 2006 – expected time of arrival to the stations and phases, data was analyzed semi-automatically

9. Expected arrival time of Is Data Analysis by PMCC • Data recorded at CHNAR on 01 June 2007

10. Speed: 352 m/s Azimuth: 116.5±013° Speed: 363 m/s Azimuth: 115.3±0.2° Speed: 358 m/s Azimuth: 119.2±0.4° Speed: 355 m/s Azimuth: 118.4±0.3° Speed: 358 m/s Azimuth: 116.8±0.2° Speed: 352 m/s Azimuth: 118.6±0.1° Speed: 358 m/s Azimuth: 116.6±0.2° Data Analysis by PMCC (01 June 2007) • Unexpected Problem : many signals coming from the same direction in a limited time window Back-Azimuth : 115.8 °

11. Blastings at the nearby mines

12. Autumn time Spring time Spring time Winter time Summer time November Seasonal Variation of Celerity • Exactly measured celerity(Is) from mine to CHNAR: 0.27 km/s (Apr.)0.29 km/s (Aug.)0.27 km/s (Oct.) 0.26 km/s (Jan.) 0.27 km/s (Apr. 2008) • The experiment shows first arrival is close to Is phase, guided wave propagating between the stratopause and the ground. • Celerity is strongly depending on seasonal velocity structure in atmosphere. Seasonal dependency of celerity should be considered in the infrasonic location method

13. Slowness Wind corrected Slowness Modelling with ECMWF (in Summer) 2007/06/01_ECMWF91_UVTSPQZ (From mine to CHNAR) TauP & WASP 3D Z=44.1km, X=192.1/180.4, T=682.04s, Vt=354.55m/s, Vapp=281.65m/s, Daz=0.31° Z=43.5km, X=199.5/180.4, T=702.95s, Vt=351.55m/s, Vapp=283.80m/s, Daz=0.35° We thank IDC for providing the ECMWF91 data and Dr. Alexis for doing ray-tracing with them.

14. Slowness Wind corrected Slowness Modelling with ECMWF (in Winter) 2007/12/22_ECMWF91_UVTSPQZ (From mine to CHNAR) TauP & WASP 3D Z=0.2km, No=36, X=182.1/180.4, T=540.68s, Vt=339.13m/s, Vapp=336.77m/s, Daz=-0.53° Z=0.2km, No=35, X=177.0/180.4, T=525.66s, Vt=339.13m/s, Vapp=336.77m/s, Daz=-0.53° Z=0.2km, No=37, X=187.1/180.4, T=555.70s, Vt=339.13m/s, Vapp=336.77m/s, Daz=-0.53°

15. Detected Not detected Why detectability is increasing in Summer? The experiment also shows detectability in summertime is higher than autumn-winter-spring seasons. It implies that infrasound propagation and detection are related with seasonal wind in and around the Korean Peninsula. We interpret that lower detectability in autumn-winter-spring is due to prevailing NW wind, its direction is opposite to infrasound propagation from the mine to CHNAR. Observed up to 30km at Sokcho Observatory (47090)

16. Surface Winds Affects the Detectability? Do surface wind fields near the source affect the detectability? No! The wind speed and direction are value averaged over 1 minute window at the time of blasting Detectability CHNAR : ~55% (in Summer) ~40% (all over the season) KSGAR : less than 5% At least one station : ~ 40% Two stations : ~16% Three stations : ~10% Four stations : < 5%

17. Localization by SALoc Seismo-acoustic location method Location method for small-magnitude surface explosions - generating both seismic and infrasonic signals Grid search method - finding a location where the time residual between observed and model predicted is minimum at the grid points with initial conditions : origin time, celerity, wind-corrected back-azimuth An example : location results for 6 tons of ripple-firing shot ,

18. Comparison of localization results Comparison of localization results for 21 ground-truth events The seismo-acoustic location shows a mean location error of 5.7 km, which represents a substantial improvement in location accuracy of 58.7% and 46.7% compared to the seismic location and infrasonic-azimuth intersection methods, respectively.

19. Summary & General Remarks • Detectabilty is increasing in Summer • Long-term measurement over the years will be helpful to clarify seasonal variation of celerity and to study atmospheric effects on infrasound propagation in mid-latitude Northern Hemisphere around the Korean Peninula • Celerity (Is) between CHNAR and mine changes as follows; - 0.27 km/s in April 2007 - 0.29 km/s in August 2007 - 0.27 km/s in October 2007 - 0.26 km/s in January 2008 - 0.27 km/s in April 2008 • Most frequently detected phase is Is at the stations of interest

20. Summary & General Remarks • SALoc significantly reduces location error for small magnitude explosion, M < 2.0

21. In the Future • Recovering source waveform saturated • Empirical relationship between the amplitude of the source wavelet/energy/measured amplitude • Continue this experiment to the next April to see seasonal change is consistent from year to year