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 Presented at 2008 Infrasound Technology Workshop Bermuda, November 3 – November 7, 2008 With the contribution of Dr. Alexis Le Pichon

2. Korea Institute of Geoscience and Mineral Resources 516 SA events in 2006649 SA events in 2007 Increase accuracy of localization of the seismo-acoustic events 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 169 Events Seismic Data Infrasound 25 194 341 79 10 0 50 100 150 200 250 300 350 Numbers ~< 0.5~<1.0~<1.5~<2.0~<2.5 Local magnitude (M L ) Seismic magnitude (M L ) of SA in 2007

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

5. Korea Institute of Geoscience and Mineral Resources Experiment at a Quarry Mine (2007-2008) Station information 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. Korea Institute of Geoscience and Mineral Resources Source Time Function (1) Source signals measured near the blasting point (2007/06/01) 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. Origin Time : 02:55:45.4 Seismic Infrasound Air wave

7. Korea Institute of Geoscience and Mineral Resources Source Time Function (2) Source signals measured near the blasting point (2007/06/15) Source signals measured near the blasting point (2007/06/15) - Charge Size : 8.0 t - Dominant Frequency ~ 1.8Hz - P-to-P Amplitude : Clipped A seismic ↔ A infrasound Seismic Infrasound Air wave

8. Korea Institute of Geoscience and Mineral Resources Data Analysis Detection & Parameter Estimation Detection & Parameter Estimation Phase Identification 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. Korea Institute of Geoscience and Mineral Resources Data Analysis by PMCC Data recorded at CHNAR on 01 June 2007 Data recorded at CHNAR on 01 June 2007 Expected arrival time of Is

10. Korea Institute of Geoscience and Mineral Resources Speed: 358 m/s Azimuth: 116.8±0.2° Speed: 352 m/s Azimuth: 118.6±0.1° Speed: 352 m/s Azimuth: 116.5±013° Speed: 358 m/s Azimuth: 116.6±0.2° 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° 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. Korea Institute of Geoscience and Mineral Resources Blastings at the nearby mines Distances in Km (Back Azimuth in Degree) Study Mine Mine A Mine B Remarks BRDAR 383.0 95.6 391.5 97.0 404.3 97.4 < 2.o° CHNAR 180.4 115.8 191.4 117.6 204.5 117.2 < 2.o° KSGAR 126.9 155.2 139.4 154.4 149.4 151.0 < 5.o° TJIAR 192.7 47.2 191.3 50.9 198.0 54.2 <7.2°

12. Korea Institute of Geoscience and Mineral Resources 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 Spring time Summer time Autumn time Winter time November Spring time

13. Korea Institute of Geoscience and Mineral Resources 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. Slowness Wind corrected Slowness

14. Korea Institute of Geoscience and Mineral Resources 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° Slowness Wind corrected Slowness

15. Korea Institute of Geoscience and Mineral Resources 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. Detected Not detected Observed up to 30km at Sokcho Observatory (47090)

16. Korea Institute of Geoscience and Mineral Resources Surface Winds Affects the Detectability? 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% Do surface wind fields near the source affect the detectability? No!

17. Korea Institute of Geoscience and Mineral Resources 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. Korea Institute of Geoscience and Mineral Resources 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. Korea Institute of Geoscience and Mineral Resources 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. Korea Institute of Geoscience and Mineral Resources Summary & General Remarks SALoc significantly reduces location error for small magnitude explosion, M < 2.0

21. Korea Institute of Geoscience and Mineral Resources 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