The Journal of Engineering Geology (지질공학)

The Korean Society of Engineering Geology (대한지질공학회)
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Discrimination of Local Microearthquakes and Artificial Underground Explosions on the Basis of Time-Frequency Domain

시간-주파수 영역에서의 국지 미소지진과 지하인공폭발의 구별

  • Published : 1997.04.01
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Abstract

In this study, our purpose is to develop a technique to discriminate artificial explosions from local microearthquakes on the basis of time-frequency domain. To obtain spectral features of artificial explosions and microearthquakes, we used 3-d spectrograms(frequency, time and amplitude) because this is a useful tool to study the frequency content of entire seismic waveforms observed at local and regional distances (e. g., Kim et al., 1994). P and S waves from quarry blasts show that frequency content of dominant amplitude appeared above 10 Hz and Rg phases that are observed at near distance ranges. But P and S waves from microearthquakes have more broad frequency content as well as below 10 Hz. And for discrimination, Pg/Lg spectral ratio is performed below 10 Hz. In order to select time windows we computed group velocity using multiple filter method(MFM) and removed free surface effects from all 3-components data for improving on data quality. Next step, we computed Fast-Fourier transform, and a log average spectral amplitude over seven frequency bands : 0.5 to 3, 2 to 4, 3 to 5, 4 to 6, 5 to 7, 6 to 8 and 8 to 10 Hz. The best separation is observed from 6 to 8 Hz.

본 연구의 목적은 시간-주파수 영역에서의 미소지진과 인공폭발을 구별하는 것으로 미소지진과 인공폭발의 주파수특성을 연구하기 위해서 3차원 스펙트로그램(주파수, 시간, 진폭)을 이용하였다. 3차원 스펙트로그램은 국지 및 광역 거리에서 관측된 자료에 대하여 각각의 위상에 대한 주파수대역의 연구에 매우 유용한 방법이다. 채석장 발파로부터 관측된 P파와 S파는 Hz 이상에서 큰 진폭을 가졌고 또한 가까운 거리에서는 뚜렷한 Rg파가 관측되었다. 미소 지진의 경우 P파와 S파는 넓은 주파수 대역에서 큰 진폭이나타났다. 인공 폭발과 미소지진의 구별을 위해서는 10Hz 이하에서 Pg/Lg 스펙트럼 비를 이용하였고 각각의 위상에서의 정확한 시간창(time window)을 구하기 위해서 다중 필터 방법(MFM)을 이용하여 군속도를 계산하였다. 또한 3 성분 자료에 관해서는 자료의 순수한 P, SV, SH 성질을 구하기 위해 자유 표면에서의 영향을 보정하고 각각의 위상에 대하여 FFT을 실시하여 7개의 주파수 대역(0.5-3, 2-4, 3-5, 4-6, 5-7, 6-8, 8-10Hz)에서 Pg/Lg 스펙트럼 비를 계산하였다. 위의 과정을 통해 6-8Hz 대역에서 미소지진과 인공폭발이 가장 잘 구별되었다.

Keywords

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