• The Journal of the Acoustical Society of Korea
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• v.23 no.3
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• pp.197-205
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• 2004

This paper represents the results of interpretation on the cause of sign changing of the striation slopes appearing in the range-frequency domain spectrogram of ship-radiated noise measured in a shallow sea. Striation patterns and dispersion characteristics simulated from a numerical model based on mode theory at various seabed conditions show that the sign changing of the striation slopes appearing in measured signal is caused by the shear property of seabed. more specifically by the shear property of the basement lying below the sediment which is estimated about 3±1m thick.

• The Journal of Engineering Geology
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• v.7 no.1
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• pp.63-79
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• 1997

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.