Discussion on the Unusual Oscillation of Dispersion Curve for Surface Wave Testing
| Autor: | Ching-Lin Huang, 黃清林 |
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| Rok vydání: | 2004 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 92 In the area of the nondestructive testing, the Spectral Analysis of Surface Waves (SASW) Method belongs to the category that utilizes Rayleigh waves. Both the realization of frequency characteristics of soil profiles and the study on the unusual phenomenon in dispersion curves will be important tasks in improving the accuracy as well as the stability of the SASW method. In the literature, there are three key phenomenons being observed in the dispersion curve: "incorrect phase unwrapping", "discontinuity jump", and "strong oscillation". Among them, the first phenomenon will be the most important matter in this research. In the field-testing, there are many factors that will directly or indirectly affect the phase angle obtained in the receivers. Direct factors include the type of seismic source, background noise, source distance and the receiver distance. Indirect factors include the goodness of data acquisition, amplitude range of low or high frequencies, coherence coefficient from multiple impacts versus the difference of phase angles, and the correctness of unwrapping of the difference of phase angles. In this research, an impulsive type of seismic source was used in the filed and three factors including coherence coefficient, decibel (dB), and minimal unwrapping frequency ( ) were taken as threshold values for selecting frequency points in order to produce dispersion curves for different receiver arrangement. The results in this research showed that the wave velocity in the shallow depths oscillates in an unusual manner due to a large discontinuity jump in certain frequency range so that wrong unwrapping of the difference of phase angles causes wrong unwrapped angle, even in hundred degrees. On the other hand, due to high background noise and low amplitude from seismic source, the unwrapped angle will possibly be distorted. Thus, the correct selection of the minimal unwrapping frequency ( ) will be vital for correct unwrapping of the phase angles at the right beginning. The exponential filtering can increase coherence coefficient for short distances, whereas it will decease coherence coefficient for long distances so that some dispersion curves cannot be drawn. Key Words: Spectral Analysis of Surface Waves (SASW) Method, coherence coefficient, dispersion curve |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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