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| | Abstract | | As the Spatial Auto-Correlation Method (SPAC), which is a main method for dispersion analysis of surface wave from microtremors, requires the phase velocity to be a monodromy function of frequency, the presence of higher modes could damage this precondition and lead to a wrong result. The authors studied this matter with field data and confirmed that predominance of higher modes in low-frequency microtremors can be observed in various types of sedimentary plain with different depth of seismic bedrock. The study also shows that though the predominance of higher modes may distort the dispersion curve, the SPAC method still gives correct phase velocities provided that there is only one predominant mode for each frequency. For inversion, if the frequency band of higher modes can be recognized, the conventional inversion still gives good a result, otherwise predominant mode inversion will be required. However, if two or more modes have similar energy level, the SPAC method will not work and the analysis algorithm for this case remains a problem. |
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