On the noise level of the ambient noise cross-correlation function and its applications
Date Issued
2014
Date
2014
Author(s)
Chen, Ying-Nien
Abstract
Retrieving the Empirical Greens function (EGF) between two receivers by cross-correlating continuous records is now a well-recognized technique and the derived EGFs have been applied to various fields of seismology. However, little attention has been given to a more quantitative description on the noise behavior of the noise-derived cross-correlation functions (CCF), for its signal-to-noise ratio (SNR) can be improved easily by increasing the total correlation time. In this thesis, we propose a method to measure the noises within the CCFs and demonstrate the relationship between noises and the corresponding sources properties. We evaluate the original noise level (ONL) for CCFs in Taiwan and Korea. With the measured ONL, we can estimate data quality for any portion of the CCF in the time domain. Moreover, since the ONL is closely related to the noise source population and EGF’s amplitude is sensitive to the excitation strength, combination of both measurements allows us to put better constraints to the noise sources. Using the approach, we conclude that (1) The dominant microseisms of period 5~10 sec observed in Taiwan and Korea are mostly contributed by Primary microseisms (PM), rather than long period secondary microseisms (LPSM) proposed by previous studies; (2) The high short period secondary microseisms (SPSM) level in Taiwan Strait is mainly caused by the bathymetry effect; (3) The low ONL in the SPSM band implies that sources for these dominant signals in CCFs are likely confined in the near-coast region; (4) The expected high source population of PM around Taiwan is well demonstrated by the strong ONL in the period ~6-9 seconds, although the PM signals are not present in the CCF records or the background seismic noises.
Subjects
ambient noise
cross-correlation function
microseisms
original noise level
Type
thesis
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