探討周遭噪訊交互相關函數中雜訊的特性以及其應用

On the noise level of the ambient noise cross-correlation function and its applications

陳映年(Ying-Nien Chen)

指導教授 ： 喬凌雲 ； 龔源成

國立臺灣大學/理學院/海洋研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.00916

摘要

地震紀錄中的噪訊主要來自於海浪與海床之間的交互作用所產生的微震訊號(microseisms)。近年來利用周遭噪訊法所得到的經驗格林函數已經被廣泛的應用在地震學的研究上，進而大幅增加地殼以及上部地函的解析能力。由於經驗格林函數的訊噪比可以簡單藉由增加地震連續紀錄的長度來提升，因此對於該函數中雜訊的特性至今尚未有量化的描述或系統性的探討。本研究首先提出測量經驗格林函數中「初始雜訊強度」的流程， 藉此我們可以客觀的量化經驗格林函數的資料品質。理論上，「初始雜訊強度」與噪訊源的數量分佈息息相關，而經驗格林函數的強度則與噪訊數量以及其產生機制有關，因此，藉由比較經驗格林函數的強度的以及相對應的「雜訊」特性，我們可以進一步了解噪訊源產生的機制。分析了台灣以及韓國的寬頻地震站的資料之後，我們發現台灣短周期(3~5秒)的噪訊強度主要與周遭海域的水深有關。此外，多數研究認為7~9秒的噪訊主要來自於遠方長浪引起的長周期的次級微震(long period secondary microseisms)，而本研究提出證據指出台灣以及韓國的噪訊主要是來自於近岸的海浪。過去研究必須同時仰賴地震資料以及海洋的觀測才能探討噪訊的產生機制，而利用本研究提出的「初始雜訊強度」，我們有能力可以單從地震噪訊的資料來探討噪訊源的特性。

關鍵字

周遭噪訊 ； 雜訊強度 ； 經驗格林函數

並列摘要

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.

並列關鍵字

ambient noise ； cross-correlation function ； microseisms ； original noise level

參考文獻

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