噪音交互相關函數之時變化成因探討：以臺灣東部外海海底地震儀資料為例

Abstract

本研究利用 2008 臺灣大地動力學國際合作計畫佈放於臺灣東部外海約一年期深海差壓計所記錄到的環境噪訊，以計算每日的噪聲交互相關函數 (Noise Cross-correlation Functions, NCF) 進行環境噪音法之分析。由環境噪聲求得之廣義格林函數的統計結果發現于測站間的訊號走時約有 2.9 秒的時變化，由頻譜得知其能量主要集中在 0.1 Hz 到 0.4 Hz 的微震 (Microseism) 頻段。為瞭解造成此時變化的機制，1. OASES (Ocean Acoustic and Seismic Exploration Synthesis) 數值模擬：發現所觀察到的訊號為行進於海水-彈性底床間的界面波，其傳播速度較不受到海水聲速影響；2. 由廣義格林函數的頻譜時間序列發現部分走時變化來自於訊號主導頻率隨時間改變，此乃為界面波的頻散現象：不同頻率的波傳在深度方向上對速度變化的敏感程度各有不同，使得各個頻率的波的傳遞速度不同；3. 噪訊源並非均勻散射場：當噪訊源分布受具方向性的海面波浪影響時，可能會造成與噪訊傳播介質無關的走時變化。根據 Wavewatch 所得的颱風浪場分佈，部分走時變化與海面波浪空間分佈相關。本研究亦希望能透過比較實際觀測結果與理論預測量，驗證是否能透過理論的方式修正於噪聲場具有方向性時對於 NCF 的影響，然而受限於實際噪訊場不易直接觀測，雖然理論修正的趨勢與實際觀測接近，但在修正量上仍有改進空間。

The one-year ambient noise recorded by the OBS system deployed off the east coast of Taiwan in the TAIGER (TAiwan Integrated GEodynamics Research) project was employed to calculate the Noise Cross-correlation Functions (NCF) of the noise field. The data recorded between the stations located at Yaeyama Ridge (water depth about 4000 m), starting from May of 2008 for a period of more one year, have been processed using the noise cross correlation technique. The results of NCF have shown that there exists strong microseism energy in the frequency band between 0.1 Hz and 0.4 Hz all year around, and has also demonstrated a large temporal variations as much as 3 seconds. To understand the temporal variations of NCF, we conduct the following analyses: 1. Numerical simulations using OASES: the result suggests that the generalized Green’s function found between stations is the interface wave traveling between the ocean and the elastic seafloor. The propagating speed of the interface wave is mainly dominated by the shear velocity of the seafloor, and is little affected by the ocean sound speeds. 2. Variable frequency content of NCFs: due to wavelength-dependent nature (dispersion) of the interface wave, the travel time will be a function of the frequency. 3. Non-diffusive noise distribution: the travel time obtained from NCF between stations requires that the noise field is diffusive. The directionality of noise distribution will affect the NCFs dramatically. A theoretical formula is adopted to correct the offset of observed travel time by directional noise fields using the wavewatch III model.