臺灣博碩士論文加值系統

海底地震儀通常被使用於記錄天然地震資料或是接收主動式震源，如空氣槍、炸藥等的訊號。而事實上地震儀所記錄到的不單只有震波訊號的波形，還有風浪、潮汐和一些其它的外力所造成的影響，通常被稱為環境噪訊(microseisms)。在2011年8月15日至9月7日，15個海底地震儀佈設在台灣東北部海域進行為期約20天的海底地震資料收集，我們利用台灣地震科學中心（TEC）所提供的「Antelope」程式來分析地震訊號和背景雜訊特性。在同年8月底時，南瑪都颱風於西太平洋形成。其移動路徑起於菲律賓東方，向西北方前進，最後登陸台灣本島。在移動過程中，由於颱風在海面上所造成的風浪或壓力變化，會直接或間接的產生可以在海底傳播的彈性波，於海床上的海底地震儀所紀錄下來。因此，本研究希望藉由海底和陸上地震測站所收到的震波訊號變化，如振幅、頻率等，來探討颱風的動向和強度對於海底環境噪訊的影響。
結果顯示颱風活動對於震波訊號的主要影響波段位於0.2-0.5 Hz，屬於相對較低頻的訊號。此低頻噪訊的振幅與颱風和地震儀之間的距離則呈現了負相關的關係；兩者距離減少，則地震儀所收到的低頻雜訊振幅將增加。並且我們也發現，颱風的強度變化對於震波振幅的影響程度相對於距離來的小。為了進一步瞭解波浪變化和地震儀紀錄到資料的關係，我們將海底地震儀的震波資料與測站附近的氣象浮標資料來加以比對，進行迴歸分析後能得到兩者呈現正相關，並在資料分析過程中，我們並發現了重覆週期約12小時的低頻震波訊號，推測是與潮汐運動有關。此外，結果也顯示海底地震儀記錄之訊號的能量大小主要是受到海底地震儀所在的海床深度影響，可能是由於訊號能量會隨著傳播深度而衰減與擴散所造成；其次則是場址效應，不同的地形條件也會造成測站接收到的訊號能量的改變。
由於受到颱風影響的高能量訊號在海底地震儀中的水聽器與陸上寬頻地震站的資料也都有記錄到，但颱風靠近時振幅的放大效應在海底地震儀的感震器垂直分量上最明顯，推論可能是由於此低頻地表微地動是由附近水壓變化，於海床耦合成彈性波後透過海床傳遞至地震站，因而感震器垂直方向可同時收到兩者的效應並增加所接收到的訊號強度。

Ocean-bottom seismometer (OBS) is usually used to record active and passive sources, such as air guns, explosives, earthquakes and other signals. In fact, the seismometer records not only the seismic waveforms but also noises generated by winds, waves, tides and other external forces. From the end of August to early September in 2011, 15 OBSs were deployed offshore northeastern Taiwan for about 20 days. At the end of August, the typhoon Nanmadol formed in the western Pacific and moved northwestward from the East Philippines and finally made landfall on the island of Taiwan. Due to storms or pressure changes caused by the typhoon, elastic waves would be directly or indirectly produced and recorded by the seismometers. In this study, by analyzing the seismic signals collected by the OBSs and the BATS stations, we investigate the influence induced by the changes of typhoon path and intensity on the submarine seismic noises.
Preliminary results indicate that the seismic energy change related to the typhoon occurred mainly at 0.2-0.5 Hz, which is a relatively low frequency compared to that of earthquakes. The amplitude of this low-frequency noise increased when the distance between the typhoon and seismometer decreased. By comparing the seismic waves with the data collected from the marine weather buoy, we observed a positive correlation between the power of the low frequency microseisms and the wave height. This clearly indicates that the typhoon was the main source of microseisms during their passing. Owing to the ocean waves generated by the typhoon, the pressure changes in the water column is recorded by the seismometers before being transmitted to the seafloor. The spectrum analysis shows the presence of a high energy signals at 0.2-1 Hz with a period of about 12 hours which could be related to the tidal movements. In addition, the amplitude of the recorded microseisms is also affected by the depth of seismometers. In general, the deeper the seismometer is located, the smaller the amplitude of microseisms it recorded. All these observations show the seismic signal can respond to the wave and wind changes. However, some exceptions, probably induced by site effect, are observed.
Analysis based on the data recorded by hydrophones and inland stations displays consistent results with that of geophones, showing that ocean wave heights appear to be the main origin of the low frequency microseisms signals. Therefore, we suggest that the low frequency ground motions are mostly induced by nearby water pressure fields, and transmitted through the rock to the stations.

摘要 i
Abstract iii
致　謝 v
目　錄 vi
圖　目　錄 viii
表　目　錄 xi
第一章 緒論 1
1-1 研究動機與目的 1
1-2 前人研究 2
1-3 本文內容 5
第二章　儀器介紹及資料處理 9
2-1 儀器介紹 9
2-1-1 海底地震儀 9
2-1-2 陸上寬頻地震站 11
2-1-3 海氣象監測網 12
2-2 資料收集 13
2-2-1 地震波形資料 13
2-2-2 氣象資料 14
2-3 資料處理 15
2-3-1 處理軟體介紹 16
2-3-2 處理方法 17
第三章 資料分析結果 28
3-1 時域分析 28
3-2 頻譜分析 30
3-2-1 海洋波浪 31
3-2-2 潮汐 33
3-2-3 其它訊號 33
第四章 結果與討論 51
4-1 海床地震動背景雜訊之比對 51
4-1-1 水深深度 52
4-1-2 場址效應 52
4-1-3 颱風強度 53
4-2 感震器與水聽器之比較 53
4-3 海底地震儀與陸上寬頻地震站之比較 55
第五章 結論 66
參考文獻 68

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