| 摘要: | 關鍵詞：天然氣水合物、側掃聲納、底質聲納剖面儀、磁力儀、多音束水深 摘要在天然氣水合物賦存的海域，甲烷與硫化氫等氣體可能從沉積物深部藉由斷層或其他通道往海床上冒出，天然氣往海床的上昇或噴出經常伴隨地質構造形成許多樣貌如泥貫入體、泥火山或麻坑等，因此瞭解海床地貌聲納影像及海床下淺層沉積物的構造和特徵是了解海床下流體及天然氣水合物形成極重要的研究項目。根據第一年的綜合資料分析結果顯示，永安海脊海床上具有聲波的高反射係數特徵，推測可能與岩層遭到侵蝕裸露或自生碳酸塩礁的形成有關。此外，永安線形西側的沈積地層中有廣泛的聲波透明層的震測相出現，而此區也有廣泛的麻坑同時存在，因此，推測此區可能是濁流岩流堆積形成的高孔隙率地層，後來成為甲烷氣體的儲集層，而在此溫壓條件下，事實上可能已經形成天然氣水合物的賦存。而在永安海脊東側也有局部的麻坑存在，雖然沒有明顯的聲波透明層現象，但卻有明顯的泥貫入體上衝構造，意味此區可能也有天然氣水合物的存在。在第二年成果中，我們發現好景海脊西側的近海床特徵，包括海床地貌特徵與淺部地層構造等，皆發現與天然氣有關的地質構造，例如麻坑、自生碳酸塩礁等，磁力資料逆推的結果，也發現有些區域與地化資料底水高甲烷的反應可以相搭配。綜合解釋的結果，我們推測在好景海脊西側的區域，天然氣的移棲可能非常靠近地表，幾處大規模的崩塌區，也可能在溫、壓條件合適下，成為天然氣水合物儲集的地方。在此第三年成果中，我們發現許多甲烷氣活動的徵兆，滲流構造與氣噴現象，水深分布大約在500 m附近及其以上的區域，推測此深度可能是台灣西南海域天然氣水合物穩定與不穩定的界限。強回散射區主要分佈在泥火山與枋寮海脊區域，我們認為可能與碳酸塩礁的生成與覆蓋有關。根據多音束水深資料處理結果，第一年與第二年作業區內的海底地形架構相同，是以海底峽谷為主，其中伴隨多處塊體滑移的運動與海底崩塌，然而，區域內的侵蝕與搬運作用非常明顯，因此在峽谷的主要河道上並沒有發現崩塌物的堆積。在此第三年的多音束水深測繪方面，我們將所收集的水深資料與作業區內的舊有水深資料，整編出新的西南海域海底地形圖，對於未來高屏峽谷以東海域的探測工作，提供詳實的海底地形資訊。 透過新的海底地形圖，我們發現在研究區內有許多泥火山構造，枋寮峽谷完整的地貌特徵也被顯示出來。另外，新的水深資料在精確度與解析度上，也大幅超越舊有資料的水準，對於日後地調所或是其他工作團隊在調查與規劃上的應用，能提供非常大的幫助，對於繼續擴充研究用的高解析海底地形圖資也有非常重要的貢獻。側掃聲納處理軟體的開發也有重大的進展，尤其在資料位置的回置步驟上，利用既有EK500水深與位置的資料比對，成功的提高了沿航線影像嵌合的準確度，並進一步達到底質剖面影像也自動化速度修正的功能，這部分將是底拖聲納系統資料處理的重要進展。海底地電阻的發展方面，嚐試利用海底地電阻模擬分析、逆推程序，進而實際處理海底地電阻資料，以深化我們的海洋地球物理技術。也透過緊密互訪、聯繫與協調日本JAMSTEC之DCR系統在2012年來台進行實際探測的可行性。為了延續前三年的成果並進一步瞭解其他台灣西南海域天然氣賦存和可能鑽井的考量，我們提出此第四年計畫書，規劃在台灣西南外海的永安海脊與好景海脊間的區域，進行底拖聲納系統與多音束水深資料的收集，繼續提供天然氣水合物賦存區內的高解析地質、地貌與海底地形資料，作為將來天然氣水合物研究的重要參考資料。為了落實國內自主發展海域地電探勘技術，及利用海底直流電阻法之資料模擬技術，作為更有效地輔助天然氣水合物賦存量的評估。In gas hydrate resources areas, fluid such as methane or sulfide may go up to the sea floor through sediment layers by using faults or other venting passages. The upward flow of fluid may form distinctive features at seafloor such as mud diapir, mud volcanoes or pockmarks. Therefore, understanding the morphological characteristics of the sea floor through acoustic images of deposited or structural characteristics of shallow sediment layers is important to study the mechanism of fluid flow and gas hydrate formation. According to the results of first year survey in the Yung-An Ridge (YAR), we found wide distribution of carbonate in terms of strong acoustic reflection. In this case, the acoustic penetration is very limited and diffraction phenomenon is clear in sub-bottom profiler data. In contrast, to the west part of YAR, the sedimentary layers can be clearly identified. The seismic blanking zone can be traced along a layer as thick as 30 m, which suggests that this area is a high-porosity sedimentary layer and could reserve potential gas-hydrates. In some places, the possible gas-hydrates (wipe out zones) even outcrop to the seafloor. In the east of YAR, there are also pockmarks at the same place as the seismic blanking zones. Based on the side-scan sonar data, we found seafloor features of pockmarks and authigenic carbonate in the west of Good-Weather Ridge (GWR). The analysis of magnetic data is also consistent with the high methane concentration from geochemical data. We infer that in the west of GWR methane could migrate upward near seafloor and the area of several submarine landslide deposits could be good place for the formation of gas-hydrate if the PT condition is suitable. This year, we found many sites of methane gas activities, seeps with the gas plume, distributed around 500 m water depth in ours, we suggest the water depth of 500 m in the southwestern Taiwan may be gas hydrate instability depth. Besides, the strong back scattering images mainly in the mud volcanoes and Fangliao Ridge, which could be authigenic carbonate cover on these formations. The multi-beam survey in the northern margin of South China Sea shows features of several canyons and possible submarine landslides as shown in the first year. In the survey area, the phenomenon of erosion and transportation is obvious. The multi-beam survey in the off shore of Taiwan shows the features of several canyons, ridge and possible submarine landslides. In the survey area, we compile the new collected bathymetric data and old data to make a new topographic map off southwest Taiwan. The new map is much better than the previous old data in terms of resolution. This data set is very helpful for further multi-discipline gas-hydrate studies in the near future. Based on this new topographic map, we found many mud volcanoes. A complete topographic image of the Fangliao Canyon is also displayed. In the development of side-scan sonar data processing software, we have adopted the EK500 bathymetry to improve the layback of the deep-towed side-scan and sub-bottom profiler data. This part is an important result of the deep-tow sonar system data processing. For the CSEM (Controlled-Source Electromagnetics), we have tried to build the data analysis and inversion flow for the processing of the resistivity data. We have also contact JAMSTEC in order to use JAMSTEC’s DCR instrument off SW Taiwan in 2012. In order to have a better understanding of gashydrate formation and the possible drilling off SW Taiwan. We propose to collect multi-beam bathymetry, deep-tow acoustic and magnetic data around Yung-An Ridge and Good-Weather Ridge regions off SW Taiwan in the fourth year. These data will provide important information for in-depth study of Taiwan gashydrate project in the future. The reason to implement the development of marine DC system is to build domestic ability to conduct marine electrical resistivity data, which is helpful in characterize the potential gas-hydrate formation off SW Taiwan. 研究期間：10001 ~ 10012 |
