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，天然氣往海床的上衝或噴出經常伴隨地質構造形成各式樣貌如泥貫入體、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1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〝白化〞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，雖然沒有明顯的地層〝白化〞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1年的計畫中，我們也完成南海北部大陸邊緣約1,750 km2的多音束水深資料收集與分析，並已經完成後處理與數值化海底地形模型的繪製工作。此計畫的另一項重要工作是延續去年海域地電阻探測研究之後續發展，繼續與日本JAMSTEC機構或美國WHOI等機構洽談國際合作，透過雙邊人員互訪，以暸解日本JAMSTEC(直流電法) 或美國WHOI (交流電法) 等在海底地電量測及其應用在天然氣水合物調查的情形，並規劃底拖式海底地電阻探測來台探測之籌備工作。 ; In gas hydrate resources areas, fluid such as methane or sulfide may come to the sea floor through sediment layers by means of faults or other venting passages. The upward flow of fluid may form distinctive features 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 sediment fluid flow and gas hydrate formation. According to the results of first year, to the east of YAL, the area belongs to active accretionary prism. Here we found wide distribution of carbonate in terms of strong acoustic reflection which suggests large methane gas had interacted with sulfate. In this case, the acoustic penetration is very low and diffraction phenomenon is obvious in sub-bottom profiler data. In contrast, to the west of YAL, the sedimentary layers can be clearly identified. The acoustic blanking can be traced along a layer as thick as 30 m, which suggests that this area is not only a high-porosity sedimentary layer but also reserve potential gas-hydrates. In some places, the possible gas-hydrates (blanking areas) even outcrop to the seafloor. In the west of YAL, there are also pockmarks existing at the same place as the seismic blanking zones. In first year, we also collected about 1750 km2 multibeam bathymetry data and already made DTM topography. Another task of this project is, through two-side collaboration between JAMSTEC (Japan) and Taiwan, or WHOI (US) and Taiwan, to establish the deep-towed controlled source EM methods for the geophysical community of Taiwan and expect to introduce such kind of marine EM surveys in the offshore area of SW Taiwan. ; 研究期間 9801 ~ 9812
