台灣西南海域位於被動大陸邊緣及台灣造山帶交界，為馬尼拉隱沒系統之增積楔形體延伸。自1990年起，多種探測結果顯示地層中含有大量天然氣水合物，震測剖面也具有明顯的海底仿擬反射訊號。因此，2004年10月海研一號及海研三號選定20X20公里範圍，施行密集反射震測實驗，並同時運用海底地震儀接收寬角折射訊號。海底地震儀擺放為四條相互交錯測線，每條測線使用六顆，各相距約兩公里，共計24顆站次。本研究利用四條測線所接收到的多條炸測測線訊號，利用震測剖面及陸源沉積物深度速度曲線建立一維初始速度模型，並挑選初達波及海底仿擬反射訊號走時，重新定位海底地震儀位置。並利用PStomo_eq程式，建立三維P波速度模型，探討速度變化與分佈情形。在研究區域深度約200至400公尺之間可以觀察到明顯高速轉為低速的變化，推測是含有天然氣水合物及游離氣造成。本研究推估天然氣水合物含量的方式是基於Weighting equation，由各物質所占比例與孔隙率計算出理論速度與水合物在孔隙中飽和度之對應關係，便能由速度構造轉為飽和度。所推估出孔隙中天然氣水合物飽和度約為10％?30％，大多集中於永安海脊與構造背斜以下。 Interest in natural gas hydrates has grown in recent years with the recognition of the gas-hydrate bearing sediments offshore the southwestern Taiwan. Several oceanographic cruises were conducted in the aim of acquiring more information about the presence of gas hydrate. The collected reflection and refraction seismic data are extensively used to map and interpret the gas hydrate zones for the area of the Hengchun Ridge deformation front, accretionary wedge of the eastern subducting Maniala subduction zone, and the passive China continental Margin. Among them, a wide angle seismic experiment consisted of 24 recovered Ocean Bottom Seismometers (OBSs) were acquired along 4 in-line transects offshore southwestern Taiwan in November 2004 and several off-lines and cross-lines data were recorded simultaneously. In this study, we use PStomo_eq software to perform a 3-D acoustic inversion velocity analysis of the first arrivals records of the OBS by using all the in-lines, off-lines and cross-lines information. By analysing the inverted 3-D dimensional Vp models, we estimate the density and the thickness of the marine sediments and crust. Based on the core data, the porosity, the lithology and other physical properties were assumed and the Weighting equation is applied for the estimation of the gas hydrate concentration. In our study , the gas hydrate saturation is about 10%~30%, most of them are concentrated below the Yuan-An ridge and the anticlines.
