Sediment Velocity Structures in the Taiwan Strait and the Coastal Plain of west Taiwan

• Main Authors: Mei-Ling Chen, 陳美伶
• Other Authors: Andrew Tien-Shun Lin
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/05196469270688784417

碩士 === 國立中央大學 === 地球物理研究所 === 95 === The method of reflection seismic imaging displays subsurface structures in time rather than in depth. One therefore needs to know the 3-D distribution of subsurface velocity in order to convert seismic time into depth and obtain 3-D subsurface depth structures. I use borehole and seismic data to compute a 3-D sediment velocity structure in the Taiwan Strait and the coastal plain in west Taiwan by using V0-k method. The V0-k method assumes that velocity increases with increasing depth in a linear form, in which V0 is the initial velocity at the seabed or on the ground surface, and k represents the rate of increase of the velocity with increasing depth. Fitting velocity with depth to a linear form for each data set obtains V0 (intercept) and k (slope) pairs, and gridding these values obtains the spatial distribution of V0 and k. The lateral variation of V0 reflects sediment characteristics with lower values in the basin and higher values in the western foothills. The variation of k correlates to the lithology difference and lateral structural variation. The value of k is exceptionally high in the areas of hardly compactible basement highs. Parameters of V0 and k were gridded in a 3-D depth volume. Comparing the velocity depth structures to seismic images, one finds that (1) the vertical velocity variation is mainly controlled by burial compaction of sediments; (2) the lateral velocity variation is caused by stratal offset resulted from major normal or reverse faulting; and (3) the isovelocity curves shown on cross-sections may reflect the geometry of the basement structure. Finally, I use the drilled depths of the Oligocene breakup unconformity (generally < 4 km in depth) and its corresponding depths as converted from seismic data and using the proposed velocity model to test the validity of the 3-D velocity model. The comparison yields a value of 4.78% of mean percentage error between drilled and predicted depths, indicating that the proposed velocity model predicts subsurface velocity reasonably well down to, at least, 4 km in depth.