碩士 / 臺灣大學 / 海洋研究所 / 98 / This study uses high-resolution subbottom profiles (Chirp Sonar) data to map the distribution of seafloor echo types off northeastern Taiwan, core sample analyses results are used to correlate the sub-bottom echo characters to seafloor sediments. Multichannel seismic reflection data have also been used to derive sediment velocity structures and to provide seismic images for understanding the velocity variations beneath sea floor, and to know the geological structures in the study area.
The echo characters observed on the East China Sea Shelf area are different from those observed in the East China Sea continental slope and Okinawa Trough area. Three types of echo character patterns (flat, mound and irregular gullied) and six sub-types are recognized on the East China Sea Shelf. Flat echo pattern is the dominant echo pattern in the shelf area, flat seafloor with regularly spaced wavy bedforms (echo type 1-4), mounded seafloor with smooth surface (echo type 2) and various-scale eroded seafloor (echo type 3-1 and 3-2) are observed in the offshore area. The developments of echo types in the shelf area are suggested to be influenced by tidal currents. In the area of East China Sea Slope and southern Okinawa Trough, four types of echo character patterns (distinct, indistinct, hyperbolic and irregular) and eight sub-types are identified. Distinct echoes with no or few subbottom echoes and nearly flat seafloor (echo type 1-a) are distributed in the upper East China Sea Slope, and distinct echoes with continuous and parallel internal reflectors (echo type 1-b) are distributed in the lower East China Sea Slope. Very prolonged bottom echoes occur in the Mien-Hua Canyon area, and the sediments are mostly clay there. The echo characters of stair-shaped seafloor, appear near the North Mien-Hua Canyon are suggested resulting from submarine landslides. The hyperbolic echo characters are presented at the upper East China Sea Slope and other steep slope areas. The seafloor in the southern Okinawa Trough is nearly flat, the sedimentary processes here are primarily influenced by turbidity current or pelagic sediments. The area between lower East China Sea Slope and west wall of southern Okinawa Trough presents echo characters of turbidites.
To establish sediment velocity models, we use three large-offset multichannel seismic reflection lines on the East China Sea Shelf to derive sediment velocity structures. From the established velocity profiles, the velocity of the sediment from seafloor to 200 meters in depth is around 1500 m/s~1900 m/s, the sediment velocity from 200 meters to 800 meters deep is around 1900 m/s~2700 m/s, and the sediment velocity from 800 meters to 2000 meters deep is around 2700 m/s~3500 m/s. In order to derive a general velocity function for the shelf sediments, we use Hamilton’s velocity relation to fit all the seismic velocity values of the profiles, and obtained a velocity curve: V(t)= 1510 + 1796.71× t – 720.114 × t2(t: one way travel time, V(t):interval velocity at t).
By correlating the sediment properties from core sample analyses with chirp sonar echo characters, we can understand acoustic properties of different sediments. Coarse-grained sediments can attenuate sound waves easily, and the sound penetration depth is shallow. On the contrary, the sound penetration depth of fine-grained sediments is deeper and the echo signal is clearer. Therefore, we are able to describe the distribution of sedimentary types in the study area from echo character pattern distribution map generated in the study.
| Identifer | oai:union.ndltd.org:TW/098NTU05274005 |
| Creators | Yi-Hsin Chen, 陳怡心 |
| Contributors | 劉家瑄 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 89 |
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