HR: 1340h
AN: OS33B-1481    [Abstracts]
TI: Velocity structure in marine sediments with gas hydrate in offshore SW Taiwan from wide-angle seismic tomography
AU: * Cheng, W
EM: wbin@jwit.edu.tw
AF: Jin-Wen Institute of Technology, 99 An-Chung Road, Hsin-Tein City, Taipei, 231 Taiwan
AU: Lee, C
EM: leecs@mail.ntou.edu.tw
AF: National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Keelung, 202 Taiwan
AU: Schnurle, P
EM: schnurle@oc.ntu.edu.tw
AF: National Center for Ocean Research, No. 1 sec, 4 Roosevelt Road, Taipei, 102 Taiwan
AU: Liu, C
EM: csliu@ccms.ntu.edu.tw
AF: National Center for Ocean Research, No. 1 sec, 4 Roosevelt Road, Taipei, 102 Taiwan
AU: Tsai, C
EM: chtsai@oc.gep.ncu.edu.tw
AF: National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Keelung, 202 Taiwan
AU: Lin, S
EM: vivico15@yahoo.com.tw
AF: National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Keelung, 202 Taiwan
AB: In order to explore gas hydrate, a wide-angle seismic experiment was conducted in the offshore area, southwestern Taiwan jointly by National Taiwan University, National Taiwan Ocean University, and the Central Geological Survey in November 2004. Four transects were located at longitude and latitude of approximately 119.83›XE and 22.33›XN to systematically sample the shallow structure of the study area. During this experiment, the R/V Research I air gun array, totaling about 225 cubic inches, was used as the primary seismic source for wide-angle recording. The nearly 600 air gun signals generated by the R/V Research I were recorded by 6 ocean bottom seismometers (OBSs) deployed at 2 km intervals along all four of the onshore lines. Separate passes over the OBS-deployment lines were performed with a long air gun repetition rate (50 seconds) to minimize acoustic-wave interference from previous shots in the OBS data. Wide-angle reflections from BSRs were clearly observed at offsets up to ~4 km. A strong positive-polarity reflector marks the top of this velocity anomaly, and could be interpreted as the top of the gas hydrate layer. Similarly, a negative polarity reflector coincides with the base of the high-velocity layer, and is interpreted as the base of the hydrate zone. Joint travel time inversion was performed with wide-angle and vertical-incidence data in order to estimate the 2-D average velocity model of the study area. For the first layer beneath the seafloor, it shows a value of ~1.58 km/s for the sediments below transect B. The second layer shows about 1.70 km/s at this site. Geophysical evidence for gas hydrates consists of a shallow zone of pronounced high velocity (~2.3 km/s) as compared with the surrounding sediments. The low velocities beneath the BSRs (~1.35 km/s) probably reflect the presence of free gas. Decreasing amplitude with offset at the base of the gas hydrate layer may also indicate the accumulation of free gas beneath this interface.
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005