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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/40689

Title: Preliminary result of deep-towed resistivity investigation in a potential gas hydrate area off southwest Taiwan
Authors: Chiang, C.
Hsu, S.
Goto, T.
Chen, C.
Chen, S.
Contributors: 國立臺灣海洋大學:應用地球科學研究所
Keywords: Instruments and techniques
Marine electromagnetics
Gas and hydrate systems
Date: 2012-12
Issue Date: 2017-02-06T05:29:50Z
Publisher: American Geophysical Union, Fall Meeting 2012
Abstract: Abstract: Gas hydrate (GH) is thought to be widely distributed within the active accretionary wedges passive continental margins and permafrost regions of the Earth. These deposits were formed in sub-seafloor sediments at relatively high pressure, low temperature conditions and have the potential to become a significant energy resource in the future. Bottom simulating reflectors (BSRs) from marine seismic reflection surveys are generally associated with the occurrence of gas-hydrate-bearing formations. The presence of BSRs could correspond to the base of the phase boundary between free gases and the solid hydrates. A wide-spread distribution of BSRs, high methane concentrations in the bottom water, shallow sulfate/methane interface, submarine mud volcanoes, diapirs and gas seepages significantly indicate a high potential of gas hydrates existence in the offshore area of southwest Taiwan. BSR is an indicative of a GH deposit and is often used to determine the base of gas hydrate stability zone of the solid-gas boundary where the upper bound of the GH in shallow sediments is still uncertain in the potential areas. The marine resistivity techniques are considered as one of powerful geophysical tools to image GH in upper bound of the GH in shallow portion of the seabed. In the study, we have rebuilt our new deep-towed marine resistivity system based on Goto et al. (2008) experiment. The new system is consisted of three components: 1. deep-towed frame, 2. receivers, and 3. side-scan sonar. The transmitter is mounted on the deep-towed frame and a long cable is towed behind the deep-towed frame in length of 240 m. The maximum of output electric current amplitude is up to 50 amperes. A dipole source of 15 m and two Ag-AgCl electric receivers are respectively installed in 210 m, 223 m and 226 m on the cable. Moreover, eight electric receivers (Ag-AgCl) are set from 0 m to 120 m with a dipole length of 15 m on the cable. A side-scan sonar equipment is attached below the system for the seafloor images. An important function of the side-scan sonar provides a stable towing of the whole system near the seafloor with altitude in range of 10 - 30 m. This configuration of the multidisciplinary geophysical system can roughly provide within 200 m of depth resolution beneath the seafloor according to the numerical modelling. A first multiple channels of deep-towed marine resistivity survey was carried out by R/V OR1-1001 from 21 to 23 June, 2012 to understand the detailed distribution of GH off SW Taiwan. Three survey lines were discovered in total length about 30 km. The towed speed was kept in 1.5 - 2.5 knots for both mapping of resistivity profiles and side-scan sonar images. An extremely high apparent resistivity anomaly appears in pockmark areas that corresponds to the side-scan sonar image. This high resistivity anomaly implies gas and/or gas hydrate reservoirs beneath the seafloor. The preliminary result has shown that our new resistivity system is a useful tool to explore GH deposits.
URI: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/40689
Appears in Collections:[應用地球科學研究所] 演講及研討會

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