Development of a method to measure the amount of gushing gas using MBES-comparison of volume back scattering strength observed by 2 tools-
Abstract
Two types of naturally seeping methane hydrate (MH) are confirmed to exist: sand-layer type and shallow type. Shallow type methane hydrate is discovered with high purity in clumps. Quite recently, research and development of shallow type methane hydrate has started and is drawing attention as a potential to be a new energy resource. One of the elements that characterizes shallow-type MH is the gushing methane gas from seafloor called methane seeps. It is important to obtain the amount of methane seeps, not only for the purpose of energy resources, but also its effects such as greenhouse gas in the atmosphere and ocean deoxygenation. One of the instruments for searching methane seeps is marine acoustic equipment. Aoyama et al. (2009,2020) revealed the effectiveness of searching methane seep by using quantitative echo sounder (UQES) and measuring volume backscattering (SV). Although UQES specializes in measuring and observed SV is standardized, its observation area is narrow and it is time consuming to survey wide areas. Therefore, it is not realistic to survey all ocean area. Multibeam echo sounder (MBES) is one of the wide range oceanic observing equipment. However, SV observed by MBES is not standardized among other type models, and seep analysis method has not been established. The purpose of this study is to develop the method for measuring amount of gushing gas by using MBES. In order to achieve this, we consider conversion formula of MBES SV value to UQES SV value.
Data used in this study were obtained in Umitaka Spur, in offshore Joetsu Sea of Japan where it is known for active methane seeps. Survey voyage are ① annual MH survey conducted by Japan's Independent Institute Inc. using MBES EM122(KONGSBERG) and UQES EA600(KONGSBERG) and ② annual voyage of Shinyo Maru (Tokyo University of Marine Science and Technology) using UQES EK80. By using the obtained data from Echoview, MBES data, UQES data, location of each methane seep, and its SV mean were calculated by grid of every 50m depth. SV mean of every depth of each plume were plotted on the scatter plot. Each plume was determined by its location. Using least-squares method, we calculated linear coefficient of each area. Lastly, the transition of coefficients were calculated according to its depth and SV, and the conversion formula and its validity were considered.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMOS0430006O
- Keywords:
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- 3002 Continental shelf and slope processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3050 Ocean observatories and experiments;
- MARINE GEOLOGY AND GEOPHYSICS