9:20 AM - 9:35 AM
[MIS18-02] High-resolution 3D seismic reflection survey in offshore area north of Tango Peninsula, Hyogo Prefecture, Japan
Keywords:shallow methane hydrate, high-resolution survey, 3D seismic reflection survey
Methane hydrate (MH), which is expected as promising unconventional next-generation natural gas resource, is known to exist in the seas around Japan. The National Institute of Advanced Industrial Science and Technology has been conducting comprehensive research and development of shallow MH, involving the identification of resource amounts, the study and development of recovery methods and the evaluation of environmental impacts. Currently, locations for offshore production tests have been explored.
In order to understand the precise subsurface structure from the seafloor to several hundred meters below the seafloor, the high-resolution three-dimensional seismic reflection survey was conducted in an area within ~8 km (north-south) and ~6 km (east-west) off the northern part of the Tango Peninsula, the selected area as one of the model survey areas for the offshore production test. The area is the northern edge of a topographic high extending northward from the marginal plateau and shares the western side with the part of the southern margin of the Oki Trough and the eastern side with the Wakasa Basin with multiple mounds at the top and many pockmarks. The presence of shallow MH field has been suggested by logging while drilling and confirmed by core samples.
The vessel used for data acquisition was the Kairi, with gross tonnage of 1951 tons and hull dimensions of 63 m by 14 m by 6.6 m. A GI gun with a chamber size of 150 cu. in. was triggered at 6.25 m intervals and the seismic signal was received by 96 hydrophone sensors in total attached at 6.25 m intervals to six 100 m long streamer cables towed in parallel. For velocity analysis, the two-dimensional seismic reflection survey was conducted at a line spacing of approximately 1 km and the estimated velocity was used to process the three-dimensional seismic survey data.
In data processing, common midpoint bin-size was set to 6.25m by 12.25m, and after sorting the data for each common reflection point, the bins with small data density were supplemented with values from the surrounding bin data. The quality of the stacked record was improved by suppressing various noises that contaminated the reflected waves, such as noise caused by machine and environment associated with vessel operation, wave noise, and cable propagation noise. In addition, errors in the traveltime of reflected waves caused by environmental changes (seawater sound wave velocity changes, tidal changes) and geometry errors (changes in streamer cable depth) between seismic survey lines were also suppressed. As a result, significant improvement in the spatial continuity of reflected wave events and then the result from pre-stack time migration has significantly improved. This study provides high-resolution three-dimensional structures of the area down to several hundred meters below the seafloor, illustrating the stratified sedimentary layers, localized high-amplitude reflections, small-scale faults and so on. It is considered that the results will be fundamental data for understanding the accumulation and reserves of shallow MH.
This study was conducted as part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan). Acquisition and analysis of seismic reflection data were conducted by JGI, Inc.
In order to understand the precise subsurface structure from the seafloor to several hundred meters below the seafloor, the high-resolution three-dimensional seismic reflection survey was conducted in an area within ~8 km (north-south) and ~6 km (east-west) off the northern part of the Tango Peninsula, the selected area as one of the model survey areas for the offshore production test. The area is the northern edge of a topographic high extending northward from the marginal plateau and shares the western side with the part of the southern margin of the Oki Trough and the eastern side with the Wakasa Basin with multiple mounds at the top and many pockmarks. The presence of shallow MH field has been suggested by logging while drilling and confirmed by core samples.
The vessel used for data acquisition was the Kairi, with gross tonnage of 1951 tons and hull dimensions of 63 m by 14 m by 6.6 m. A GI gun with a chamber size of 150 cu. in. was triggered at 6.25 m intervals and the seismic signal was received by 96 hydrophone sensors in total attached at 6.25 m intervals to six 100 m long streamer cables towed in parallel. For velocity analysis, the two-dimensional seismic reflection survey was conducted at a line spacing of approximately 1 km and the estimated velocity was used to process the three-dimensional seismic survey data.
In data processing, common midpoint bin-size was set to 6.25m by 12.25m, and after sorting the data for each common reflection point, the bins with small data density were supplemented with values from the surrounding bin data. The quality of the stacked record was improved by suppressing various noises that contaminated the reflected waves, such as noise caused by machine and environment associated with vessel operation, wave noise, and cable propagation noise. In addition, errors in the traveltime of reflected waves caused by environmental changes (seawater sound wave velocity changes, tidal changes) and geometry errors (changes in streamer cable depth) between seismic survey lines were also suppressed. As a result, significant improvement in the spatial continuity of reflected wave events and then the result from pre-stack time migration has significantly improved. This study provides high-resolution three-dimensional structures of the area down to several hundred meters below the seafloor, illustrating the stratified sedimentary layers, localized high-amplitude reflections, small-scale faults and so on. It is considered that the results will be fundamental data for understanding the accumulation and reserves of shallow MH.
This study was conducted as part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan). Acquisition and analysis of seismic reflection data were conducted by JGI, Inc.