2:45 PM - 3:00 PM
[MIS26-03] Characteristics of acoustic blanking, pockmarks, and mounds based on AUV survey in a shallow methane hydrate bearing area, off Tango Peninsula, Japan
Keywords:shallow methane hydrate (MH), autonomous underwater vehicle (AUV), acoustic mapping, acoustic blanking, pockmark, mound
Methane hydrate (MH), a non-conventional fuel resource, is present worldwide, including offshore waters of Japan with two types of depositional style; MH-bearing subseafloor sandy sediment, and in the mud-dominant sediment less than 100 m beneath the seafloor. The latter one is called "shallow MH" and is known to present in areas along the eastern margin of the Sea of Japan and around the off Hokkaido. To understand the detailed distribution and volume of shallow MH, the Ministry of Economy, Trade, and Industry (METI) of Japan has conducted several projects. Further investigations are conducting in the three model areas (off Sakata, off Joetsu, and off Tango Peninsula). Investigations such as acoustic mapping, 3D seismic survey, controlled-source electromagnetic survey, logging-while-drilling (LWD) measurement, sediment coring, and optical observation using ROVs have been conducted in the area off Tango Peninsula. Here we present results from acoustic mapping using a research vessel Kaiyo Maru No. 7 (50 m-gridding bathymetry) and an autonomous underwater vehicle (AUV) Deep1 (3 m-gridding bathymetry, 1.2 m-gridding backscatter strength distribution, and 6–10 cm resolution of sub-bottom profiles).
There is a north-south extending spur which is a part of a marginal terrace extending into the Oki Trough. The spur has steep landslide scar on the western scarp connecting to the Oki Trough, and a relatively moderate landslide scar to the east connecting to the Wakasa Basin. A small ridge of mounds is present on a gentle top. Shallow MH deposits appeared below the mounds based on LWD measurements and was sampled by shallow drilling cores collected. Pockmarks are widely distributed from the top of the spur to the gentle eastern slopes. The diameters of the pockmarks vary from several tens to 500 m. The pockmarks make lines at the top and are scattered along the eastern slope. Several pockmarks are accompanied with mounds. Pockmarks are visible in the bathymetry; however, they are not clearly shown in the backscatter strength data, implying that they are extensively covered by soft sediment. Mounds, on the other hand, are shown by high-backscatter strength.
Well-stratified reflectors are recognized in the sub-bottom profiler (SBP) cross sections, and they are continuously observed just below the pockmarks. The reflectors are flexing downward below the pockmarks, with no significant faulting on the periphery. The flexing patterns can be divided into several units according to the common reflective layers. The lateral succession of the flexural strata beneath the pockmarks suggests that their formation was not caused by destructive events.
Acoustic blanking is prominent just below the mounds and is also observed below the seafloor with no bathymetric features, however, it is rarely seen under pockmarks. There are several types of boundary images between the acoustic blanking and surrounding reflectors; "fuzzy" or "sharp" boundary images. Diffracted waves sometimes generated from the sharp boundaries. In terms of various images between acoustic blanking zones and adjacent reflectors, the "fuzzy" or "sharp" ones, the former one tends to be appeared in northern area and the latter one in the southern area of our target area. The fuzzy one is interpreted as a boundary between sediment and areas where acoustic signals are easily attenuated. Conversely, the areas with sharp boundaries suggesting that acoustic signals are shielded by a material (possibly a shallow MH and/or carbonate crust) which is distributed in the shallow part of the acoustic blanking zone. Diffracted waves are observed particularly in the northeastern part of the target area. The various boundary conditions suggest that the shallow submarine environment may locally affect the presence of shallow MH-related materials.
This study was conducted as part of the methane hydrate research project funded by METI.
There is a north-south extending spur which is a part of a marginal terrace extending into the Oki Trough. The spur has steep landslide scar on the western scarp connecting to the Oki Trough, and a relatively moderate landslide scar to the east connecting to the Wakasa Basin. A small ridge of mounds is present on a gentle top. Shallow MH deposits appeared below the mounds based on LWD measurements and was sampled by shallow drilling cores collected. Pockmarks are widely distributed from the top of the spur to the gentle eastern slopes. The diameters of the pockmarks vary from several tens to 500 m. The pockmarks make lines at the top and are scattered along the eastern slope. Several pockmarks are accompanied with mounds. Pockmarks are visible in the bathymetry; however, they are not clearly shown in the backscatter strength data, implying that they are extensively covered by soft sediment. Mounds, on the other hand, are shown by high-backscatter strength.
Well-stratified reflectors are recognized in the sub-bottom profiler (SBP) cross sections, and they are continuously observed just below the pockmarks. The reflectors are flexing downward below the pockmarks, with no significant faulting on the periphery. The flexing patterns can be divided into several units according to the common reflective layers. The lateral succession of the flexural strata beneath the pockmarks suggests that their formation was not caused by destructive events.
Acoustic blanking is prominent just below the mounds and is also observed below the seafloor with no bathymetric features, however, it is rarely seen under pockmarks. There are several types of boundary images between the acoustic blanking and surrounding reflectors; "fuzzy" or "sharp" boundary images. Diffracted waves sometimes generated from the sharp boundaries. In terms of various images between acoustic blanking zones and adjacent reflectors, the "fuzzy" or "sharp" ones, the former one tends to be appeared in northern area and the latter one in the southern area of our target area. The fuzzy one is interpreted as a boundary between sediment and areas where acoustic signals are easily attenuated. Conversely, the areas with sharp boundaries suggesting that acoustic signals are shielded by a material (possibly a shallow MH and/or carbonate crust) which is distributed in the shallow part of the acoustic blanking zone. Diffracted waves are observed particularly in the northeastern part of the target area. The various boundary conditions suggest that the shallow submarine environment may locally affect the presence of shallow MH-related materials.
This study was conducted as part of the methane hydrate research project funded by METI.