9:30 AM - 9:45 AM
[HQR04-03] Estimation of sedimentary condition in Kinu River and Kokai River lowland using micro tremor survey
Keywords:sedimentary environment, Kinu River and Kokai River lowland, micro tremor, boring data, Misaka-Shinden and Oki-Shinden
The Misaka-Shinden and Oki-Shinden districts are located in the back marsh environment in the central part of the Kinu River and Kokai River lowland in the southern part of Ibaraki prefecture. These districts are located in an environment extremely vulnerable to flood disasters, and are flooded even during the 2015 Kanto and Tohoku Heavy Rain.
The authors attempted to estimate the sedimentary environment in these areas as basic information for examining the historical reasons for the settlements located in such places. Micro tremor survey is performed around the Ken-oudo High Way where there is boring data, and the boring data are compared with the S wave velocity structure obtained by micro tremor survey. From the results, the underground ground structures are estimated using the micro tremor survey at the point where there is no boring data. We also performed boring excavation with a hand auger and particle size analysis of the collected samples.
The authors set an approximate S-wave velocity range with respect to the ground from the comparison between the boring data and the micro tremor survry results. S wave velocity of less than 100 m / s is mud, around 200 m / s is sand, and over 300 m / s is gravel. Using these data, the ground conditions of the Misaka-Shinden and Oki-Shinden Districts and the surrounding areas, where there is no boring data, were estimated, and the estimated columnar sections of these areas were created.
As a result, it was estimated that there was a thick sand layer at a depth of about 5 to 15 m just below Misaka-Shinden, and that the river channel of that era was flowing around it. According to the results of boring by hand auger, it is presumed that the southern part of Oki-Shinden was a back marsh environment where flood sediments were difficult to reach, as the surface layer was almost a soft mud layer up to about 5 m.
The authors attempted to estimate the sedimentary environment in these areas as basic information for examining the historical reasons for the settlements located in such places. Micro tremor survey is performed around the Ken-oudo High Way where there is boring data, and the boring data are compared with the S wave velocity structure obtained by micro tremor survey. From the results, the underground ground structures are estimated using the micro tremor survey at the point where there is no boring data. We also performed boring excavation with a hand auger and particle size analysis of the collected samples.
The authors set an approximate S-wave velocity range with respect to the ground from the comparison between the boring data and the micro tremor survry results. S wave velocity of less than 100 m / s is mud, around 200 m / s is sand, and over 300 m / s is gravel. Using these data, the ground conditions of the Misaka-Shinden and Oki-Shinden Districts and the surrounding areas, where there is no boring data, were estimated, and the estimated columnar sections of these areas were created.
As a result, it was estimated that there was a thick sand layer at a depth of about 5 to 15 m just below Misaka-Shinden, and that the river channel of that era was flowing around it. According to the results of boring by hand auger, it is presumed that the southern part of Oki-Shinden was a back marsh environment where flood sediments were difficult to reach, as the surface layer was almost a soft mud layer up to about 5 m.