9:30 AM - 9:45 AM
[HGM03-05] Estimating the ultra-shallow underground structure of the coastal lowland of Enshunada Sea by GPR survey - Toward elucidation of the development process of the beach levee topography
Keywords:GPR survey, Enshunada Sea, costal landform, beach levee, ultra-shallow underground structure
1. Introduction
In order to clarify the diversity of earthquakes in the Nankai Trough, we have typified the geomorphic development process of coastal landforms (beach levees and coastal plains) in the coastal lowlands of the Enshu Sea, and estimated the influence of seismic crustal movements.
Whereas, the geomorphic development of coastal landforms has not been fully clarified. Therefore, in order to reconstruct the paleogeography of this area, borehole surveys, shallow reflection surveys (Inasaki et al., 2021), and GPR surveys (Nakano et al., 2020) have been conducted in the Kikugawa lowland and the beach levees plain in the west of the Kikugawa lowland. These surveys have indicated the existence of buried valleys at several sites.
In this study, GPR (Ground Penetrating Radar) survey was conducted to clarify the two-dimensional distribution of the buried valleys and the ultra-shallow subsurface structure shallower than 10 m underground.
2. Study outline
Fig. 1 shows the survey area and GPR survey lines. The GPR survey was conducted from December 12 to 13, 2021. GPR survey was performed using Noggin Plus system of Sensors & Software Inc. with an antenna center frequency of 250 MHz.
The study area consists of three sites: Obuchi site (Site OB), located on the beach levee plain in the west of the Kikugawa lowland; Ohama Junior High School site (Site OH), located on the beach levee on the right bank of the Kikugawa River; and Naka site (Site NK), located on the side of a hill composed of basement rock siltstone between the Kikugawa and Shimo-ogasa rivers.
3. Result & Discussion
Representative results are reported for the Obuchi site and the Ohama junior high school site.
At the survey line OB-L1 of the Obuchi site, a buried valley was identified at a horizontal distance of 150 to 250 m by shallow reflection survey, and a concave reflection was also identified at a horizontal distance of 155 to 205 m by GPR survey. The GPR survey may have captured the structure of the sedimentary layers filling the buried valley. On the east side of this survey line, OB-L6, reflection structures indicating small-scale buried valleys were detected at two adjacent locations at distances of 225-270 m and 280-315 m. In the aerial photograph taken in 1962, Naka-arai Pond extended to the position of the survey line, and the buried valley structure at a horizontal distance of 225-270 m is considered to be a reclaimed pond.
In the north-south direction of the Ohama Junior High School site, a clear and generally flat reflection was detected near the top of the beach levee and south slope 2-3 m underground level (elevation 5-6 m and 4-5 m, respectively). In the borehole data near the survey line OH-L4, fine sand is present from the surface to a depth of about 5 m, with thin layers of silt at a depth of about 1.5 m and 2.1 m, respectively. Considering that the surface of this boring site is about 0.7 m lower than the nearest ground point of survey line OH-L4, it is possible that the clear reflection around 2 to 3 m underground captured by the GPR survey detects the silt-mixed sand layer interbed in the fine sand.
4. Summary
The GPR survey in the beach levees area was able to detect very shallow structures that are difficult to identify by shallow reflection survey, and small buried valleys and ponds were detected. The GPR survey at the beach levee also revealed the distribution of thin layers of different soil types interbed in the fine sand. In the future, we will attempt to reconstruct the paleogeography of the study area by combining the results of other surveys.
Acknowledgements: This work was supported by JSPS KAKENHI Grand Number JP18H00765. The GPR devices used for the survey was borrowed from Prof. Yasuhiro Suzuki of Nagoya University.
References: Inasaki et al. (2021): JpGU2021, HTT17-03; Nakano et al. (2020): JpGU-AGU Joint Meeting 2020, HGM03-P09.
In order to clarify the diversity of earthquakes in the Nankai Trough, we have typified the geomorphic development process of coastal landforms (beach levees and coastal plains) in the coastal lowlands of the Enshu Sea, and estimated the influence of seismic crustal movements.
Whereas, the geomorphic development of coastal landforms has not been fully clarified. Therefore, in order to reconstruct the paleogeography of this area, borehole surveys, shallow reflection surveys (Inasaki et al., 2021), and GPR surveys (Nakano et al., 2020) have been conducted in the Kikugawa lowland and the beach levees plain in the west of the Kikugawa lowland. These surveys have indicated the existence of buried valleys at several sites.
In this study, GPR (Ground Penetrating Radar) survey was conducted to clarify the two-dimensional distribution of the buried valleys and the ultra-shallow subsurface structure shallower than 10 m underground.
2. Study outline
Fig. 1 shows the survey area and GPR survey lines. The GPR survey was conducted from December 12 to 13, 2021. GPR survey was performed using Noggin Plus system of Sensors & Software Inc. with an antenna center frequency of 250 MHz.
The study area consists of three sites: Obuchi site (Site OB), located on the beach levee plain in the west of the Kikugawa lowland; Ohama Junior High School site (Site OH), located on the beach levee on the right bank of the Kikugawa River; and Naka site (Site NK), located on the side of a hill composed of basement rock siltstone between the Kikugawa and Shimo-ogasa rivers.
3. Result & Discussion
Representative results are reported for the Obuchi site and the Ohama junior high school site.
At the survey line OB-L1 of the Obuchi site, a buried valley was identified at a horizontal distance of 150 to 250 m by shallow reflection survey, and a concave reflection was also identified at a horizontal distance of 155 to 205 m by GPR survey. The GPR survey may have captured the structure of the sedimentary layers filling the buried valley. On the east side of this survey line, OB-L6, reflection structures indicating small-scale buried valleys were detected at two adjacent locations at distances of 225-270 m and 280-315 m. In the aerial photograph taken in 1962, Naka-arai Pond extended to the position of the survey line, and the buried valley structure at a horizontal distance of 225-270 m is considered to be a reclaimed pond.
In the north-south direction of the Ohama Junior High School site, a clear and generally flat reflection was detected near the top of the beach levee and south slope 2-3 m underground level (elevation 5-6 m and 4-5 m, respectively). In the borehole data near the survey line OH-L4, fine sand is present from the surface to a depth of about 5 m, with thin layers of silt at a depth of about 1.5 m and 2.1 m, respectively. Considering that the surface of this boring site is about 0.7 m lower than the nearest ground point of survey line OH-L4, it is possible that the clear reflection around 2 to 3 m underground captured by the GPR survey detects the silt-mixed sand layer interbed in the fine sand.
4. Summary
The GPR survey in the beach levees area was able to detect very shallow structures that are difficult to identify by shallow reflection survey, and small buried valleys and ponds were detected. The GPR survey at the beach levee also revealed the distribution of thin layers of different soil types interbed in the fine sand. In the future, we will attempt to reconstruct the paleogeography of the study area by combining the results of other surveys.
Acknowledgements: This work was supported by JSPS KAKENHI Grand Number JP18H00765. The GPR devices used for the survey was borrowed from Prof. Yasuhiro Suzuki of Nagoya University.
References: Inasaki et al. (2021): JpGU2021, HTT17-03; Nakano et al. (2020): JpGU-AGU Joint Meeting 2020, HGM03-P09.