3:30 PM - 5:00 PM
[HDS10-P05] Investigation for improvement of reliability of methods for evaluating the risk of deep-seated landslides in the Kii mountainous area using airborne electromagnetic survey
Keywords:Deep-seated landslide, Helicopter Electromagnetics, Drone-GREATEM, Electrical prospecting, Fault, Groundwater
We identified slopes at very high risk of deep-seated landslides in the large Kii mountainous area through geophysical techniques including Helicopter Electromagnetics (HEM), "Drone-Grounded Electrical source Airborne Transient Electromagnetics" (D-GREATEM), and electrical prospecting. After Typhoon Talas in 2011, the Ministry of Land, Infrastructure, Transport and Tourism regularly used LiDAR(Light Detection and Ranging) to acquire detailed topographic data. HEM was performed from 2012 to 2014 over a 280 km2 area via laser profiling.(1) Based on the topographic maps and aerial photographs taken by the LiDAR, slopes exhibiting deformations (such as linear depressions and double ridges near the upper ridge lines) were extracted.(2) The slope survey lines were set near the centers of deformed slopes, and vertical cross-sections were obtained during the HEM.(3) Based on the results of (1) and (2), deformed slopes were assigned a risk score of 1to 3; a score of 1 was associated with a scarp ratio over 5% (obtained by dividing the length of the sliding cliff by the slope length). Risk level 2 is the same as level 1, except for the additional requirement that the zone of resistivity (slip surface) runs in the direction of the slope. Risk level 3 is the same as level 2, except for the additional requirement that the resistivity zone runs vertically relative to the direction of the slope (thus forming a fault fracture zone that crosses the slope). Fourteen slopes had level 3 scores, which were associated with mass rock creep. All such slopes that could be accessed were surveyed on the ground, and were all in the Tochio district of Nara Prefecture. The size of the target slope is about 14.7 ha and the strain rate is about 5.5%. The resistivity profiles obtained by helicopter-borne electromagnetic survey of the slope in the Tochio area showed gravity deformation and the existence of a slope directional isostatic concentration zone that appears to be a slip surface, and two vertical isostatic concentration zones that appear to be faults that cross the slope. For the purpose of confirming the impact of faults on groundwater, we conducted drone aerial electromagnetic surveys in the Tochio area in three periods: October 2021, December 2021, and September 2022. Resistivity change rates were calculated for each combination of two of the three periods, and their applicability to the investigation of the internal structure of the slope was verified. The resistivity concentration zone in the upper part of the slope showed low resistivity during the outflow period, suggesting that groundwater flowed into or accumulated in this area. The iso-resistivity concentration zone in the lower part of the slope is not as low resistivity as the upper part, and is considered to be an area where groundwater does not tend to inflow or accumulate continuously as much as in the upper part. In addition, the resistivity change rates for the September 2022 and December 2021 periods, which have high 28-day prior cumulative rainfall, can be compared to the resistivity change rates for the October 2021 and December 2021 periods, and the areas of lower resistivity can be seen to the near surface, which is considered to represent the effect of recent rainfall on the groundwater distribution. As described above, geophysical surveys can visualize the influence of faults on groundwater behavior and are highly applicable to structural investigations of the slope interior. In addition, by utilizing data from two different periods of preceding accumulated rainfall in the runoff and non-runoff periods, it is possible to estimate the distribution of groundwater in the interior of the slope after rainfall.