4:30 PM - 4:45 PM
[STT37-05] Identification of slopes at very high risk of deep-seated landslides in the Kii mountainous area through a Helicopter electromagnetics and Drone electromagnetics
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 a laser profiler to acquire detailed topographic data. HEMs were 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 laser profilers, 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 HEMs.
(3) Based on the results of (1) and (2), deformed slopes were assigned a risk score of 1–3; a score of 1 was associated with a scarp ratio >5% (obtained by dividing the length of the sliding cliff by the slope length). Chigira et al. (2013) found that most deep-seated landslides caused by Typhoon Talas had scarp ratios of 5–21%. 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. As the clay content increases near the slip surface, resistivity tends to decrease, leading to a concentrated band of resistivity lines. 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). Thirteen 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 total slope area is about 14.7 Ha and the scarp ratio is 5.5%. Eleven resistivity zones run in the direction of the slope (slip surface); the remaining two zones run vertically (faults crossing the slope).
We compared the electrical prospecting results of the main survey line in the Tochio area, obtained in December 2021 (3 months prior to a cumulative rainfall amount of 272mm), to the longitudinal resistivity diagrams obtained via the HEMs. The high resistivity bands ran vertically; thus, these areas have numerous voids. To investigate the effects of faults on groundwater, DATEMs were conducted on the main Tochio survey line in October 2021 (3 months prior to a cumulative rainfall amount of 610mm) and December 2021 (3 months prior cumulative rainfall 296mm). The change in resistivity was obtained by dividing the October resistivity value by that for the December. In October, resistivity was lower in the “constant resistivity zone”, into which groundwater had flowed and accumulated.
(1) Based on the topographic maps and aerial photographs taken by the laser profilers, 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 HEMs.
(3) Based on the results of (1) and (2), deformed slopes were assigned a risk score of 1–3; a score of 1 was associated with a scarp ratio >5% (obtained by dividing the length of the sliding cliff by the slope length). Chigira et al. (2013) found that most deep-seated landslides caused by Typhoon Talas had scarp ratios of 5–21%. 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. As the clay content increases near the slip surface, resistivity tends to decrease, leading to a concentrated band of resistivity lines. 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). Thirteen 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 total slope area is about 14.7 Ha and the scarp ratio is 5.5%. Eleven resistivity zones run in the direction of the slope (slip surface); the remaining two zones run vertically (faults crossing the slope).
We compared the electrical prospecting results of the main survey line in the Tochio area, obtained in December 2021 (3 months prior to a cumulative rainfall amount of 272mm), to the longitudinal resistivity diagrams obtained via the HEMs. The high resistivity bands ran vertically; thus, these areas have numerous voids. To investigate the effects of faults on groundwater, DATEMs were conducted on the main Tochio survey line in October 2021 (3 months prior to a cumulative rainfall amount of 610mm) and December 2021 (3 months prior cumulative rainfall 296mm). The change in resistivity was obtained by dividing the October resistivity value by that for the December. In October, resistivity was lower in the “constant resistivity zone”, into which groundwater had flowed and accumulated.