4:30 PM - 4:45 PM
[STT34-05] Study on the behavior of the groundwater in the slope in the heavy rain using Drone Airborne TEM
Keywords:Drone Airborne TEM, groundwater, deep-seated landslide
In the survey by Drone Airborne Transient Electromagnetics (TEM), a cable of about 100 to 1000 m is laid on the ground and the cable is energized. By causing a rapid time change of the magnetic field (primary magnetic field) generated there, an induced current proportional to the resistivity of the underground is generated on the ground. The resistivity value in the underground is obtained by measuring and analyzing the time change (transient response) of the new magnetic field (secondary magnetic field) generated by this. With the development of this technology, we could verify the landslide mechanisms after the sediment-related disasters and the design method of the countermeasures would change significantly in the future. The reasons for this are that it is cheaper than the electrical prospecting and the Airborne Electromagnetic Survey by helicopter that have been often used so far, and that it can be measured immediately after the heavy rain. In this study, we conducted a survey in the Iya district in Tanabe City, Wakayama Prefecture, where the deep-seated landslide was occurred due to Typhoon Talas in 2011. Drone Airborne TEM was conducted to clarify the behavior of groundwater in the slope during the heavy rain.
A survey line of about 700 m length was set at the upper right of the landslide slope. And Drone Airborne TEM was conducted in the flood season and in the dry season. From the results of the field survey, multiple fault crush zones were distributed around the area where the survey line was set. The measurement day in the flood season was three days after the end of rainfall of 217 mm in total rainfall of Typhoon CHAN-HOM in 2020 (October 13, 2020). The measurement day in the dry season was set on December 2, 2020, when there was no rainfall for 11 days. By taking the difference between the measurement results of the resistivity values, the behavior of the groundwater during Typhoon CHAN-HOM in 2020 was estimated.
By Drone Airborne TEM, we could estimate the behavior of groundwater in the slope during the heavy rain, which was previously unknown without observing the water level under the ground using the boring hole. As a result, it was found that the four fault crush zones distributed in the Iya area induce groundwater and block the flow of the groundwater in the slope. We think that the deep-seated landslide in the 2011 disaster was caused due to the effects of these fault crush zones. In this way, we would evaluate the roles of the fault crush zones during the heavy rains can be evaluated by using Drone Airborne TEM.
A survey line of about 700 m length was set at the upper right of the landslide slope. And Drone Airborne TEM was conducted in the flood season and in the dry season. From the results of the field survey, multiple fault crush zones were distributed around the area where the survey line was set. The measurement day in the flood season was three days after the end of rainfall of 217 mm in total rainfall of Typhoon CHAN-HOM in 2020 (October 13, 2020). The measurement day in the dry season was set on December 2, 2020, when there was no rainfall for 11 days. By taking the difference between the measurement results of the resistivity values, the behavior of the groundwater during Typhoon CHAN-HOM in 2020 was estimated.
By Drone Airborne TEM, we could estimate the behavior of groundwater in the slope during the heavy rain, which was previously unknown without observing the water level under the ground using the boring hole. As a result, it was found that the four fault crush zones distributed in the Iya area induce groundwater and block the flow of the groundwater in the slope. We think that the deep-seated landslide in the 2011 disaster was caused due to the effects of these fault crush zones. In this way, we would evaluate the roles of the fault crush zones during the heavy rains can be evaluated by using Drone Airborne TEM.