09:45 〜 10:00
[STT39-03] Drone magnetic survey for the landslide risk assessment- A case of Hatakura area, Yufu City, Oita Pref., Kyushu, Japan
キーワード：ドローン磁気探査、地すべり、リスク評価、熱水変質
An aeromagnetic survey using a UAV (drone) was conducted by the Geological Survey of Japan (GSJ) over the Hatakura area, Yufu City, Oita Pref. in April 2024. A year before, a landslide occurred in the center of the survey area on June 30, 2023 by torrential rain resulting in the loss of human life as well as the damage of infrastructures.
Before the drone survey, regional magnetic anomalies including reduction to the pole anomalies and apparent magnetization intensities were compiled for the area of the geologic map of Beppu on a scale of 1:50,000 (Hoshizumi et al., 1988) using the Aeromagnetic Database of Japan (GSJ, 2005) and compared with the geology in detail (Okuma et al, 2024). The area is covered generally by the Early-Pleistocene Kawanishi Andesite (Hoshizumi et al., 1988) and lies in the regional north-trending hydrothermal alteration belt caused by geothermal and hot spring activities (Sakaguchi and Hoshizumi, 2023). Magnetic lows coincide with the hydrothermal alteration belt, suggesting the demagnetization of the Kawanishi Andesite by hydrothermal activity as well as the reverse magnetic polarity of the volcanic rocks. A preliminary ground magnetic survey was also conducted in the study area to better understand general characteristics of magnetic anomalies. In addition, rock samples were collected from outcrops in and around the collapsed area and were shaped into cylindrical specimens, each measuring 25 mm in length and 25 mm in diameter. Rock magnetic properties including magnetic susceptibility and natural remanent magnetization (NRM) were measured on these specimens.
Based on the preliminary studies above, an ultra-high resolution aeromagnetic survey using a multicopter (DJI Matrice 600 Pro) was conducted in April 2024 in the Hatakura area to clarify the detail relationship between the magnetic structure and hydrothermal alterations (Miyakawa et al., 2024; Okuma et al., 2024). While flying, an airborne magnetometer (Geometrics MagArrow) was suspended 3 m below the multicopter to keep away from the magnetic noise caused by the drone. The survey was successfully flown with narrow flightline spacing of 25 m and low flying altitude of 25 m above ground covering a 500 × 500 m area. As the corruption site was under emergency countermeasure during the magnetic survey, a LiDAR measurement with aerial photography was conducted at the same time to create the detailed DEM as well as ortho-image data.
Data processing of the observed magnetic data was conducted using the data processing software developed by the Geological Survey of Japan, AIST (Nakatsuka and Okuma, 2014a, b). Magnetic anomalies were, then reduced to the smoothed observation surface, assuming equivalent anomalies below the observation surface. Magnetic lows are dominant over the landslide area and its surrounding areas, suggesting the hydrothermal alteration of volcanic rocks which constitute the survey area. However, a local magnetic high was observed near the source area of the corruption site, corresponding to the newly constructed concrete wall with reinforcement. This artificial magnetic anomaly can be mitigated by synthetic magnetic modeling of the concrete wall using the detailed DEM and aerial photos.
Acknowledgements:
We would like to express our sincere gratitude to the Erosional Control Division of Civil Engineering and Construction Division, and the Oita Civil Engineering Office both at Oita Pref., the Disaster and Prevention and Crisis Management Division, Yufu City for their help to conduct the drone magnetic survey. We extend our appreciation to local residents in Hatakura area, Yufu City, Oita Pref. for their permission when flying drones.
Before the drone survey, regional magnetic anomalies including reduction to the pole anomalies and apparent magnetization intensities were compiled for the area of the geologic map of Beppu on a scale of 1:50,000 (Hoshizumi et al., 1988) using the Aeromagnetic Database of Japan (GSJ, 2005) and compared with the geology in detail (Okuma et al, 2024). The area is covered generally by the Early-Pleistocene Kawanishi Andesite (Hoshizumi et al., 1988) and lies in the regional north-trending hydrothermal alteration belt caused by geothermal and hot spring activities (Sakaguchi and Hoshizumi, 2023). Magnetic lows coincide with the hydrothermal alteration belt, suggesting the demagnetization of the Kawanishi Andesite by hydrothermal activity as well as the reverse magnetic polarity of the volcanic rocks. A preliminary ground magnetic survey was also conducted in the study area to better understand general characteristics of magnetic anomalies. In addition, rock samples were collected from outcrops in and around the collapsed area and were shaped into cylindrical specimens, each measuring 25 mm in length and 25 mm in diameter. Rock magnetic properties including magnetic susceptibility and natural remanent magnetization (NRM) were measured on these specimens.
Based on the preliminary studies above, an ultra-high resolution aeromagnetic survey using a multicopter (DJI Matrice 600 Pro) was conducted in April 2024 in the Hatakura area to clarify the detail relationship between the magnetic structure and hydrothermal alterations (Miyakawa et al., 2024; Okuma et al., 2024). While flying, an airborne magnetometer (Geometrics MagArrow) was suspended 3 m below the multicopter to keep away from the magnetic noise caused by the drone. The survey was successfully flown with narrow flightline spacing of 25 m and low flying altitude of 25 m above ground covering a 500 × 500 m area. As the corruption site was under emergency countermeasure during the magnetic survey, a LiDAR measurement with aerial photography was conducted at the same time to create the detailed DEM as well as ortho-image data.
Data processing of the observed magnetic data was conducted using the data processing software developed by the Geological Survey of Japan, AIST (Nakatsuka and Okuma, 2014a, b). Magnetic anomalies were, then reduced to the smoothed observation surface, assuming equivalent anomalies below the observation surface. Magnetic lows are dominant over the landslide area and its surrounding areas, suggesting the hydrothermal alteration of volcanic rocks which constitute the survey area. However, a local magnetic high was observed near the source area of the corruption site, corresponding to the newly constructed concrete wall with reinforcement. This artificial magnetic anomaly can be mitigated by synthetic magnetic modeling of the concrete wall using the detailed DEM and aerial photos.
Acknowledgements:
We would like to express our sincere gratitude to the Erosional Control Division of Civil Engineering and Construction Division, and the Oita Civil Engineering Office both at Oita Pref., the Disaster and Prevention and Crisis Management Division, Yufu City for their help to conduct the drone magnetic survey. We extend our appreciation to local residents in Hatakura area, Yufu City, Oita Pref. for their permission when flying drones.