1:45 PM - 2:00 PM
[STT40-01] 3D aeromagnetic imaging of the shallow subsurface structure of western flanks of Aso volcano, Kyushu Japan for evaluation of risks of slope failures
Keywords:Aso volcano, airborne magnetic survey, 3D imaging, magnetic structure, hydrothermal alteration, slope failures
To map hydrothermal alteration zones for evaluation of risks on slope failures, the Geological Survey of Japan (GSJ) has started a study on drone magnetic surveys. At first, we have conducted a re-processing of magnetic data observed by airborne geophysical surveys in Aso volcano, Kyushu (MLIT, 2014 a, b) and subsequent 3D magnetic imaging of the western flank of the volcano. The results of the study will be reported in this presentation.
Kyushu Technical Office, Kyushu Regional Development Bureau of the Ministry of Land, Infrastructure, Transportation and Tourism had conducted two airborne EM and magnetic surveys over Aso volcano, one in December 2013 and the other in March 2014 (MLIT, 2014 a, b). The survey covers the area except for over present and past craters such as Nakadake, Ojyodake, Kijimadake, Kusasenrigahama and their northern flanks. The surveys were flown by a manned helicopter AS 350B3 at an altitude of 60 m above ground along survey lines spaced 100 m apart, using the frequency-domain Fugro RESOLVE system (Exploration depth ~ 150 m below ground). Apparent resistivity data corresponding to each frequency were analysed and the resistivity structure was constructed for the survey areas.
Magnetic data were observed by Scintrex CS-3 Cesium magnetometer and processed without taking into accounts of variations of observation altitudes and a reduction to pole anomaly map was compiled in addition to a total magnetic intensity map. However, magnetic grid data with the information of observation altitudes are necessary to conduct quantitative analyses of subsurface structures. Therefore, observed magnetic data were re-processed by the method developed by the GSJ (Nakatsuka and Okuma, 2006; Nakatsuka and Okuma, 2018a, b). Magnetic anomalies were reduced on a smoothed observation surface, assuming equivalent anomalies below the observation surface. According to the total magnetic intensity anomaly map, magnetic highs occupy the southern and south-eastern flanks of Nakadake, Takadake and Maruyama in the eastern part and the southern flanks of Okamadoyama and Yominesan in the western part. On the contrary, low and calm magnetic anomalies are distributed dominantly in Yunotani, Yoshioka and Jigoku-Tarutama areas in the western flank of the volcano.
Next, 3D magnetic imaging (Nakatsuka and Okuma, 2014) was applied to the area of 5.5 km × 6 km in the western flank of the volcano using the complied aeromagnetic anomalies on the reduction surface. The magnetic model was assumed to be comprised of an ensemble of 23 sub-layers parallel to ground surface with a total thickness of 1,000 m. According to the resultant magnetization intensity map of the shallowest layer (ground surface ~ 20 m deep), magnetization highs are distributed over the area covered generally by volcanic products like Kusasenrigahama from volcanic centres in the western parts of Aso volcano (Geologic map of Aso Volcano (Ono and Watanabe, 1985)). Magnetization lows are obvious next to the magnetization highs in the hot springs and fumarolic areas of Yunotani, Yoshioka and Jigoku-Tarutama areas. By comparison with the hydrothermal alteration map (NEDO, 1995), these low magnetization areas correspond to the alteration zones with clay minerals such as smectites and kaolinites. As smectite is famous for its high swelling and known as a cause of degradation of shear strength of rocks, it is very important to estimate locations of subsurface hydrothermal alteration zones bearing those clay minerals in addition to surface ones. It is expected that more detailed structures of these alteration zones can be revealed by super high-resolution drone magnetic surveys with narrower line spacing (~25m) and lower altitude flights (~25m above ground) than manned surveys.
Kyushu Technical Office, Kyushu Regional Development Bureau of the Ministry of Land, Infrastructure, Transportation and Tourism had conducted two airborne EM and magnetic surveys over Aso volcano, one in December 2013 and the other in March 2014 (MLIT, 2014 a, b). The survey covers the area except for over present and past craters such as Nakadake, Ojyodake, Kijimadake, Kusasenrigahama and their northern flanks. The surveys were flown by a manned helicopter AS 350B3 at an altitude of 60 m above ground along survey lines spaced 100 m apart, using the frequency-domain Fugro RESOLVE system (Exploration depth ~ 150 m below ground). Apparent resistivity data corresponding to each frequency were analysed and the resistivity structure was constructed for the survey areas.
Magnetic data were observed by Scintrex CS-3 Cesium magnetometer and processed without taking into accounts of variations of observation altitudes and a reduction to pole anomaly map was compiled in addition to a total magnetic intensity map. However, magnetic grid data with the information of observation altitudes are necessary to conduct quantitative analyses of subsurface structures. Therefore, observed magnetic data were re-processed by the method developed by the GSJ (Nakatsuka and Okuma, 2006; Nakatsuka and Okuma, 2018a, b). Magnetic anomalies were reduced on a smoothed observation surface, assuming equivalent anomalies below the observation surface. According to the total magnetic intensity anomaly map, magnetic highs occupy the southern and south-eastern flanks of Nakadake, Takadake and Maruyama in the eastern part and the southern flanks of Okamadoyama and Yominesan in the western part. On the contrary, low and calm magnetic anomalies are distributed dominantly in Yunotani, Yoshioka and Jigoku-Tarutama areas in the western flank of the volcano.
Next, 3D magnetic imaging (Nakatsuka and Okuma, 2014) was applied to the area of 5.5 km × 6 km in the western flank of the volcano using the complied aeromagnetic anomalies on the reduction surface. The magnetic model was assumed to be comprised of an ensemble of 23 sub-layers parallel to ground surface with a total thickness of 1,000 m. According to the resultant magnetization intensity map of the shallowest layer (ground surface ~ 20 m deep), magnetization highs are distributed over the area covered generally by volcanic products like Kusasenrigahama from volcanic centres in the western parts of Aso volcano (Geologic map of Aso Volcano (Ono and Watanabe, 1985)). Magnetization lows are obvious next to the magnetization highs in the hot springs and fumarolic areas of Yunotani, Yoshioka and Jigoku-Tarutama areas. By comparison with the hydrothermal alteration map (NEDO, 1995), these low magnetization areas correspond to the alteration zones with clay minerals such as smectites and kaolinites. As smectite is famous for its high swelling and known as a cause of degradation of shear strength of rocks, it is very important to estimate locations of subsurface hydrothermal alteration zones bearing those clay minerals in addition to surface ones. It is expected that more detailed structures of these alteration zones can be revealed by super high-resolution drone magnetic surveys with narrower line spacing (~25m) and lower altitude flights (~25m above ground) than manned surveys.