15:45 〜 16:00
[STT34-02] Comparison of airborne magnetic surveys by different types of aircrafts - A case of Izu-Oshima Volcano
キーワード：磁気探査、ドローン、自律型無人ヘリコプター、飛行機、火山、伊豆大島
The Geological Survey of Japan (GSJ) has been developing drone magnetic survey systems for geophysical survey in volcanic areas these years. In 2019, we introduced a newly developed UAV-enabled magnetometer "MagArrow" and conducted practical test flights of the magnetometer in Izu-Oshima Volcano, south of Tokyo (Okuma et al., 2020). On Izu-Oshima Island, many airborne magnetic surveys have been conducted by using various types of aircrafts so far. In this study, we compare the results of magnetic surveys using an unmanned autonomous helicopter and a drone (multicopter).
The Earthquake Research Institute (ERI), Univ. of Tokyo conducted airborne magnetic surveys using an unmanned autonomous helicopter in March and November 2008 inside the summit caldera of the volcano. The former and latter surveys cover the northern and southern parts of the summit caldera including Mt. Mihara, respectively. The survey was flown using a portable Cesium magnetometer at an altitude of around 50 m above terrain along survey lines spaced around 50 m apart. They conducted apparent magnetization mapping from the observed anomaly and indicated magnetization highs in the northwestern flank of Mt. Mihara (Kaneko et al., 2011) and in the southeastern flank of Mt. Mihara (ERI and Chiba Univ., 2009) both trending NW aligned to a major tectonic direction on the island. These results of magnetization mapping suggest subsurface intrusions overlain by surface volcanic products, which are very interesting interpretation about the subsurface structure. However, the evaluation of magnetic anomalies itself might be insufficient since they did not produce a detailed magnetic anomaly map. Hence, a compilation of such a magnetic anomaly map has been newly conducted. Magnetic anomalies (total magnetic intensity) were reduced onto a smoothed observation surface assuming equivalent anomaly below the observation surface and reduction to the pole anomalies are also calculated on the same surface. The resultant reduction to the pole anomaly map clearly shows magnetic highs on the northern and southern flanks of Mt. Mihara. Magnetic lows are dominant over the eruptive crater and the northeastern foot of Mt. Mihara. A local magnetic high seems to lie over one of B craters of the 1986 eruption. 3D magnetic imaging was then applied to terrain-corrected magnetic anomalies and the result was compared with preceding studies. These newly compiled aeromagnetic maps and interpreted subsurface structures have finer space resolution than those of airborne magnetic surveys using airplanes at a higher altitude (e.g. Makino et al., 1988).
According to the compilation of detailed magnetic anomaly maps using ERI's data, GSJ specified a survey area of drone magnetic survey using MagArrow on the island. In 2019, a drone magnetic survey was flown at an altitude of 25 m above terrain along E-W survey lines spaced 25 m apart in the northern part of the summit crater (Okuma et al., 2020). Unfortunately, many typhoons attacked Izu-Oshima Island in 2019, causing difficulties in surveying on calm days with a wind speed less than 2 m/s. Because of the harsh weather, the flights covered only a half of the provisional survey area. Magnetic anomalies (total magnetic intensity) were reduced onto a smoothed observation surface assuming equivalent anomaly below the observation surface and reduction to the pole anomalies are also calculated on the same surface. Many magnetic highs and lows are distributed on the maps. Comparing the result of the drone magnetic survey with that of the unmanned autonomous helicopter survey, it is obvious that the former is superior to the latter in spatial resolution. Many short-wavelength anomalies can be attributed to topography consisted of volcanic products on magnetic maps of the drone magnetic survey. Even the magnetization heterogeneity of the 1986 lava can be estimated on the same maps.
The Earthquake Research Institute (ERI), Univ. of Tokyo conducted airborne magnetic surveys using an unmanned autonomous helicopter in March and November 2008 inside the summit caldera of the volcano. The former and latter surveys cover the northern and southern parts of the summit caldera including Mt. Mihara, respectively. The survey was flown using a portable Cesium magnetometer at an altitude of around 50 m above terrain along survey lines spaced around 50 m apart. They conducted apparent magnetization mapping from the observed anomaly and indicated magnetization highs in the northwestern flank of Mt. Mihara (Kaneko et al., 2011) and in the southeastern flank of Mt. Mihara (ERI and Chiba Univ., 2009) both trending NW aligned to a major tectonic direction on the island. These results of magnetization mapping suggest subsurface intrusions overlain by surface volcanic products, which are very interesting interpretation about the subsurface structure. However, the evaluation of magnetic anomalies itself might be insufficient since they did not produce a detailed magnetic anomaly map. Hence, a compilation of such a magnetic anomaly map has been newly conducted. Magnetic anomalies (total magnetic intensity) were reduced onto a smoothed observation surface assuming equivalent anomaly below the observation surface and reduction to the pole anomalies are also calculated on the same surface. The resultant reduction to the pole anomaly map clearly shows magnetic highs on the northern and southern flanks of Mt. Mihara. Magnetic lows are dominant over the eruptive crater and the northeastern foot of Mt. Mihara. A local magnetic high seems to lie over one of B craters of the 1986 eruption. 3D magnetic imaging was then applied to terrain-corrected magnetic anomalies and the result was compared with preceding studies. These newly compiled aeromagnetic maps and interpreted subsurface structures have finer space resolution than those of airborne magnetic surveys using airplanes at a higher altitude (e.g. Makino et al., 1988).
According to the compilation of detailed magnetic anomaly maps using ERI's data, GSJ specified a survey area of drone magnetic survey using MagArrow on the island. In 2019, a drone magnetic survey was flown at an altitude of 25 m above terrain along E-W survey lines spaced 25 m apart in the northern part of the summit crater (Okuma et al., 2020). Unfortunately, many typhoons attacked Izu-Oshima Island in 2019, causing difficulties in surveying on calm days with a wind speed less than 2 m/s. Because of the harsh weather, the flights covered only a half of the provisional survey area. Magnetic anomalies (total magnetic intensity) were reduced onto a smoothed observation surface assuming equivalent anomaly below the observation surface and reduction to the pole anomalies are also calculated on the same surface. Many magnetic highs and lows are distributed on the maps. Comparing the result of the drone magnetic survey with that of the unmanned autonomous helicopter survey, it is obvious that the former is superior to the latter in spatial resolution. Many short-wavelength anomalies can be attributed to topography consisted of volcanic products on magnetic maps of the drone magnetic survey. Even the magnetization heterogeneity of the 1986 lava can be estimated on the same maps.