HR: 0830h
AN: GP11C-0277    [PDF]
TI: High-Resolution, Low-Altitude Helicopter-Borne Aeromagnetic Survey over Unzen Volcano, Kyushu, Japan
AU: * Okubo, A
EM: ayako@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokanosho, Uji, Kyoto, 611-0011 Japan
AU: Tanaka, Y
EM: tanaka@aso.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Choyo-son, Aso, Kumamoto, 869-1404 Japan
AU: Utsugi, M
EM: utsugi@aso.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Choyo-son, Aso, Kumamoto, 869-1404 Japan
AU: Kitada, N
EM: kitada@aso.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Choyo-son, Aso, Kumamoto, 869-1404 Japan
AU: Shimizu, H
EM: shimizu@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shinzan, 2-5643-29, Shimabara, Nagasaki, 855-0843 Japan
AU: Matsushima, T
EM: mat@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shinzan, 2-5643-29, Shimabara, Nagasaki, 855-0843 Japan
AB: We try to use repeated high-resolution aeromagnetic surveys at low altitudes to detect the geomagnetic field changes associated with volcanic activity. Previous magnetic studies in volcanic areas using fixed station distributions have detected small temporal changes, however, they do not have the spatial resolution to detect spatial changes. It may be possible to make repeated magnetic surveys even during active volcano eruptions using, for example, unmanned helicopters. On September 18, 2002, we conducted a high-resolution and low-altitude helicopter-borne magnetic surveys in and around Unzen Volcano in Kyushu, Japan. Unzen is an active volcano that had a sequence of eruptions from November, 1990 to 1995, after a quiescence of 198 years. The first flight covers an area over the Futsu, Chijiwa, and Kanahama faults, which are major normal faults that form the Unzen graben system. The second andthird flights cover the summit area of Unzen volcano with spiral trajectories at altitudes of 1000 and 500 ft, respectively. The spacing between the survey lines is about 50 m. The total geomagnetic was recorded by an optical pumping magnetometer installed in the sensor bird and the sampling intervals are 0.1 sec. Precise positioning data of the sensor bird was obtained by a differential GPS technique, with a time resolution of 1 sec. Diurnal magnetic variations of extra-terrestrial origin were removed by subtracting the total field data recorded at a nearby temporary station. In order to eliminate the effects of topography, the average terrain magnetization was estimated using a statistical correlation method (Grauch, 1987). Finally, an inversion was carried out for the terrain corrected anomalies, after removing the linear regional trend. From the results of this inversion, a low magnetized area was seen around the lava dome, while high magnetization is distributed around Mt.Fugen. The low magnetized area suggests that the rock bodies with remanent magnetization is fractured into pieces, and the pieces were then oriented into random directions. Another possibility is that the shallower region under the lava dome was not completely cooled, after the rock magnetization was reduced by the hight temperatures of the eruption. In addition, lava flows in the vicinity of a lava dome can also be recognized by magnetization lows. This study shows the spatial distribution of the magnetization intensity in and around Unzen Volcano and will provides important information regarding the temporal change in the geomagnetic field associated with the volcanic activity.
DE: 1517 Magnetic anomaly modeling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting