Apparent magnetization intensity map on Sakurajima Volcano, Kyushu, Japan, inferred from low-altitude, high-density helicopter-borne aeromagnetic surveys

In November 2005, we conducted the first low-altitude, high-density helicopter-borne aeromagnetic survey over Sakurajima Volcano, Kyushu, Japan to provide a baseline for subsequent measurement of changes in magnetic anomalies related to volcanic processes, because previously acquired aeromagnetic data cannot resolve small-scale geological feature. Although the observation error was considered to be < 10 nT, the harmonic component is extracted by the reduction procedure. We applied an apparent magnetization intensity mapping method to the newly obtained aeromagnetic data assuming that magnetization intensity varies only laterally. Results showed the following features: (1) Apparent magnetization intensities of lava flows were from 4.0 to 6.0 A/m. The most remarkable highs (> 5.0 A/m) were located on the lava around An'ei craters and on the northeastern flank of Kita-dake, (2) the lowermost values of apparent magnetization (< 2.0 A/m) are locally distributed around the active craters of Minami-dake, suggesting high temperature in the shallow part and (3) apparent magnetization lows (~ 2.5 A/m) lie over the northwestern flank of Kita-dake, reflecting debris flows and/or volcanic fans. To examine the validity of the obtained map, we carried out rock-magnetic studies. The total magnetization intensities for the surface-rocks showed reasonable correlation with the mapped values.