Development of the ACTIVE system : an Array of Controlled Transientelectromagnetics to Image Volcanic Environment

这项为期三年的项目是为了开发一种利用受控电磁方法监测活火山的准确系统(主动)。2000年，进行了数值模拟研究，以确定监测伊豆-大岛火山活动引起的结构变化的最佳阵列配置。设计了发射机电路、接收机灵敏度、记录长度等观测系统，制作了1台发射机和2台接收机的样机，2001年在伊豆-大岛火山中心锥体宫原山附近进行了两次野外试验。第一个实验的目的是调试系统，第二个实验的目的是探索中心锥体周围的三维电导率分布。除了这些实验工作外，我们还开发了一种方案来成像电导率结构的时间变化。2002年8月，我们开始自动重复测量，使用去年安装的发射器偶极子和中央凹坑周围新安装的五个接收器。数据采集每两天重复一次(从凌晨2点到3点)。时间序列数据记录在连接在接收器中的PC卡中。所有仪器的PC卡每两个月回收一次，并采用数据处理方法监测电导电荷。对实验数据也应用了成像程序，但到目前为止没有得到明显的变化。我们的重复测量表明没有明显的变化，因为伊豆-大岛火山的活跃度仍然很低。另一方面，我们重新研究了1986年峰顶喷发前观测到的视电阻率的变化，发现这些变化可以用喷发前岩浆在管道中的迁移来解释。进一步的数值实验清楚地表明，该活动系统将能够以高精度成像1986年喷发前的岩浆迁移。

This three-year project was carried out to develop an accurate system (ACTIVE) to monitor active volcanoes by a controlled electromagnetic method.In 2000, numerical simulation studies were made to determine the optimal array configuration to monitor the structural change due to volcanic activity of Izu-Oshima. Observation system was designed, such as transmitter circuits, sensitivity of the receiver, and record length, and then prototype instruments (one transmitter and two receivers) were manufactured.In 2001, field test experiment was carried out twice around the Mihara-yama, the central cone of Izu-Oshima volcano. The first experiment was intended to debug the system, while the second experiment aimed to explore 3-D conductivity distribution around the central cone. In addition to these experimental efforts, we developed a scheme to image time changes in the electrical conductivity structure.In 2002, automatic repeated measurement was started in August, 2002, using the transmitter dipole installed last year and five receivers newly installed around the central pit crater. Data acquisition have been repeated once every two days (from 2-3 AM). Time series data are recorded in a PC card that is attached in the receiver. PC cards from all instruments were retrieved once every two months and data processing were adopted to monitor the conductivity charge. Imaging program was also applied to the experimental data but so far no significant change was obtained.Our repeated measurement indicated no significant change, because the activity of Izu-Oshima volcano still remains very low. On the other hand, we re-examined changes i n the apparent resistivity observed before the 1986 summit eruption and found that they can be explained by pre-eruptive migration of magma in the conduit. Further numerical experiments clearly showed that the ACTIVE system would be able to image the magma migration before the 1986 eruption with a high accuracy.