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

这个为期三年的项目是为了开发一个精确的系统（ACTIVE），通过可控的电磁方法监测活火山。2000年，通过数值模拟研究确定了监测伊豆大岛火山活动引起的结构变化的最佳阵列配置。设计了发射电路、接收机灵敏度、记录长度等观测系统，并制作了样机（一台发射机、两台接收机）。2001年，在伊豆大岛火山中心锥体三原山附近进行了两次实地试验。第一个实验旨在调试系统，而第二个实验旨在探索中心锥体周围的三维电导率分布。除了这些实验之外，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，利用去年安装的发射机偶极子和中央坑坑周围新安装的5台接收机，开始了自动重复测量。数据采集每两天重复一次（从上午2-3点）。时间序列数据记录在附在接收器上的PC卡上。每两个月从所有仪器中取出PC卡一次，并进行数据处理，监测电导率电荷。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。我们的反复测量表明没有明显的变化，因为伊豆大岛火山的活度仍然很低。另一方面，我们重新分析了1986年峰顶喷发前观测到的视电阻率变化，发现这些变化可以用岩浆在喷发前在管道内的迁移来解释。进一步的数值实验清楚地表明，ACTIVE系统能够以较高的精度对1986年喷发前的岩浆迁移进行成像。

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.