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

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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.