Active System For Monitoring Volcanic Activity- A Case Study Of The Izu-Oshima Volcano, Central Japan

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Journal Article: Active System For Monitoring Volcanic Activity- A Case Study Of The Izu-Oshima Volcano, Central Japan

A system is proposed for the monitoring of changes in the underground structure of an active volcano over time by applying a transient electromagnetic method. The monitoring system is named ACTIVE, which stands for Array of Controlled Transient-electromagnetics for Imaging Volcano Edifice. The system consists of a transmitter dipole used to generate a controlled transient electromagnetic (EM) field and an array of receivers used to measure the vertical component of the transient magnetic field at various distances, with automatic operation of both units. In order to verify the performance of the proposed system, numerical and field experiments were carried out by application of the system to the Izu-Oshima volcano, where a remarkable change in the apparent DC resistivity over time had been detected in association with the eruption in 1986. Assuming that the next eruption will follow a scenario similar to that of 1986, an array of five receivers was constructed around the summit crater of the central cone, Mihara-yama (Mt. Mihara) and a transmitter dipole with a 700 m long grounded cable was installed approximately 1 km southeast of the summit crater in 2002. A long-term field experiment was then carried out. With the transmitter and receivers both synchronized by a global positioning system (GPS) clock, this field experiment has shown that over approximately 4 years, the daily value of the accuracy of the system was as high as 1% or better, while the accuracy of the monthly mean of the response functions was approximately 0.1%. Many problems with the instruments, the device software, and/or the electronic circuits have also been solved during this field experiment. For data interpretation, a three-dimensional (3-D) forward modeling code was built to calculate theoretical responses. By applying this forward code, a sensitivity analysis was performed for a realistic 3-D conductivity distribution model, which showed that the receiver array of the ACTIVE system in Izu-Oshima has a relatively high sensitivity to conductivity changes below the summit crater. A simple and rapid imaging method based on the Born approximation was developed for monitoring purposes, and a numerical test showed that the change in conductivity over time, supposed to have occurred before the 1986 eruption, could be imaged by the present system, which features improved spatial resolution. During the 4-year-long field experiment, changes in the response function over time of up to 7% were observed, from which two source regions, R1 and R2, were imaged at a shallower part of the caldera between the transmitter and the crater, and at approximately 500 m below the crater, respectively. A preliminary interpretation suggests that the present receiver array does not have resolution for R1, although the time-lapse change due to R1 is statistically significant, while the time-lapse change due to R2 is shown to be statistically insignificant, even though the receiver array has sufficient resolution for recording the time-lapse change.

Hisashi Utada, Yuji Takahashi, Yuichi Morita, Takao Koyama and Tsuneomi Kagiyama

Published Journal 
Journal of Volcanology and Geothermal Research, 2007

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Hisashi Utada,Yuji Takahashi,Yuichi Morita,Takao Koyama,Tsuneomi Kagiyama. 2007. Active System For Monitoring Volcanic Activity- A Case Study Of The Izu-Oshima Volcano, Central Japan. Journal of Volcanology and Geothermal Research. (!) .