A simulation study of electromagnetic wave reflection and refraction to monitor subsurface electrical resistivity time variation

2019 Fiscal Year Final Research Report

• Project/Area Number: 17K05624
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Solid earth and planetary physics
• Research Institution: Tohoku University
• Principal Investigator: Ichiki Masahiro 東北大学, 理学研究科, 助教 (80359182)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: 地震発生予測・火山噴火予測 / 観測手法開発

Outline of Final Research Achievements

Electrical resistivity time variation of the earth's crust reflects transport of crustal fluids, which more or less involves phreatic eruption or seismicity. To develop a technique to monitor the resistivity variation, this project aims to study loop-loop transient electromagnetic method. The objective is (1) to develop a finite element time domain simulation code to calculate horizontal transient electromotive force in the 3D resistivity distribution, and (2) to calculate expected transient electromotive force in the 3D resistivity distribution model of Zao volcano, NE Japan by use of the code.
We took a finite element time domain approach to develop the code, and found still insufficient numerical precision of loop source formulation to be composed line dipole segments. In uniform half-space resistivity, we can expect 0.987 mV at 1 ms, 0.987 us at 10 ms on 100×100 m^2 in 100 Ohm-m, and 98.7 mV at 1 ms, 98.7 uV at 10 ms in 10 Ohm-m, induced by 100×100 m^2 loop of 10 A.

Free Research Field

地球惑星内部電磁気学

Academic Significance and Societal Importance of the Research Achievements

2014年の御嶽山の噴火や2016年の阿蘇山の噴火によって、地震計や傾斜計、GNSSによる力学的変動の観測では水蒸気噴火の短期的噴火時間予測が難しいケースが再認識された。水蒸気噴火の予測には火山体下の熱水流体の動きを捕捉すると考えられる電気比抵抗の変化の監視で精度を向上させることが期待されており、本研究では時間領域電磁法と呼ばれるうちLoop-Loop法と呼ばれる手法を向上させることで、地下の電気比抵抗が変化した場所を特定できるだけでなく、時間、空間的に分解能を改善させ、火山噴火予測、さらには流体駆動型の内陸地震などの予測を多項目で実装できることが期待できる。