A procedure for stable electrical measurements on a rock sample against high contact resistance as a prerequisite for electrical tomography

令和4年度 Conductivity Anomaly研究会日時：令和4年12月26日（月）09:25-18:30, 12月27日（火）09:00-16:30場所：京都大学防災研究所連携研究棟3階301号室およびZoo

Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

Three sub-Plinian eruptions took place on 26–27 January 2011 at Shinmoe-dake volcano in the Kirishima volcanic group, Japan. During this event, GPS and tiltmeters detected syn-eruptive ground subsidence approximately 7 km to the WNW of the volcano. Starting in March 2011, we conducted broad-band magnetotelluric (MT) measurements at a site located 5 km NNW of the volcano, beneath which the Shinmoe-dake magma plumbing system may exist. In addition, temperature monitoring of fumaroles and hot-springs near the MT site was initiated in July 2011. Our MT data record changes in apparent resistivity of approximately ±5%, along with a ±1◦ phase change in the off-diagonal component of the impedance tensor (Zxy and Zyx ). Using 1-D inversion, we infer that these slight changes in resistivity took place at relatively shallow depths of only a few hundred meters, at the transition between a near-surface resistive layer and an underlying conductive layer. Resistivity changes observed since March 2012 are correlated with the observed temperature increases around the MT monitoring site. These observations suggest the existence beneath the MT site of pathways which enable volatile escape

Seismicity controlled by resistivity structure : the 2016 Kumamoto earthquakes, Kyushu Island, Japan

The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL

ヘン ヨウソ ユウゲン ヨウソホウ オ モチイタ グローバル スケール デンジ ユウドウ シミュレータ

京都大学0048新制・課程博士博士(理学)甲第9327号理博第2462号新制||理||1265(附属図書館)UT51-2002-G85京都大学大学院理学研究科地球惑星科学専攻(主査)教授 大志 万直人, 教授 家森 俊彦, 教授 町田 忍学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDA

地殻活動地磁気現象検出のための地域地磁気モデルの精度 : 北陸・近畿・中国地方における観測点の効果

地球主磁場の空間分布についてのいくつかの仮定にもとづき，観測所データをもとに構成可能な日本周辺の地域地磁気モデルの決定精度を評価した。地域地磁気モデルは，地磁気変化のなかから地殻活動起源のわずかな変化を抽出するための基準として用いられるものであり，1 nT よりも高い精度が求められる。評価の結果，西日本におけるモデルの精度が不十分であることが示された。防災研究所が展開する地磁気観測点のなかでは，鳥取観測点の位置におけるモデル決定精度がもっとも低く，同観測点における地磁気観測が日本周辺の地域地磁気モデルの精度向上のために最重要であることが結論される。Accuracy of the regional geomagnetic field model based on geomagnetic observatories in Japan has been examined under some assumptions on the spatial distribution of the Earth's geomagnetic main field variation. The geomagnetic field model is used as a reference to distinguish small tectonomagnetic signals which are as small as 1 nT from the main field variation, so that the model should be more accurate than 1 nT. This criterion is satisfied in east Japan, whereas it is not satisfied in the vast part of west Japan. It is clarified that the model has the least accuracy at the place of the TOT site among all stations deployed by Disaster Prevention Research Institute (DPRI). This means that the TOT site has a great importance to determine more accurate reference field models.地球主磁場の空間分布についてのいくつかの仮定にもとづき，観測所データをもとに構成可能な日本周辺の地域地磁気モデルの決定精度を評価した。地域地磁気モデルは，地磁気変化のなかから地殻活動起源のわずかな変化を抽出するための基準として用いられるものであり，1 nT よりも高い精度が求められる。評価の結果，西日本におけるモデルの精度が不十分であることが示された。防災研究所が展開する地磁気観測点のなかでは，鳥取観測点の位置におけるモデル決定精度がもっとも低く，同観測点における地磁気観測が日本周辺の地域地磁気モデルの精度向上のために最重要であることが結論される。Accuracy of the regional geomagnetic field model based on geomagnetic observatories in Japan has been examined under some assumptions on the spatial distribution of the Earth's geomagnetic main field variation. The geomagnetic field model is used as a reference to distinguish small tectonomagnetic signals which are as small as 1 nT from the main field variation, so that the model should be more accurate than 1 nT. This criterion is satisfied in east Japan, whereas it is not satisfied in the vast part of west Japan. It is clarified that the model has the least accuracy at the place of the TOT site among all stations deployed by Disaster Prevention Research Institute (DPRI). This means that the TOT site has a great importance to determine more accurate reference field models

A procedure for stable electrical measurements on a rock sample against high contact resistance as a prerequisite for electrical tomography

Abstract As a basis for the electrical tomography of laboratory-scale rock samples (~ 10 cm), we developed a procedure for stable, multi-point, electrical measurement on rock samples that is effective even at high contact and sample resistance. Electrodes were strongly attached to the surface of high-resistivity rock using conductive and adhesive epoxy. Sustained current injection for long periods into high-resistance rocks was fulfilled using a constant direct current source with high internal resistance. Accurate voltage measurement across the high-resistance rock was accomplished by differential measurement using two high input resistance voltmeters. Measurements of high resistance also require a stable measurement environment: the temperature and humidity in the laboratory were controlled using an air conditioner, a humidifier, a dehumidifier, and a vinyl tent. Signal noise arising from human activities was eliminated by the remote operation of the measuring equipment and switching terminal. The proposed measurement procedure was evaluated in terms of the stability and accuracy of measured values and its applicability to electrical tomography. To assess measurement stability, we performed multiple measurements of a dry granite sample at various levels of absolute humidity. Our procedure recorded highly reproducible measurements under each humidity condition. The observed changes in measured values with absolute humidity indicate the importance of stabilising the temperature and humidity conditions in the laboratory. Applying our technique to multiple plastic samples with known resistivity confirmed its accuracy. To evaluate its applicability to electrical tomography, we measured the potential distribution on a dry granite surface in response to an injected current using a simple 40-electrode array. The potential distribution measured by our procedure agreed well with that predicted by forward modelling, demonstrating the robustness of our procedure in array measurements, and thus indicating its potential applicability to tomographic measurements for a variety of targets even under severe conditions including the relative dryness of ambient humidity