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

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