交流インピーダンス法を用いた比抵抗測定の精度および地質試料における有効性の検討実験

Abstract

交流インピーダンス法による室内比抵抗測定は, 電気探査や電気検層の比抵抗データの解釈, あるいは岩石や堆積物の基本的な電気的性質を明らかにするための有効な手段である. 本研究では, 交流インピーダンス法(二極法および四極法)による比抵抗測定の精度およびその有効性を検討するため, 抵抗が既知の抵抗器(実験1), 比抵抗が既知の電解質溶液(実験2), および海洋掘削コアを含む地質試料(実験3)を用いた実験を行った. 実験1では, 本測定システムの精度が装置の基本精度と同等であることを確認した. 実験2は二極法で試料測定を行い, 周波数が低いほど, また試料の比抵抗が低いほど, 分極の影響で電流－電極の位相差が大きく, 比抵抗が高くなる傾向が見られた. また, 位相差が小さいほど正確な比抵抗値が得られることを示した. 実験3では, 試料表面の乾燥・湿潤状態が比抵抗に与える影響を検討し, 試料の乾燥による比抵抗の上昇, および表面の付着水による比抵抗の低下を防ぐために, 試料にパラフィルム巻いた測定方法が有効であることを示した. また, 間隙率の異なる試料を二極法と四極法で測定し, 四極法の方が低周波域における分極の影響が小さく幅広い周波数での測定に適用できること, 二極法でも間隙率の小さい試料や高い周波数での測定であれば信頼できる比抵抗値が得られることを明らかにした. さらに, 二極法も四極法も, 幅広い間隙率の試料に対して有効であることを示した.

Laboratory experiment of electrical resistivity measurement using an alternative current (AC) impedance method is important to interpret the electrical survey and logging data, and to understand the electrical properties of rocks and sediments. This study investigated the accuracy and applicability of the AC impedance method by measuring resistors of known resistance (Experiment 1), electrolyte solutions of known resistivity (Experiment 2), and rocks and deep-sea core samples (Experiment 3). We confirmed that the accuracy of the measurement system was the same as that provided by manufacture in the Experiment 1. Two-electrode method was used for the Experiment 2, and the results showed that resistivity and phase angle increased with lowering of frequency and decreasing of the sample resistivity. The resistivity data was more accurate when the phase angle was close to zero. In the Experiment 3, we examined the way to prevent overestimation and underestimation of resistivity by drying and wetting of the sample surface, respectively, and wrapping a sample with parafilm was the most effective. In addition, we measured the samples of various porosity using two- and four-electrode methods. Four-electrode method had an advantage that is applicable to wide range of frequency (102 to 105 Hz) because of its less influence of electrode polarization at low frequency, but reliable data ware also obtained using two-electrode method by measuring highly resistive samples or measuring with high frequency. Our data showed both twoand four-electrode methods were applicable to samples of various porosity (0.3 to 67%).