(1)応力を受けた岩石の電位差変化(第2部室内岩石実験による電磁波放射特性に関する研究, 地殻破壊の前兆としての電磁波放射の特性に関する研究)

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We measured change of electric potential during uniaxial compression of Inada granite saturated by water or electrolytic solutions. Measured was the potential difference between two electrodes located at different sites of the cylindrical rock surface : one at the middle part and the other, close to one of the ends. We expect many microfractures at the middle part of the sample when the rock sample is under large differential stresses. The main target of this experimental study is to observe change of the electric potential associated with generation of microfractures and to interpret mechanisms of the potential change. We also aim to observe effects of ion species on the change of electric potential because the fresh surface of microfracture is chemically very active and reacts with ions in the electrolytic solutions. It is expected that microcracks radiate charged particles and thus affect the change of electric potential, and electrokinetic phenomena caused by the ζ-potential (electrokinetic potential) result in changes of the electric potential. Samples were coarse grained Inada granite with 50 mm in diameter and 100 mm in length. The samples were put into a vessel evacuated by rotary pump for 4-6 hours and then water was replaced into the vessel to saturate the rock sample. The surface of the sample was coated with silicon rubber to prevent evaporation of water. We used silver-silver chloride electrodes which consist of silver mesh coated with silver chloride and immersed into 0.1 mol/l KCl solution. The electric potential between two electrodes was measured by an electrometer with internal resistance 10^<12>Ω. Load strain and acoustic emission activities as well as the electric potential were measured simultaneously. A cyclic loading of three differential stress levels were performed. Stress levels were kept constant for 20-30 min : the low level where most of the pre-existing microcracks in the rock sample tend to close, the intermediate level where new fractures appear accompanying acoustic emissions, and the high level where acoustic emission activity is very high and is close to the strength of the rock sample. The absolute value of the change of the electric potential suggests that it is affected by the concentration of solution; high concentrations gave small changes whereas low concentrations gave large changes. In the low stress levels the potential changes only small amount enhancing the difference between two electrodes. In the intermediate stress levels the potential changes to both positive and negative directions affected by ion species and concentrations of the solutions. ζ-potential plays an important role in these changes. This suggests that some electrochemical reactions are occurring at the interface between solutions and fracture surfaces. Since the change of electric potential associated with microcrack generation is affected by ion species of the solution, we must consider various combinations of rock species and electrolytic solutions. This obviously results in very complicated features in the changes of electrical potential and also affects the magnitudes of the potential change. The selection rule on which local points are assigned as sensitive regions of earthquake precursor is often manifested in observing changes of electric potential. We consider that this rule is one of the manifestation of the electrochemical reaction or electrokinetic phenomena associated with microcrack generation which is resulted from interactions between rock and groundwater.
1993-03-29

(1)応力を受けた岩石の電位差変化(第2部室内岩石実験による電磁波放射特性に関する研究, 地殻破壊の前兆としての電磁波放射の特性に関する研究)

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(1)応力を受けた岩石の電位差変化(第2部 室内岩石実験による電磁波放射特性に関する研究, 地殻破壊の前兆としての電磁波放射の特性に関する研究)

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(1)応力を受けた岩石の電位差変化(第2部室内岩石実験による電磁波放射特性に関する研究, 地殻破壊の前兆としての電磁波放射の特性に関する研究)