地震前兆としての蒸気ゾーン形成に伴う電磁場発生
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概要
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The flow of a fluid through a porous medium can transport electric charge along the flow path by the interaction of the moving pore fluid with the electrical double layer at the pore surface. This process is known as electrokinetic coupling. If the fluid flow is confined to depth, electric (φ) and/or magnetic (B) anomalies will not appear at the earth's surface unless the cross-coupling coefficient is non-uniform. If underground fluid sources or sinks are present, the resulting distribution of φ (B) is equivalent to the φ (B) produced by surface distributions of current dipoles along boundaries between regions of differing streaming potential coefficients (C). The surface density of current dipoles along a C boundary is expressed as L_<ee>PΔC, where L_<ee> is the electrical conductivity, P is the fluid pressure (above hydrostatic) at the boundary, and ΔC is the difference in C across the boundary. If the C boundary is a closed surface, L_<ee>PΔC must vary along the boundary for φ (B) to appear at the earth's surface. A deep liquid/vapor two-phase zone may develop at depth if the rock pore volume suddenly increases or the rock formation is heated sufficiently. When the liquid-phase saturation drops below the residual saturation (typically 〜0.3), the liquid phase becomes immobile and the effective value of C becomes zero within the vapor-dominated zone, since steam alone cannot carry electrical charge. L_<ee>PΔC may vary along the closed C boundary surrounding the vapor zone, since P (the difference between the water/steam saturation pressure and the hydrostatic pressure) is ordinarily not constant along the boundary. If enough fluid pressure decrease is induced within a dilatant volume prior to an earthquake, a vapor-dominated zone will develop. If the vapor (i.e. zero C) zone is H meters high and the temperature (and pressure) is uniform within it, the difference in P is about 10^4H Pa between the top and bottom of the zone. If the vertical surface area of the vapor zone is HW m^2, the associated integrated current dipole, which can cause an anomaly on the earth's surface, is roughly L_<ee>(10^4H^2W/2)ΔC. Experimental evidence indicates that ΔC will probably lie between 10^<-7> and 10^<-6> V/Pa. A current dipole of 10^4 to 10^5 A-m may appear under typical crustal conditions if H=W=10^3 m. An observable φ anomaly (and B anomaly if the distribution of all current dipoles is not axisymmetric) can be produced by this process prior to an earthquake. The liquid saturation is not necessarily uniform within the vapor-dominated volume. There-fore, the surface anomaly may show step changes with time as the liquid saturation of various subregions passes through the residual saturation value. If the liquid-phase saturation subsequently recovers (above the residual saturation) in the two-phase zone due to rapid fluid inflow from nearby higher-pressure formations, the anomaly may disappear before the earthquake occurs.
- 1995-03-31
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関連論文
- 地震前兆としての蒸気ゾーン形成に伴う電磁場発生
- (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)
- (2)自然電位から推定される火山地域の地下流体流動(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)