Several kinds of Induced Polarization (IP) effects occur and will be reviewed in the present paper, namely electrosmosis effect, membrane polarization, MaxwellWagner effect and electrolytical polarization. All effects are based on different physical phenomena. The electrosmosis processes occur in all rocks/sediments. However the amount of double electric layer plays the major role. This phenomenon is described by HelmholtzSmoluhowsky equation and is linear. Decay constant of electrosmosis process is usually in range 10-6- 10-2 s and can be mostly observed on Transient Electro-Magnetic (TEM) signals. The membrane polarization is based on constrictivity of pore. When an electrical current flows through rocks containing channels and pores with different sizes, an excess/loss of ions accumulates at the boundaries. The homogeneous diffusion equation, with specified (nonlinear) boundary conditions, has been used for solving this problem. This type of polarization is non-linear, depends on applied current and may depend on current pulse length (in TEM method). Duration of membrane polarization can reach 10 s and more. Maxwell-Wagner model consists of isolated pores. The homogeneous diffusion equation also has been used for solving the problem of ions distribution in the pores. However the boundary conditions are linear. The ions move in the pores with constant velocity and accumulated the neighbour of the pore ends. Duration of process of redistribution is comparable with duration of the electro osmosis process. The pores with unallocated ions may be represented by an equivalent electrical circuit (a capacitor). The Cole-Cole or capacitor discharging formula can be used to describe the effect. Numerous laboratory measurements of different types of rocks and minerals and some field TEM data demonstrate different kinds of IP effects.
Different kinds of IP effects and laboratory measurements samples
SANTARATO, Giovanni;
2016
Abstract
Several kinds of Induced Polarization (IP) effects occur and will be reviewed in the present paper, namely electrosmosis effect, membrane polarization, MaxwellWagner effect and electrolytical polarization. All effects are based on different physical phenomena. The electrosmosis processes occur in all rocks/sediments. However the amount of double electric layer plays the major role. This phenomenon is described by HelmholtzSmoluhowsky equation and is linear. Decay constant of electrosmosis process is usually in range 10-6- 10-2 s and can be mostly observed on Transient Electro-Magnetic (TEM) signals. The membrane polarization is based on constrictivity of pore. When an electrical current flows through rocks containing channels and pores with different sizes, an excess/loss of ions accumulates at the boundaries. The homogeneous diffusion equation, with specified (nonlinear) boundary conditions, has been used for solving this problem. This type of polarization is non-linear, depends on applied current and may depend on current pulse length (in TEM method). Duration of membrane polarization can reach 10 s and more. Maxwell-Wagner model consists of isolated pores. The homogeneous diffusion equation also has been used for solving the problem of ions distribution in the pores. However the boundary conditions are linear. The ions move in the pores with constant velocity and accumulated the neighbour of the pore ends. Duration of process of redistribution is comparable with duration of the electro osmosis process. The pores with unallocated ions may be represented by an equivalent electrical circuit (a capacitor). The Cole-Cole or capacitor discharging formula can be used to describe the effect. Numerous laboratory measurements of different types of rocks and minerals and some field TEM data demonstrate different kinds of IP effects.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
