Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 385

Fig. 10.7 A model of sealed underground compartments V with high pore pressure P .Thefluid
filtration through the surface S towards the low pressure region V may be triggered by weak
seismic events to generate electrokinetic effects (Bernard 1992 ; Fenoglio et al. 1995 )

0 C jr P j V
4r 2

ıB : (10.39)

Taking the notice of the numerical values of the parameters: jr P jD 10 2 -10 4 Pa/m,
V D 10 3 -10 5 m 3 , C D 10 6 -10 8 V/Pa, D 10 2 -10 3 S/m and distance r D
10 km, we come to the following upper estimate ıB max 10 2 pT. Certainly, it is
only a rough estimate which depends on the chosen values of the parameters.
In the model, the duration of the electrokinetic phenomena is determined by the
time scale of the underground fluid diffusion through the volume V of the crashed
rock. The typical time of the fluid diffusion is of the order of l 2 =D, where
D kK f = is the fluid diffusion coefficient (Frenkel 1944 ). Unfortunately, at
higher depth the fluid viscosity coefficient , the compressibility modulus of the
fluid K f and especially the rock permeability k are not well known to provide
reliable estimates of both the time scale of the process and typical frequencies.
One more model is based on the assumption that the electromagnetic pertur-
bations observed at the ground recording station can be due to the variations of
streaming potential coefficient in the vicinity or inside the EQ focus (Surkov 2000a ;
Fedorov et al. 2001 ). This large-scale effect can be observable around the EQ
epicenter but not only near the water-bearing zones or near the faults coming to
the ground surface.
In this case the electrokinetic current moment ( 7.1 ) can be estimated as follows:

LJ LJ LJ LJ LJ LJ LJ LJ LJ LJ LJ LJ

Z

j d jD j e dV C jr P j V; (10.40)

V

where C is the variation of the streaming potential within the volume V of
the EQ focal zone. For simplicity, we have ignored the changes in pore pressure
gradient and conductivity. Substituting Eq. ( 10.40 )for j d j into Eq. ( 7.3 ), we obtain
the estimate for magnetic variations