Electrokinetic Effect in Water-Saturated Rock - Ultra and Extremely Low Frequency Electromagnetic Fields - page 324

8.2 Seismoelectric Effect Due to Propagation

of Seismic Waves

To all appearance Ivanov ( 1939 , 1940 ) was the first who detected electromagnetic
effect associated with the propagation of seismic waves in the ground. High
explosive charges with mass 1:5 kg were detonated just under the surface of
the moist soil (surface explosions) in order to examine the acoustic and electric
properties of the soil. During the explosion the appearance of an electric potential
drop between two grounded electrodes was recorded at the distance of 120 m from
the explosion point. This phenomenon was called the seismoelectric effect of the
second kind in contrast to seismoelectric effect of the first kind that lies in the
fact that the seismic wave changes the electric current flowing in the moist soil
between two grounded electrodes. Frenkel ( 1944 ) and Martner and Sparks ( 1959 )
have shown that this phenomenon can be explained by the electrokinetic effect
in fluid-filled cracks and channels contained in the surface layer of the ground.
In the previous section we have studied the MHD mechanism which is capable
of explaining such a kind of phenomena whereas this section will focus on the
seismoelectric effect as an alternative way to explain the observation.
The main cause of the seismoelectric effect is that the seismic wave creates the
deformations of porous rock followed by the generation of groundwater pressure
gradient in pores and cracks. As is seen from Eq. ( 8.7 ), this results in the generation
of electrokinetic current in porous rocks. According to the linear theory of porous
water-saturated medium (Frenkel 1944 ) the excess of pore pressure ıP over the
hydrostatic level and the volume deformation are related by the following
equation:

@ t ıP
K f C

:

@ t ıP
K f C

.LJ 1/@ t

2 ıP
Ǜ

Ǜ D r k

LJ@ t
Ǜ

(8.23)

Here the following abbreviations are made

1 ;

Ǜ D 1 C .LJ 1/ K f

1
n

K
K s K s ;LJ D

(8.24)

where K f , K s , and K are the compression modules of the fluid, solid matrix, and
dry porous rock (rock skeleton without fluid), respectively, n is the porosity, k is the
rock permeability, and and are the fluid viscosity and density, respectively.
Below we focus on the large-scale seismic waves with typical wavelengths about
1 km and frequencies of the order of several Hertz. In this low frequency limit one
may neglect the second order temporal and spatial derivatives in Eq. ( 8.23 ). As a
result, this equation is reduced to the following one

ıP D K f LJ=Ǜ: (8.25)