Geoscience Reference
In-Depth Information
Compressional P-wave propagation
Direction of wave propagation
Compression
Equilibrium
Wavelength
Dilation
Dilation
Co-seismic electrical ield
E
(a)
Flow in the pore network
Drag of the excess of charge
Figure 1.19
The coseismic electrical field and
the coseismic (streaming) electrical current.
a)
The propagation of a compressional (pressure
or P-)wave through a porous material generates
areas of compression and dilation (expansion).
b)
In response to the change in the mechanical
stresses, the pore water flows from the
compressed regions to the dilated regions.
c)
The
flow generates a streaming current density that
is locally counterbalanced by the conduction
current density creating, locally, an electrical
field
E
of electrokinetic nature.
S
o
lid
Solid
Pore
(b)
(c)
the interface. Finally, Type III corresponds to the EM
fields associated with the seismic source itself.
We first describe Type I and II seismoelectric effects.
When seismic waves propagate in a linear poroelastic
porous material, two types of electrical disturbances are
observed (Figures 1.19 and 1.20). The propagation of
compressional P-waves generates an electrical source
current associated with the displacement of the electrical
diffuse layer in a Lagrangian framework attached to the
solid phase (Figure 1.19). Inside a wavelength of a com-
pressional wave, there are areas of dilation and compres-
sion (Figure 1.19a). These dilations and compressions of
the solid skeleton are responsible for the flow of the pore
water from the compressed regions to the regions where
expansion occurs (Figure 1.19b). As the result of the flow
of the pore water, there is the advective drag of a fraction
of the excess of charge of the pore water. This advective
drag is responsible for the streaming current density.
Shear (S-)waves do not create coseismic electrical field.
These coseismic electrical signals travel at the same speed
as the seismic waves (Pride, 1994). The amplitudes of the
coseismic EM signals are controlled by the properties of
the porous material (the formation factor) and by the
properties of the pore fluid/solid interface (the zeta poten-
tial in the theory of Pride (1994), the excess charge per
unit pore volume in the formulation used in this topic).
In addition to the coseismic signals, another phenom-
enon occurs when a seismic wave moves through a sharp
interface characterized by a change in the textural prop-
erties or a change in salinity or clay content or mineral-
ogy (Figure 1.20). In this situation, a fraction of the
mechanical energy is converted into EM energy, and a
dipolar EM excitation is produced at the interface. The
resulting EM disturbances diffuse very quickly away
through the interface and can be recorded, nearly instan-
taneously, by electrodes or antennas located at the
ground surface, in boreholes, or at the seafloor, for
instance. In the seismoelectric method, we are mostly
