Geoscience Reference
In-Depth Information
consists in writing this field as the sum of the gradient of a
scalar potential and the curl of a vector potential. Using
Equations (1.169) and (1.170) in Equation (1.168) and
using the properties
effect was inspired by Louis Statham in the same com-
pany (see Blau & Staham, 1936). They made indeed
the first documented observations of electric potential
changes between a pair of electrodes when seismic waves
passed through these electrodes. Thompson (1936) was
looking for the effect of seismic waves on the electrical
resistivity of Earth materials. His goal was to detect seis-
mic waves bymeasuring continuously the electrical resis-
tivity between a set of electrodes. His paper starts as
follows:
∇ ∇
×
Ψ
=0
1 171
∇
×
∇
φ
=0
1 172
we end up with two equations:
The seismic electric effect is the name which
has been given to the variation of earth resistivity with
elastic deformation.
“
c
p
∇
2
φ
=
φ
1 173
”
effect was modeled in the same issue of
Geophysics
by Slot-
nick (1936) on the base of an equivalent electrical circuit.
In the case of Thompson (1936), he measured clear fluc-
tuations in the electrical potentials that could not be
explained by a change in the value of the resistance (or
apparent resistivity) between the electrodes. In these
works, there were no concept that the propagation of
the seismic waves could create a source current density
or an electrical field on their own and no ideas related
to electrokinetic effects and the associated electrical
double layer theory.
Ivanov (1939) was the first to record seismoelectric
effects in USRR completely passively (no current
injection). His pioneering work was followed few years
later by the seminal paper of Frenkel (1944). Frenkel
developed the first electrokinetic theory of the coseismic
electric field in water-saturated porous rocks to explain
the observations made by Ivanov. Ivanov and Frenkel
should be therefore considered as the pioneers of the
seismoelectricmethod in geophysics. A rigorous treatment
of wave propagation in water-saturated porous media
was, however, only introduced more than a decade later
by Biot (1962a, b). A huge number of field and theoretical
works were done in USRR after WWII in using the
seismoelectric effect to determine the thickness of the
weathered zone to help in interpreting seismic data or
for mineral exploration.
Decades after, Frenkel (1944) and Thompson and Gist
(1991, 1993) presented a case study for the exploration of
oil and gas reservoirs using seismoelectric converted
electrical signals. They used adapted data processing
and common midpoint (CMP) techniques to produce a
seismoelectric image of the subsurface to depths on the
order of a few hundred meters. They concluded that
seismoelectric conversions could be detected from a
depth of 300 m. Thompson and Gist (1993) suggested
”
The concept of
“
seismic electric
c
S
∇
2
Ψ
=
Ψ
1 174
The first mode corresponds to the compressional wave
with velocity
c
p
, while the second mode corresponds to
the shear mode with velocity
c
S
. The velocity ratio is such
that we have
c
p
c
S
= 2+
λ
u
G
≥
1
1 175
Therefore, the P-wave propagates faster than the shear
wave. We will show in the next chapters that there are
more than two wave modes in poroelastic media. The dif-
ferent wave modes have different EM signatures in seis-
moelectric theory, and we will discuss in the next
chapters these different signatures and their potential
applications to probe the Earth using the seismoelectric
method.
1.6 Short history
Now that we have discussed some of the key concepts
required to understand the seismoelectric theory, we
can provide a short introduction to the history of the seis-
moelectric method. The physics of the streaming poten-
tial takes its roots in the experimental work done initially
by Quincke (1859) who discovered that the flow of water
through a capillary generates a measurable difference of
electrical potential. Helmholtz (1879) obtained theoreti-
cal expressions for the streaming current density for glass
capillaries. The first observations of the seismoelectric
effect itself were made by Thompson (1936). Thompson
was a geophysicist working for the Humble Oil and Refin-
ing Company. His work on the so-called
“
seismic electric
”