Geoscience Reference
In-Depth Information
full waveform inversion is possible. An alternative is to
enhance the seismoelectric conversion effects through
the beamforming approach discussed in Chapter 6.
Additionally, various strategies can be used to separate
the coseismic effects from the seismoelectric conver-
sions using, for instance,
k
near-surface applications is nearly unheard of, for
example, in environmental-related applications. The
use of high-precision lasers can be used to monitor
extremely small fluctuations of the displacement of
the ground surface due to electroseismic influences.
A lot of recent research regarding the inversion of
ground displacement in terms of mechanical sources
(for the monitoring of active volcanoes) could be used
to interpret electroseismic effects. There is a lot of work
to do in this direction.
6
While the present topic has been focused on the occur-
rence of electrical fields, the seismoelectric method can
be based on the magnetic field as well. Similarly, elec-
trical resistivity tomography is usually based on the
record of the electrical field, but the magnetic field
associated with the flow of the current in the subsurface
can bemeasured as well (magnetoresistivity). We know
that the magnetic field is measurable in the laboratory,
but it remains uncertain if the magnetic field of electro-
kinetic nature can be measured in field conditions. If so,
what is the value of using the magnetic field alone or in
combinationwith the electric field? Once generated, the
magnetic field is not sensitive to the heterogeneous dis-
tribution of the electrical conductivity of the subsurface.
In fact, the relative permittivity of the subsurface is usu-
ally one or very close to one (except in some iron-based
deposits such as magnetite and other similar minerals).
This implies that it is, in principle, easier to invert the
magnetic field fluctuations relative to the inversion of
the electrical field fluctuations. However, the magnetic
field strength of magnetic dipoles decreases with dis-
tance faster (proportional to 1/
r
3
) than the electric field
strength of electric dipoles (proportional to 1/
r
2
). This
means that there may be an effective range of use for
magnetic field-based applications that is different from
the electric field effective range of use. Even so, it is very
likely that the joint inversion of electromagnetic fields
can provide more information relative to the inversion
of just the electrical field alone. There ismuch to explore
here as well.
7
We have seen in Chapter 6 that the seismoelectric
approach can be used to produce spectral seismoelec-
tric signals. The presence of cracks should have a very
specific signature on these spectra. In other words,
we could foresee the use of the seismoelectric
methods to detect cracks and to determine their pref-
erential orientations and possibly their hydraulic
conductances.
-
f
(wave number
-
frequency)
filters.
3
The seismoelectric theory needs to be extended to ani-
sotropic formations especially in the case of transverse
isotropy. In principle, such an extension should be
rather straightforward since the Biot
Frenkel theory
has already been extended in anisotropic conditions.
Also, actual electrical conductivity data and models
(including the effect of surface conductivity and
quadrature conductivity) are available for anisotropic
formations. The same can be said about formation
permeability. It remains unclear if the charge per unit
volume used extensively in this topic should be consid-
ered as a scalar or a second-order symmetric tensor as
electrical conductivity and permeability are. As a first
step, the seismoelectric theory based on the acoustic
approximation should be easily amenable to the aniso-
tropic case. Then it would be interested to see what
valuable information the seismoelectric method could
bring on the table to characterize anisotropy.
4
Because both the seismoelectric method and the
induced polarization method are sensitive to the
electrical double layer, there is likely some interest
in formulating time-lapse joint inversion approaches
for these two methods. In addition, their spatial
sensitivities are different and complementary. The
petrophysics of these methods is now fairly well
established, so we can formulate the fully coupled
geophysical inversion that involves reactive transport
modeling codes.
5
The present topic is focused on the seismoelectric
method. We did not discuss the symmetric effect, the
so-called electroseismic effect. Here again, there are
very few undisclosed field trials of the electroseismic
effect. The use of an electrical current provides a highly
reproducible seismic source of electroosmotic nature
(the drag of the excess of charge in the pore water
provides a fluid pressure source). Thanks to Onsager
reciprocity relations in the constitutive equations of
transport, the theory discussed in the present topic
can easily be used to produce an electroseismic theory
in unsaturated conditions or in two-phase flow condi-
tions. The application of the electroseismic approach to
-
