Geoscience Reference
In-Depth Information
Nonpolarizability is maintained through the diffusion of
small amounts of the electrolyte through the membrane
into the soil immediately outside of the electrode.
The electrical measurements made during seismoelec-
tric data acquisition are made with a multichannel digital
voltmeter that makes a series of time-synchronized mea-
surements. To make accurate voltage measurements, the
input impedance of the voltmeter has to be at least ten
times higher than the impedance of the ground between
the two electrodes in order to avoid errors caused by
input bias currents in the voltmeter. A voltmeter with
an internal impedance of 100MOhm would be high
enough for most applications. However, working over
very resistive materials (>10,000 Ohm m, e.g., ice, per-
mafrost, crystalline rocks) may imply the use of a volt-
meter with much higher internal impedance (some
voltmeters can be made with an internal impedance of
10
12
Shot 1
S
S
S
Shot 2
S
S
S
Shot 3
S
S
S
....
Shot 12
S
S
S
S
Seismograph
Seismic source
Electrical dipole with ampliiers
Figure 7.1
Seismoelectric array geometry and shooting
progression (modified from Dupuis et al., 2007, reproduced with
the authorization from the SEG and the authors).
10
14
Ohm). The voltmeter, like all instrumentation
in geophysics, has to be calibrated regularly against
known resistances to check its accuracy over a broad
range of resistance values. This accuracy needs to be
known if we want to use the amplitude of the recorded
voltages, especially in some applications like the one
described in the succeeding text designed to connect
the seismoelectric signals to the water content of the
vadose zone. Note, however, that in a number of the
applications we described in the previous chapters, we
were more interested in the timing of the seismic excita-
tion relative to the resulting seismoelectric disturbances
and known electrode positions. Importantly, amplifiers
can be used to boost the voltages that are recorded.
The amplified and recorded voltages will require correc-
tion of the amplitude of the measured data with respect
to the magnitude of the source of the phenomena caus-
ing the signal.
An example of a seismic and seismoelectric super-
gathers obtained by Dupuis et al. (2007) is shown in
Figure 7.3. We can clearly see the seismoelectric conver-
sions on this seismoelectric supergather. The final result
of their survey is shown in Figure 7.4 and is compared to
a georadar (GPR) cross section. The two surveys clearly
show the position of the water table and possibly some
pockets of high water content in the vadose zone. This
confirms that the seismoelectric method can be used
for shallow surveys to potentially characterize the vadose
zone in terms of water content distribution. If the mea-
surements are repeated over time, it is possible to mon-
itor the progress of unsaturated flow in the vadose
-
Petiau non-polarising electrode
Lead
Lead chloride/NaCl
in kaolinite
Ground surface
Bentonite
Membrane
(ceramic/wood)
Ground
Figure 7.2
Petiau nonpolarizing electrodes (see Petiau, 2000).
These electrodes are composed of a metal in contact with its own
salt (e.g., Cu in CuSO
4
, Ag in AgCl, or Pb in PbCl
2
). This salt is
usually in a saturated solution (can be solid too, as in one form of
the Ag
AgCl system) and typically fully saturates a clay material
(usually kaolinite) contained inside the electrode to slow down
the fluid diffusion processes. The contact between the electrode
and the ground is made though a membrane with micropores
(some woods or ceramics can be used for this purpose).
A bentonite mud cake can be placed between the electrode and
the ground to decrease the contact resistance of the electrode and
keep the moisture content constant during the time of the survey.
Note that these electrodes have a thermal drift of 0.2mV per C.
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