Geoscience Reference
In-Depth Information
CHAPTER 7
Application to the vadose zone
In this chapter, we discuss how seismoelectric data can be
acquired to characterize the vadose zone, that is, the
unsaturated, usually shallow, portion of the ground.
Then, we present a case study in the United Kingdom,
performed by B. Kulessa and L. J. West, in which seismo-
electric signals are correlated with the water content of
the vadose zone. Finally, we apply the numerical model
discussed in Chapter 4 to this case study with the objec-
tive of reproducing the field data using the seismoelectric
theory we developed earlier in this topic that is valid in
unsaturated conditions. We show that we can reproduce
the field data fairly well, and from that, we can poten-
tially infer the water content of the vadose zone using
the seismoelectric method.
Geometrics Geode system. Four shot points spaced 1 m
apart were placed in the shot gap offset laterally, by about
2 m from the acquisition line. The seismic source was a
40 kg accelerated weightdrop source. Three to five
impacts were recorded at each point, and the data were
staked. The total length of the profile was 300 m, and
the acquisition was done using the methodology shown
in Figure 7.1.
The electrodes used for the investigation of Dupuis
et al. (2007) were stainless steel electrodes. A better
choice would be to use nonpolarizable electrodes (see
Figure 7.2), which are known to be much more stable
over time (Perrier et al., 1997, 1998; Petiau, 2000). The
Petiau nonpolarizable electrode is a two-compartment
electrode that is composed of a metal wire that is con-
nected to an ionic reservoir containing a dissociated, sat-
urated salt solution of the samemetal cation (electrolyte).
The first compartment in the electrode is a fluid reservoir
containing the electrolyte, and the second compartment
is filled with a salt solution (incorporating the electrolyte)
saturated clay mineral (such as kaolinite). The last and
critical part of the electrode, the part that makes contact
with the Earth, is the diffusion membrane, and it is usu-
ally made of wood or ceramic. The combination of salt
and clay in the second compartment maintains ionic
continuity throughout the electrode while slowing
down fluid loss from diffusion. The two compartments
of the electrode are connected to each other through a
small diameter hole that is able to supply fluid to the
second compartment as needed to replace lost fluid.
7.1 Data acquisition
In this section, we describe seismoelectric data acquisi-
tion systems that have been used to characterize vadose
zone phenomena where seismoelectric effects occur rel-
atively close to the network of electrodes used to record
the seismoelectric signals. Dupuis et al. (2007) published
an interesting case study showing clear seismoelectric
signals in the vadose zone of a sandy aquifer. Their elec-
trical data acquisition system consisted of 26 electrodes
with a takeoff separation of 4 m. These electrodes were
arranged to form 24 end-to-end dipoles except for a
4 m shot gap at the center (Figure 7.1). The seismic acqui-
sition system itself consisted of three 12-channel 24-bit
