Geoscience Reference
In-Depth Information
S 2 = 0.5, and S 2 = 0.8. The results obtained point out that
the seismoelectric conversion response can be detected
when there is a saturation contrast between the vadose
and saturated zones. The seismoelectric conversion
response can be identified in the first and the second cases
(Figure 3.7) by the increase of the amplitude of the
coseismic signature with the increase of the water satura-
tion in the vadose zone. However, for the third case,
when the saturation of the vadose zone is S 2 = 0.8, the
amplitude of the seismoelectric conversion response is
very weak relative to the coseismic signal (Figure 3.7).
We conclude that a dry vadose zone is the best
condition to identify the groundwater level, because
the coseismic signal is low relative to the seismoelectric
conversion response. This phenomenon was proven
during several seismoelectric surveys carried out at
different fields with different water content conditions
by Strahser (2006).
3.3 Extension of the acoustic
approximation
Offset (m)
-200
0
200
In unsaturated conditions, the density of the pore fluid,
ρ f ,
the bulk modulus of the pore fluid, K f , and the dynamic
viscosity of the pore fluid,
0
PML
η f , are related to the properties
of the nonwetting (subscript
Seismic source
nw
) and wetting (subscript
w
)phaseby
Unsaturated layer
ρ f =1
s w ρ n w +
s w ρ w
3 179
200
PML
PML
1
K f =
1 s w
K n w
+ s w
K w
3 180
Saturated layer
s w
η n w η w
η n w
η f =
3 181
Q V s w = Q V
s w
PML
3 182
400
Figure 3.6 Sketch of the model used for the simulations.
The electrodes are collocated at z = 60 m. All the electrodes
are assumed to be connected to a reference electrode.
This models the effect of saturation on the permeabi-
lity (through the effect of saturation on the relative
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
20
4
3
15
3
CS
CS
10
2
2
IR
IR
5
1
1
0
0
0
-5
-1
-1
-10
-2
-15
-2
-3
-20
-4
-3
-25
-100
-50
0
50
100
-100
-50
0
50
100
-100
-50
0
50
100
Offset (m)
Offset (m)
Offset (m)
(a)
(b)
(c)
Figure 3.7 The electric potential generated with our modeling code for three values of the water saturation of the vadose zone
S 2 = 0.1, S 2 = 0.5, and S 2 = 0.8. a) Case corresponding to S 2 = 0.1. b) Case corresponding to S 2 = 0.5. In both cases, the interface response
at 0.28 s can be distinguished when the contrast between the saturated and vadose zones is strong enough. c) Case corresponding to
S 2 = 0.8. In this situation, the contrast in saturation between the vadose zone and the aquifer is very weak, and consequently, it is
difficult to detect the interface response (seismoelectric conversion). IR, interface response; CS, coseismic field.
 
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