Geoscience Reference
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(19 in each borehole and 8 along the ground surface).
The first point of focus (point A; see Figure 6.1) is located
at an interface with a sharp discontinuity in both electri-
cal conductivity and permeability. Therefore, we expect
to see a seismoelectric conversion; our results confirm
this assumption (Figure 6.4). However, we need to
remove the distribution of potential recorded with only
the background to see the seismoelectric conversion
associated with the interface response. We can clearly
see that the dominant spike in the electric potential time
series (background removed) is perfectly synchronized
with the focusing time of the seismic wave field at
point A. This enables us to detect the presence of a
heterogeneity at point A.
Figure 6.5 shows the electric potential at an electrode
located at electrode E45 when the seismic waves are
focused at point B which is not located close to an inter-
face. As expected, we see no spike at the focus time,
which indicates a lack of heterogeneity at this focus point.
We conclude that this method can be used in time lapse
to follow the evolution of a saturation front over time,
for example, for CO 2 sequestration or for monitoring
water flood experiments.
6.2 Application to an enhanced
oil recovery problem
The example discussed in the preceding section uses
several strong assumptions: constant elastic background
and blocky anomalies with constant physical properties.
In this section, we relax these assumptions, and we
demonstrate that the method is applicable to heterogene-
ous geological structures with a contrast in the water
saturation. A lot of work has been done recently using
low-frequency electrical signals to detect oil water
encroachment fronts (see, for instance, Saunders et al.,
2008). We want to see if seismoelectric beamforming
can be used to localize, in a very simple way, the position
of the front. We consider two wells crossing a heteroge-
neous reservoir (see Figure 6.6). Well B is located 250 m
away from Well A, and the total geometry of the
model covers an area of 410 × 250 m. The reference
position, O(0,0), is located at the upper left corner of this
domain. The reservoir is initially saturated with oil
(oil saturation of 80%). During water flooding opera-
tions, water is injected into Well A and oil is produced
from Well B.
Electric potential (normalised)
10
0
−10
0
5
10
15
20
25
30
35
40
45
50
(a)
Time (ms)
Background corrected potential
0.2
0
−0.2
5
10
15
20
25
30
35
40
45
0
(b)
Time (ms)
Pressure ield at focus point A (normalised)
P
(
A
,
t
)
2
0
−2
5
10
15
20
25
30
35
40
45
50
0
(c)
Time (ms)
Figure 6.4 Beamforming at point A. a) Time series of the electrical potential at electrode E35 for beamforming the seismic wave
at point A. This time series has both the seismoelectric conversions and the coseismic field. The interface response is not detectable.
b) Electrical potential recorded when the medium contains the two anomalies minus the potential recorded if we only had the
background. The interface response is now clearly visible. c) Time series of the pressure field at point B, P (A, t ).
 
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