Geoscience Reference
In-Depth Information
Electrical conductivity
Mass density
Geometry
× (m)
× (m)
Well A
× (m)
Well B
0
200
400
0
200
400
0
200
400
0
0
0
200
200
200
400
400
400
Scanned
area
600
600
600
800
800
ℜ
800
1000
1000
1000
Horizontal distance
0.5
Conductivity (S
m
-1
)
1.0
2000
2000
Bulk density (kg m
-3
)
(a)
(b)
(c)
Figure 6.12
Model used for the scanning simulations.
a), b)
Heterogeneous distribution of the electrical conductivity and mass
density of the formations.
c)
Geometry of the acquisition array. The porous material is bimodal with a larger correlation length
in the horizontal direction. The seismic receivers are located in two wells (A and B). The area scanned through focusing
is indicated by the yellow rectangle (scanning region
ℜ
). (
See insert for color representation of the figure
.)
seismoelectric sources does not allow us to associate an
observed electric potential with a given position in the
subsurface. This causes interpretation ambiguity, which
can be resolved by restricting our attention to the focus
time. Figure 6.19 shows the potential observed at a given
electrode by focusing at all locations in the target area.
Although the potential is measured at a single point,
we record all of the measurements from the correspond-
ing seismic energy focus positions. The accumulation of
the stimulated seismoelectric responses at each electrode
during the scanning process generates a seismoelectric
map of the subsurface. This map highlights the locations
characterized by hydroelectric contrasts. Also, this map is
subject to the effects of
Table 6.3
Material properties used for the simulations. The
viscosity of the pore water is taken equal to 10
−
3
Pa s.
Reservoir
rock
Nonreservoir
rock
Property
22 × 10
9
7 ×10
9
Bulk modulus
K
(Pa)
Skempton coefficient
B
(
)
0.65
0.85
—
Density
ρ
(kg m
−
3
)
2300
1900
, m
2
)
Log (water-phase permeability,
k
12
16
−
−
focuses on the desired scanning point. In this case, we
know precisely the source of electric current position
and time, and we also maximize the strength of the
source. The number of focus points is determined from
the wavelength of the wave. Typically, we use two focus
points per wavelength.
We repeat the process outlined earlier and record the
electric potential at a small number of electrodes located
in the two wells. These electrodes record the electrical
potential over time, but we only use the voltage observed
at the focusing time. This is mainly because the seismo-
electric sources prior to the focus time are distributed
everywhere in the medium. This wide distribution of
from a given elec-
trode. This causes the potential to weaken with distance
from the measurement electrode, as well as a function of
the distribution of conductivity in the medium. This
means that different electrodes are sensitive to different
portions of the subsurface. Such maps are shown for
various electrodes in Figure 6.19. A cumulative map
of the electrical potential is shown in Figure 6.20. This
“
“
illumination
”
illuminates the position of all
heterogeneities in the scanning area located between
the boreholes.
seismoelectric image
”
