Geoscience Reference
In-Depth Information
×10
-7
×
10
-
6
0
0
3
Relection at
the irst interface
Relection coseismic
at the irst interface
4
Direct ield
Coseismic
100
100
2
2
200
1
200
Interface response
Top of the reservoir
0
0
300
300
-1
400
400
-2
-2
500
500
-4
-3
600
600
-200
-100
0
100
200
-200
-100
0
100
200
X
(m)
X
(m)
(a)
(b)
Offset (m)
Offset (m)
Offset (m)
-200
-200
-100
-100
0
0
100
100
200
200
-200
-100
0
100
200
×10
-6
×10
-7
0
0
8
10
Coseismic
Direct P-wave
6
8
0.1
0.1
4
IR1
6
IR2
0.2
2
0.2
IR3
4
IR4
PPr1
0
0.3
2
0.3
-2
IR5
PPr2
0
RCS1
-4
0.4
0.4
-2
-6
RCS2
0.5
-4
0.5
-8
RCS3
PPr3
-6
-10
0.6
0.6
(c)
(d)
Figure 4.11
Snapshots of seismic and seismoelectric phenomena.
a), b)
Snapshots of the seismic and electrical fields at time
t
=0.24 s.
This corresponds to the time when the P-wave reaches the top of the reservoir, which acts as a dipole radiating electromagnetic
energy.
c)
The seismograms reconstructed by the geophones (with a takeout of 10 m) present direct field and different reflections of the
P-wave: the reflection PPr1 on the interface L1
-
L2, the reflection PPr2 on the interface L2
-
top of the reservoir, and the reflection PPr3
on the interface L2
bottom of the reservoir.
d)
The electrograms show the coseismic electrical potential field associated with the
direct wave and the reflections of the P-wave (labeled RCS1, RCS2, and RCS3) and the seismoelectric conversions with a smaller
amplitude and a flat shape (labeled IR1, IR2, IR3, IR4, and IR5).
-
generated, to the receiver (quasistatic field approxima-
tion). This assumption is very good for investigations
within the first kilometer below the ground surface.
Due to constructive interferences, a significant portion
of the first Fresnel zone acts like a disk of electric dipoles
oriented normal to the interface. These dipoles oscillate
with the waveform of the seismic wave (Figure 4.11b).
Because the electromagnetic diffusion of the electrical
disturbance is very fast, the seismoelectric conversions
are observed nearly at the same time by all the electrodes
but with different amplitudes. Therefore, the seismoelec-
tric conversions appear as flat lines in the electrograms.
Note also that the polarity of the converted seismoelectric
signals depends on the contrast of electrical material
properties (volumetric charge density and electrical
conductivity) at the interface where they are generated.
On the contrary, the polarity of the coseismic electrical
signals depends on the value of the streaming potential
