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Fig. 7.8
Normalized vertical
component of the GMPs
resulted from the longitudinal
seismic wave propagation
(Surkov
1989b
). The
arrow
shows the moment of seismic
wave arrival at the
ground-recording station. The
numerical calculations are
made for a distance 5 km
from the seismic source
10
6
.
B
z
/
B
0
3
2
1
t,
s
0,6
0,8
1,2
0
-1
with period of several tenth of seconds. The normalized potential of the elastic
displacements shown in Fig.
7.7
was used for calculation of the mass velocity (
7.40
)
and the electromagnetic perturbations (
7.35
)-(
7.37
) caused by the underground
explosion (Surkov
1989b
). The numerical calculations are based on the following
parameters: R
0
D
100 m is radius of the crushing zone, P
D
5
10
8
Pa is the rock
compression strength, P
0
D
1:5
10
8
Pa is the lithostatic pressure at the depth of the
explosion, C
l
D
5 km/s is the velocity of longitudinal seismic wave,
m
C
l
D
5
10
10
Pa,
D
C
t
=C
l
D
0:2 where
m
and C
t
are the rock density and velocity of
transverse wave, respectively. A plot of the vertical component of the magnetic
perturbations on the ground surface in the acoustic zone is shown in Fig.
7.8
.
The rock and explosion parameters are the same as in Fig.
7.7
. The numerical
calculations are made for a distance 5 km from the seismic source. The ground
conductivity is taken as
D
0:1 S/m, that is a typical value for the upper layer of
moistened sedimentary rocks. Notice that the damped vibrations, arising after the
seismic wave arrival (shown with arrow), correlate in frequency with the seismic
vibrations.
7.2.7
Magneto-Dipole Approximation for the Diffusion Zone
To achieve better understanding of the phenomenon we shall now simplify the
general solution (
7.35
)-(
7.37
) for a spherically symmetric acoustic source and then
extend them to the source of arbitrary shape. First of all, notice that the factor
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