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B
, nT
2
2
1
1
r
, km
0
2
4
6
8
10
-1
-2
Fig. 11.7
Remanent changes in the Earth magnetic field resulted from the surface detonation of
HE with mass 251 t as a function of distance from the detonation point.
1
—Experimental data
(adapted from Erzhanov et al.
1985
);
2
—model calculations (Surkov
1989
)
only a rough estimate of the phenomenon because we do not take into account the
effect of medium unloading near the free surface.
The remanent changes of the Earth magnetic field measured in the case of
MASSA experiment are shown in Fig.
11.7
with line 1 (Erzhanov et al.
1985
). The
numerical calculation shown with the dotted line 2 was made under the following
parameters: P
c
D
0:1 GPa, C
m
D
1 GPa
1
, R
c
D
100 m, and J
D
0:12 A=m. It
is obvious from this figure that the theoretical and experimental dependencies are
close except for the region of r<0:5km. Actually the surface detonation generates
the non-spherically symmetric SW. The study of such a problem has shown that the
asymmetry of the rock magnetization can lead to an increase of the above estimate
(Surkov
1989
).
The additional effect can be due to the impact action on technical constructions
and installations which are magnetized under the shock and vibrations in the Earth
magnetic field. The mechanism of this effect has been discussed more fully in
Sect.
9.1
. One of such construction is the steel casing/encasement pipe which
is used in order to protect utility lines from getting damaged and for lowering
the explosive device in the rockhole. Vibrations of the casing pipe due to SW
propagation can result in the pipe magnetization thereby producing local changes
of the geomagnetic field.
One more effect can be due to the redistribution of natural and man-made telluric
currents flowing in the rock around the place of underground detonation. The major
origin for the man-made current is believed to be the electrochemical processes at
the interface between the casing pipe and surroundings. The occurrence of the con-
tact potential difference at the surface of the metallic pipes can be associated with
the difference between mechanisms of conductivity, namely electronic conductivity
in the metal and ionic one in the rock surrounding the pipe. It should be noted
that the similar effect is usually observed in the vicinity of ore deposit (e.g., see
Semenov
1974
). The contact electromotive force at the casing pipe surface depends
on the depth because the mineral content of underground water varies with depth.
As a result the currents develop in both the casing pipe and the rock around the pipe.
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