Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 438

back to the former level was lasted during 24 h. Hasbrouk and Allen ( 1972 )have
reported magnetic measurements during the underground nuclear explosion, which
is referred to as CANNIKIN experiment. The explosion with TNT equivalent of
5 Mt was conducted on the Amchitka Island (Aleutian Islands) on January 6,
1971. The proton magnetometer, which was placed at the epicentral distance of
3 km, recorded the gradual increase of the magnetic field by 9 nT in 30 s after the
detonation. The field variation about 2 nT was detected at the distance of 9 km. The
magnetic survey around the epicenter of detonation revealed that the 10 nT changes
of the geomagnetic field were kept approximately constant for 8 days.
Thus the magnetic perturbations due to the detonation in rock could be con-
ventionally divided into three stages (Erzhanov et al. 1985 ): (1) the transient
alternating-sign pulse (EMP) with duration smaller than 1 s and with magnitude
0.1-100 nT; (2) the 10-20 nT residual changes which can relax during several hours
or days; (3) the long-term residual changes with magnitude of several nT that can
last for several days or months.
It was hypothesized by Stacey ( 1964 ) and by Undzenkov and Shapiro ( 1967 ) that
the residual magnetic perturbations near the detonation site are excited by means of
changes in the natural rock magnetization which in turn are based on the occurrence
of inelastic/plastic deformations in the rock. As noted in Sect. 9.1 , the laboratory
tests with magnetite-bearing rocks have shown that the sample magnetization can
change by 1% under the stress of 10 MPa. Restoration of the local geomagnetic
field back to the former level could be resulted from the relaxation of inelastic
deformation in the rock. However this effect is likely if the rock contains sufficient
amount of the ferromagnetic inclusions.
To estimate the above effect, we consider the model in which the SW and resid-
ual stresses around the detonation site are spherically symmetric (Surkov 1989 ).
The SW magnitude exceeds the crushing strength of the rock in the vicinity of the
powerful explosion. The crushed zone is assumed to have a spherical shape with
radius of R c . As a rule this radius is of the order of several tens meters. The residual
magnetization in this zone is probably chaotic owing to the repacking of the broken
rock. Therefore the contribution of this zone to magnetic perturbations is neglected
as compared to the residual rock magnetization which occurs in the region r>R c .
In this region the SW magnitude is lower than the crushing strength but higher than
the tensile strength of the rock. The medium is monolithic in character while there
occur separate large cracks. The typical size of this region is of the order of several
hundreds meters, and the rock deformation is elastic and reversible from outside of
this zone.
The primary rock magnetization, J , is assumed to be constant. In the region
r>R c the magnetization increment, J , due to the SW is described by Eq. ( 9.29 ),
where s n D s rr denotes the magnitude of radial component of the stress tensor.
The stress magnitude depends only on the distance r from the explosion point. The
effect of residual rock magnetization ceases at the certain radius R e >R c when the
stress magnitude falls short of certain threshold stress so that in the field r>R e
the residual magnetization is absent. The magnetic permeability of the rock inside
the zone R c <r<R e can be slightly different from that of surrounding medium.