Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 344

Brovkin et al. ( 1990 ) used the electromagnetic technique based on the shock
demagnetization effect in order to distinguish individual detonations during a group
shot. The detonator bodies were magnetized prior to detonation which results in a
significant increase of the pulse amplitude. This phenomenon is caused by shock
demagnetization of the ferromagnetic components during the destruction of the
detonator body.

9.1.6 Remanent Magnetic Effects

Modification of crustal magnetization by strong explosions, EQs and by giant
impact events can be due to both the phenomena; that is, the shock-induced
remanent rock magnetization and partial or complete shock demagnetization of
pre-existing rock remanence. The irreversible changes in the local geomagnetic
field have been observed around the detonation point several minutes or hours after
the detonation moment (Stacey 1964 ; Barsukov and Skovorodkin 1969 ; Hasbrouk
and Allen 1972 ; Erzhanov et al. 1985 ). The effect of shock on the remanent
magnetization of rocks and other magnetic anomalies resulted from impact cratering
of planetary surface on the Mars (e.g., Hood et al. 2003 ; Louzada et al. 2011 ) and the
Moon (e.g., Halekas et al. 2003 ; Hood and Artemieva 2008 ) have been extensively
studied.
As one example, consider the irreversible magnetization of the granite samples
studied in laboratory conditions (Shapiro and Ivanov 1969 ). The cubic samples with
volume of 27 cm 3 were struck by the free falling weight with mass of 0.68 kg. The
height of the fall was 7 cm. The primary natural magnetization was 1.5-2.6 A/m.
A series of several tens strikes gives rise to additional irreversible magnetization
of the order of 2-5 A/m which was aligned with the vector of geomagnetic field.
Then the phenomenon of magnetic saturation was observed. Further tests with
different magnetically isotropic rocks immersed in a weak magnetic field (<1mT)
have shown that the shock remanent magnetization is linearly proportional to the
amplitude of the ambient magnetic field while the magnetization vector is parallel
to the magnetic field independently of the angle between the vectors of the shock
wave velocity and of the magnetic field (Nagata 1971 ; Pohl et al. 1975 ; Pohl and
Eckstaller 1981 ; Nagata et al. 1983 ; Gattacceca et al. 2008 ; Funaki and Syono 2008 ).
The shock remanent magnetization is different from thermoremanent magnetization
by larger magnitude and by shift of a coercivity spectrum towards lower values
(Gattacceca et al. 2008 ).
Macroscopic theory of this effect is based on the empirical parameters of rocks.
Let J be the vector of primary magnetization of a medium. It is generally accepted
to suppose that the increment of the magnetization, J , caused by elastic stress in
an anisotropic crystal can be expressed through the deviator of the stress tensor, s ij ,
in the following way (e.g., Dobrovolskiy 1991 )

3

X

3

J i D

2 C m

J j s ij ; (9.28)