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where C
m
denotes the piezomagnetic coefficient which is varied within 0.5-
1.7 GPa
1
(e.g., Stacey
1964
). More usually we deal with the isotropic magnet so
that the directly proportional dependence is used to analyze the laboratory tests
J
D
C
m
J
s
n
;
(9.29)
where s
n
is normal component of the stress tensor or pressure.
In Chap.
11
we use this equation in order to estimate the effect of irreversible
rock magnetization under large-scaled surface and underground detonations.
9.2
Electromagnetic Effects Caused by Fracture
of a Solid Dielectric
Here we dwell on the laboratory observations of HF and VHF electromagnetic
signals, particle emissions microdischarges, and other electromagnetic effect caused
by the fracture of samples. These experiments have served such purposes as
understanding of the solid fracture process, the development of nondestructive
testing of inelastic deformations during solid loading, examination of anomalous
electromagnetic phenomena possibly associated with EQs, volcano eruptions and
etc.
9.2.1
Electrical Charges on the Surface of Fractured Solid
Fracture and intensive deformation of a solid results in the formation of static
and transient electromagnetic fields in a wide band of frequencies. As usual the
surfaces of fractured samples contain chaotically distributed charged areas with
different signs. For example, the surface charge density of the order of 10
7
-
10
8
C/m
2
have been measured upon the fracture of basalt, peridotite, fine-grained
marble, polymethylmetacrylate/plexiglas, and other materials (Parkhomenko
1968
;
Shevtsov et al.
1975
; Balbachan and Parkhomenko
1983
). We notice that the results
of these experiments depend on moisture of the rock. The local charge density on the
surface of fractured monocrystals of alkaline-halogen compound reaches the value
.5-7/
10
7
C/m
2
(Urusovskaya
1968
; Kornfeld
1975
,
1978
).
Below we examine the dynamic laboratory tests with dry rocks. Heterogeneous
electric structure of the surface is typical for both fractured and undamaged crystals
except for extra pure crystals prepared in the vacuum. In the undamaged crystals the
linear sizes of negatively and positively charged macro-areas are about 50-500 m.
The rock fracture results in appearance of the new charges occurring on the newly
formed fresh surfaces of the fractured material. These fresh charges are distributed
in the form of fluctuation mosaic areas with positive and negative signs. Finkel
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