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where V is the mass medium velocity. Substituting Eq. ( 9.5 )for@ t into Eq. ( 9.4 )
we finally find the expression for the current density
" 0 " F
e r
2 V :
j d
(9.6)
This phenomenon is referred to as the piezo-galvanic effect (e.g., see Gurevich
1957 ). In order to estimate the amplitude of this effect we note that LJ LJ r
2 V LJ LJ
V= 2
D Vf 2 =C l , where and f are the wavelength and frequency, respectively.
Then we get j d " 0 " F Vf 2 = eC l while the potential difference within a
wavelength is ' " F V=.eC l /. Taking the following numerical values of the
parameters " F D 5 eV, V D 5 mm/s, f D 100 kHz and C l D 5 km/s, we find that
j d 0:1 nA/m 2 and ' 3V. Notice that the potential difference between the
edges of the metal sample can reach a much bigger value of about 10-100 mV under
the shock compression of several metals (Mineev and Ivanov 1976 ). The relaxation
time of this short-term effect is about 0.1-1 s.
The field experiments on the ore deposit of poly-metals have shown that the
presence of the conductive bodies or inhomogeneities in a rock could promote
the amplification of radio-emission caused by acoustic wave propagation (Sobolev
et al. 1980 ). In these experiments the acoustic waves were excited due to the
detonation of HE with masses of 2-6 kg. The measurements of the electric field were
performed at the distance 120 m from the explosion site and 130-140 m from the ore
body/inclusion. The electromagnetic signals in the frequency band of 0.2-3 MHz
were recorded for the time when the explosive wave crosses the ore body/inclusion.
9.1.2
Shock Polarization of a Dielectric
A similar effect arises in the course of propagation of a shock wave (SW) in a
dielectric. The schematic plot of a typical laboratory experiment is shown in Fig. 9.1
(Mineev and Ivanov 1976 ). The investigated sample (1) was sandwiched between an
electrode (2) and a metallic screen (4). The arrows indicate the direction of plane SW
propagation. The electrode and the protective ring (3) were prepared from metals
which have an acoustic impedance close to the impedance of the sample material.
Input resistance of the oscillograph and the load resistance of the protective ring
are shown by R and R 1 . The parameters of the circuit were selected in such a way
that it was equivalent to that of short-circuited capacitor, with plates consisting of
the screen and electrode. The explosive device to produce the shock and the sizes
of the sample was selected in such a way that the damping of a plane SW and
the influence of lateral unloading can be neglected for the time of SW propagation
through the sample.
An electric current is observed in the external circuit immediately upon initiation
of the SW. This signal continues for the duration of SW propagation in the sample.
The external circuit has no current supply; that means that the formation of the
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