Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 329

and Pilorz 2012 ). Effects of shock magnetization and demagnetization have been
observed in magnetic materials. Fragments of fractured solid, as a rule, carry electric
charges. The rock deformation and fracture can explain, in principle, some features
of electromagnetic perturbations arising from large-scale tectonic processes such as
EQs, volcanic eruptions and deeply buried high-yield detonations. In this section we
first review laboratory studies and then examine these electromagnetic phenomena
by analyzing the basic physical processes at microscopic level. Sometimes we will
need to extend our research field to the frequency range of several MHz to gain a
better insight into underlying mechanisms of these phenomena.

9.1.1 Electromagnetic Fields Originated from Acoustic

Waves Propagation in Samples

Pioneering investigations of strain-induced polarization and depolarization in a solid
dielectric were provided by Stepanow ( 1933 ) who observed the appearance of an
electric potential difference between opposite sides of an ionic crystal under slow
strain. This effect cannot be explained solely by pyro-electricity or piezoelectricity
because it was observed in quite different materials. Caffin and Goodfellow ( 1955 )
and Fishbach and Nowick ( 1958 ) have shown that this phenomenon in ionic crystals
can result from the motion of charged dislocations under mechanical stress. The
same effect has been observed in a variety of materials subjected to dynamic
loadings (e.g., see the review by Mineev and Ivanov 1976 ).
Consider first a typical laboratory test of electric signals excited by acoustic
waves in dielectric samples. As usual the electrical signals are recorded by a
standard radio-antenna with ferrite core or a rod antenna, which is placed several
centimeters from the samples (for example, see Khatiashvily 1981 ). For the typical
frequencies of observed signals (1-7 MHz) these antennas are situated in the near
zone. Meanwhile these data are frequently interpreted as a radio emission, i.e., the
authors proceed from theoretical conceptions that are valid only within wave zone.
Therefore in fact the estimates of source parameters found in such a way are inexact.
The experiments with monocrystals of LiF, NaCl, and KCl have shown that
the acoustic wave and electromagnetic field stimulated by the acoustic one have
practically the same frequencies. It was found that the effect has the acoustic
intensity threshold. Above the threshold the magnitude of electric signals was 1 mV
for the annealed crystals. This value increases up to 4 mV with the increase of
dislocation concentration (Khatiashvily 1981 ). These features of the effect were
supposed to be due to excitation of stress-induced vibrations of the charged edge
dislocations (Molotskiy 1980 ) or vibrations and motion of the fluctuation-charged
walls of the micro-cracks (Khatiashvili and Perel'man 1982 , 1989 ).
The same effect stimulated by acoustic wave propagation has been observed
in pure metals (Misra 1978 ). The same mechanism; that is the bend vibration of
the charged segments of dislocations has been proposed by Molotskiy ( 1980 )to