Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 348

by the same cause; that is, by the fast growth (during 1-100 s) of separate
microcracks. There are a number of indirect evidences that the sources of signals
were tension cracks but not shear ones. This conclusion is consistent with the fact
that the preparatory stage of failure is usually accompanied by accumulation of the
tension microcracks around the tip of the main shear macrocrack.
Occurrence of low-frequency electric signals prior to the failure has been
observed under the uniaxial compression of samples placed in a hydraulic machine
(Hadjicontis and Mavromatou 1994 ). The cube-shaped samples with size of 3 cm
made from natural materials (quartz, granite and limestone) were loaded with
constant velocity. The dislocation model has been proposed by the authors to explain
the dependence of the signal magnitude on the loading velocity. At normal condition
the dislocation segments are surrounded by clouds of the electrically charged point
defects. It was hypothesized that the dislocations bend and shift relative to the clouds
of the point defects. This effect,which in turn gives rise the medium polarization, can
be resulted from the stress increase.
The electromagnetic effects possibly related to the rock fracture in the field
experiments have been reported by O'Keefe and Thiel ( 1995 ). The electromagnetic
signals with the frequencies over 5 kHz have been detected during quarry detona-
tions at the distance of 60 m from the quarry. There are a few reasonable mechanisms
of this phenomenon: (1) the failure of the rock caused by the detonation, (2) the
electric discharges arising during the impact of the ground particles upon the shaft
bottom, and (3) micro-fracture at the fresh walls of the quarry due to the rock
unloading and stress relaxation. The last mechanism is supported by the fact that
the separate pulses were observed one minute after the detonation.

9.2.3 Optical Emissions

Observational evidence for optical glow under the brittle failure of certain dielectrics
(Belyaev et al. 1962 ; Deryagin et al. 1973 ; Brooks 1965 ; Thiessen and Meyer 1970 ;
Altier et al. 1979 ) and seignette-electrics/ferroelectrics (Chandra and Shrivastava
1978 ) have been reported although this phenomenon is not typical for all matters.
Brady and Rowell ( 1986 ) have studied the fracture of samples immersed in different
gases. The spectroscopic observations have shown that the glow of the fractured
samples only contains the typical emission lines of gases, which the samples were
placed in. Using the helium atmosphere they have found intensive emissions with
wavelength of 0.7065 m corresponding to emission transition between atomic
levels with energies 22.76 and 20.96 eV. The continuous spectrum due to emission
from the ionized region, which might be formed in the fracture zones, was not
revealed. Possibly, this part of the emission spectrum was outside the sensitivity
area of the sensors because the ionized atoms mostly radiate in the UV region.
It was proposed the atoms excitation is caused by bombardment of exoelectrons
emitted from the fracturing sample.