Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 346

et al. ( 1979 ) have used the electrometer with electric probes to measure the surface
charge density. It was found that in some areas the local density reaches the value
about 10 4 -10 2 C/m 2 which is much greater than that on the surface of undamaged
materials.

9.2.2 Radiowave Emission Resulted from Fracture

of Dielectric Solids

The rock fracture is frequently accompanied by intensive electrical discharge
processes (Deryagin et al. 1973 ; Klyuev et al. 1984 ; Lipson et al. 1986 ; Martelli
et al. 1989 ). The estimations show that characteristic times of the discharges are
about 0:1s and their typical linear sizes 10 2 -10 3 cm. The discharge processes
as well as a vibration of gas-discharge micro-plasma arising between crack walls
produce a wide-band electromagnetic emission from acoustic frequencies up to
UHF/EHF band. Impulsive radio-emissions have been observed under loading of
minerals, rocks, and ionic crystals (Gol'd et al. 1975 ; Nitsan 1977 ; Warwick et al.
1982 ; Bivin et al. 1982 ; Brady and Rowell 1986 ; Cress et al. 1987 ; Yamada et al.
1989 ; O'Keefe and Thiel 1995 ;Fridetal. 2000 , 2003 ; Goldbaum et al. 2003 ;
Hadjicontis et al. 2004 ). Notice that the same effect has been recorded during the
process of water crystallization possibly due to the microcracks formation in solid
phase (Kachurin et al. 1982 ).
One of possible causes of the phenomena could be piezoelectric effect in the
samples of quartz-bearing rocks (Nitsan 1977 ; Warwick et al. 1982 ). This electric
field generation is assumed to be at the moment of sharp stress drop occurring under
the fracture of sample. The experiments have been made on the samples with volume
from 1 to 10 4 cm 3 . Typical signals caused by fracture of samples look like a burst
of damped vibration with total duration from 5-10 s (tourmaline and quartz) to
200 s (sandstone, average size of grains is 1 mm).
The piezoelectric effect is not a non-unique one which is able to explain
observations. For example, the crack generation in basalt that does not include the
quartz is also accompanied by occurrences of the electric signals and of the light
flashes but these intensities are much smaller than those in the piezoelectric samples
(Brady and Rowell 1986 ; Cress et al. 1987 ). This effect has been observed by Bivin
et al. ( 1982 ) under destruction of a variety of materials such as vinyl-plastic, ebonite,
wood, sand, loam, plasticine and so on. In this experiment a striker throwing has
been employed to destroy the samples/targets. The velocity of the striker varies in
the interval 5-500 m/s. The electromagnetic field caused by the striker hit on target
was recorded by antennas placed about 1 m from the target. There are three kinds of
the antennas: pole, frame, and ferrite rod. The signals of the order of 10-50 mV have
been observed in all cases. Duration of the signals was 1-5 ms that approximately
corresponds to the time of the striker braking into target.