Characterization of acoustic and electromagnetic emission sources

Abstract

The creation of cracks is accompanied by electric charge redistribution due to loosened chemical bonds. Electric charges on crack walls create dipole moments. Vibrations of crack walls produce time-dependent dipole moments and, consequently, electric and magnetic fields are generated. An electric signal is induced on metal electrodes. Information about the vibration of crack walls was obtained from this signal analysis. For crack lengths below 1 mm the electrical signal has a frequency of over 2 MHz. In this paper the frequency analysis was performed in a frequency band of up to 5 MHz.

Studying electromagnetic signal of rock effected by acoustic impact under uniaxial compression

“…The Czech researchers confirmed in their work the dipolar mechanism of EMS generation in the course of mechanical effecting on rock [5]. They thought that crack creations are followed by electric charge redistribution because of the weak chemical bonds.…”

A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust

“…Koktavy et al (2004); Misra and Kumar, 2004) are convincing, future work with the aim to use crack-related EMR as a tool to study crustal stresses would be desirable. Hence, further methodological development has to solve the problem to identify and measure micro-crack induced B EMR and its directional properties under unshielded field conditions.…”

“…These observations strongly restrict possible source mechanisms. Furthermore, it was observed that the EMR pulses measured during these experiments correlate with acoustic emissions related to microcracking events (Yamada et al, 1989;Koktavy et al, 2004;Rabinovitch et al, 2007;Mori and Obata, 2008). Therefore, different models have been recently discussed that relate the generation of natural EMR to the growth of micro-cracks (e.g.…”

“…Nitsan, 1977;Gomshei and Templeton, 1989). However, at laboratory scale, the generation of EMR was observed during deformation experiments on different materials, such as ice (Fifolt et al, 1993;, metals (Misra and Kumar, 2004), amorphous materials like glass ceramics (Bahat et al, 2002), and different rock types (Yamada et al, 1989;Enomoto and Hashimoto, 1992;Koktavy et al, 2004;Mori and Obata, 2008), mostly independent of their water saturation (Yoshida and Ogawa, 2004). These observations strongly restrict possible source mechanisms.…”

“…Laboratory experiments by Koktavy et al (2004) and Misra and Kumar (2004) show that crack-related EMR exhibits directional properties with the strongest intensities parallel to the orientation of the crack walls. Consequently, a crack-related model has to be able to explain the generation of electric potentials (dipoles) perpendicular to the crack walls ( Fig.…”