Impact compressive and bending behaviour of rocks accompanied by electromagnetic phenomena

Abstract

It is well known that electromagnetic phenomena are often observed preceding earthquakes. However, the mechanism by which these electromagnetic waves are generated during the fracture and deformation of rocks has not been fully identified. Therefore, in order to examine the relationship between the electromagnetic phenomena and the mechanical properties of rocks, uniaxial compression and three-point bending tests for two kinds of rocks with different quartz content, granite and gabbro, have been carried out at quasi-static and dynamic rates. Especially, in the bending tests, pre-cracked specimens of granite were also tested. Using a split Hopkinson pressure bar and a ferrite-core antenna in close proximity to the specimens, both the stress–strain (load–displacement) curve and simultaneous electromagnetic wave magnitude were measured. It was found that the dynamic compressive and bending strengths and the stress increase slope of both rocks were higher than those observed in static tests; therefore, there is a strain-rate dependence in their strength and stress increase rate. It was found from the tests using the pre-cracked bending specimens that the intensity of electromagnetic waves measured during crack extension increased almost proportionally to the increase of the maximum stress intensity factor of specimens. This tendency was observed in both the dynamic and quasi-static three-point bending tests for granite.

Electromagnetic Radiation Generated by Acoustic Excitation of Rock Samples

“…It was observed that the increase in EMR signal amplitude measured for the magnetite ore containing quartz was tenfold that of the ore not containing quartz. Similar results were obtained by other researchers (Wan et al 2008, Kobayashi et al 2014. In particular, the authors of (Kobayashi et al 2014), in their experiment on dynamic compression of rocks, observed that the EMR signal amplitude for gabbro with 2 vol.% quartz content was one sixth of that for granite with 36 vol.% quartz content.…”

“…Similar results were obtained by other researchers (Wan et al 2008, Kobayashi et al 2014. In particular, the authors of (Kobayashi et al 2014), in their experiment on dynamic compression of rocks, observed that the EMR signal amplitude for gabbro with 2 vol.% quartz content was one sixth of that for granite with 36 vol.% quartz content. Crystalline quartz inclusions in magnetite ore are randomly distributed over the volume, and this may account for the great spread in EMR signal amplitude from sample to sample in our experiments.…”

“…In the last few years, a number of laboratory experiments have been performed to investigate the EMR generated on compression and fracture of rock samples (Lacidogna et al 2011, Koktavy 2009, Sobolev et al 2010, Kobayashi et al 2014. The reverse seismoelectric effect, i.e., generation of elastic vibrations by a rock in an AC electric field, is also essential to the understanding of the EMR physics (Ponomarev et al 2002, Sobotka 2004, 2009a.…”

“…The reverse seismoelectric effect, i.e., generation of elastic vibrations by a rock in an AC electric field, is also essential to the understanding of the EMR physics (Ponomarev et al 2002, Sobotka 2004, 2009a. The investigations were aimed both at verifying some EMR generation models (Frid et al 2003, Koktavy et al 2004, Koktavy 2009 and at finding relationships between the rock properties and the associated electromagnetic phenomena (Sobolev et al 2010, Wan et al 2008, Kobayashi et al 2014. Recently, neutron bursts have been detected to occur on fracture of quasi-brittle materials such as rocks.…”

Application of an Integrated and Self-contained Electromagnetic Acoustic Recorder for Monitoring the Rock Mass Structure and Development of Geodynamic Processes in Ore Mines

“…Electromagnetic radiation (EMR) occurring in the rocks under mechanical impact has been studied for many years in many countries around the world [1][2][3][4][5][6][7]. The key motive of these studies is high potential of EMR to be used for predicting geodynamic events such as earthquakes, rock bursts, landslides and others.…”