A study on piezoelectric effect of Rock

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Electromagnetic (EM) signal emitted from compressed and/or fractured rocks has been studied for many years. Some kinds of rocks contain many piezoelectric materials, for example, quartz. Therefore some researchers consider the piezoelectric effect as the dominant cause of the EM signals.An experiment was carried out in which EM signal was emitted from a block of granite which contains a lot of quartz. The block of granite was manually impacted with a hammer ; a shockless hammer or a standard steel hammer. As a result of the experiment, it was found that EM signal was emitted on hammering. The first phase of the. observed EM signal was similar to strain measured by strain gauge affixed on the steel hammer. EM signal was also observed simultaneously by the electric robes on the surface of the block. Each electric robe was located at position separated by a distance from the hammered point. No time lag or delay was observed in this case. The primary/peak value of the observed voltage was inversely proportional to the distance from the hammered point. The same results were observed in all directions through the experiments. The later part of the EM signal was similar to the elastic wave observed with an acoustic emission (AE) sensor. The elastic wave signal observed with the AE sensor was delayed in proportion to the distance from the hammered point. It can be concluded that the EM signal is relatively large and potentially used for civil or mining purposes.