Investigation of EM responses under shock impact on rock samples. II. Field experiments

The problem of the electromagnetic (EM) responses at mechanical shock impacts on rock samples of various structure and composition is studied experimentally. This part is a logical prolongation of previous laboratory investigations described in Part I, and presents the results of field investigations on studying the structure of EM emissions generated by shock and explosive (low power) impacts on large monolithic blocks of rocks of complex structure in the ULF and VLF frequency ranges with use of the field experimental complex specially developed. The mechanisms of excitation of seismic emission (for various sources) and the generation of the EM response at propagation of seismic wave through a mechanical system, in particular rocks, are considered. The fact of generation of the EM responses at impacts on various widespread quartz-containing rocks was confirmed; it has been established that the source of the EM emission in the ELF-VLF range are the inclusions of a crystalline piezoelectric excited by seismic wave. The existence of low and high-frequency parts of the response in the lower part of the VLF range of EM emission generated by impact, and the arising of modal structures in the high-frequency responses at impact to rocks in real conditions were detected. The proportionality of the piezoelectric response to seismic one, and an increase of the response amplitude due to the presence of an additional massive piezoelectric excited by means of a seismic wave are shown. No noticeable role of the structural destruction of the rock samples in the generation of responses was found, that gives a chance to their detection not only under hard impact (mechanical shocks, explosions), but also during natural seismic processes, that opens the perspectives for using the methods of prediction of seismic phenomena using the EM precursors of the earthquakes. The results obtained are in good agreement with our results obtained earlier in laboratory experiments and presented in Part I of this work. They are an intermediate stage between laboratory research and the registration of EM responses at powerful industrial explosions. A description of the results obtained when studying the structure of EM emissions caused by strong impacts at industrial explosions will be presented in the next part of the work.18-2