Some polymetallic ore bodies can produce pulsed electromagnetic radiation in radio frequency band under the action of acoustic elastic waves. This seismoelectric effect is used as a new geophysical method-focal electromagnetic radiation method to find ore bodies containing galena, sphalerite, pyrrhotite, chalcopyrite, arsenopyrite, stibnite, molybdenite and cinnabar.
As a precursor phenomenon of earthquakes, the electromagnetic radiation effect accompanying rock fracture was first studied by seismologists in China. In the indoor experimental study of specimens, all specimens are concentrated on rock samples (mainly granite, quartzite and limestone), but the published experimental results are quite different from the understanding.
According to the phenomena of electromagnetic radiation, acoustic emission and strain observed by Institute of Mineral Deposits, Chinese Academy of Geological Sciences and Hefei University of Technology during uniaxial compression of more than 100 rock and mineral samples, the following understandings can be obtained:
1) pulsed electromagnetic radiation is related to the formation, development and disintegration of the specimen after compression. Before the main rupture, there may be many electromagnetic signals corresponding to micro-rupture (sometimes as many as 5 ~ 7 times). The distance obtained by multiplying the first arrival time difference of a large number of obvious electromagnetic waves and acoustic signals by the propagation speed of acoustic waves in the air is just within the distance from the acoustic sensor to the specimen volume, so there is no doubt that the electromagnetic signal is caused by the fracture of the specimen.
2) The generation of pulsed electromagnetic radiation is related to the composition and structure of rocks and ores. The experiments show that obvious electromagnetic radiation signals are observed in most samples of rocks and ores with time or with a certain amount of time, but no electromagnetic radiation signals larger than the interference level are observed in most samples of rocks and ores without time or with a small amount of time. The mechanism of radiating electromagnetic waves in rock and mineral fractures is complex, and scholars at home and abroad have put forward many hypotheses in their continuous exploration.
3) The electromagnetic radiation produced by rocks and ores in the process of fracture is a broad-spectrum radiation in the radio band range of several hundred kilohertz to several megahertz, but lower-frequency electromagnetic radiation can also be seen.
Fig. 4.28 Comparison between interpretation results of Sullivan BC mining area and borehole lithology data.
(2) Working methods and applications
The working method of source electromagnetic radiation method is similar to piezoelectric method. But its characteristics are as follows: ① In order to avoid radio wave interference, it is generally carried out underground; ② The energy demand of elastic wave excited by seismic source is large; ③ The receiving and sampling frequency of using instruments should be high.
Figure 4.28 is the interpretation result of electromagnetic radiation method in a foreign mining area. As can be seen from the figure, the source position of seismoelectric response determined by arc method is roughly consistent with the borehole data, but some are inconsistent.
To sum up, the source electromagnetic radiation method is a new geophysical exploration method, which is still in the exploration stage. Therefore, it is necessary to further study the physical properties of rocks and ores and carry out mining experiments in order to gradually understand its mechanism and apply it reasonably in practice, so as to improve this method day by day.