Experimental study of ultralong wave band for electro－magnetic signals and acoustic emission during rock fracture

Ｅｘｐｅｒｉｍｅｎｔａｌ　ｓｔｕｄｙ　ｏｆ　ｕｌｔｒａｌｏｎｇ　ｗａｖｅ　ｂａｎｄ　ｆｏｒ　ｅｌｅｃｔｒｏ－ｍａｇｎｅｔｉｃ　ｓｉｇｎａｌｓ　ａｎｄ　ａｃｏｕｓｔｉｃ　ｅｍｉｓｓｉｏｎ　ｄｕｒｉｎｇ　ｒｏｃｋ　ｆｒａｃｔｕｒｅ（曹惠馨）（钱书清）（吕...

Experimental study of ultralong wave band for electro-magnetic signals and acoustic emission during rock fracture

The electro-magnetic (EM) and acoustic emission (AE) in the fracture process of both dry and wet rock samples including gabbro, granite, sandstone and marble are studied in the experiment in the laboratory under uniaxial pressure. Signals during the test are detected in ultralow frequency band. The experimental results may be outlined as follows: original waveforms of the electro-magnetic radiation (EMR) and AE in the range of 0-20 kHz frequencies are obtained; the EMR's intensity in fracture process is related to some factors as rock type, content of water, fracture intensity of rocks, loading rates and fracture state etc.; the EMR'S rate is proportional to that of AE during fracture statistically. About 70% of maximum values of EMR rate and AE rate are corresponded with each other; furthermore, the EM signals (EMS) are generally more developed and longer duration than AE signals; the principal energy of EMS detected by copper coin antenna as sensor in power spectra are distributed in frequencies lower than 6 kHz and otherwise those detected by EMinduction coil are at about 10 kHz, and in contrast the major energies of AE are in frequencies less than3 kHz; another notable phenomenon is that distinct electro-magnetic signals are detected before AE occurrence. Since EMR and AE signals obtained in the process of rock fracture are better corresponded to eachother, therefore it'S reasonable to propose that the EMR anormalies observed prior to some earthquakes aremainly produced by fractures of rock in source region, as for the EMS occurred before AE may be related topiezoelectric and piezomagnetic effects.