Electromagnetic Emissions During Rock-fracturing Experiments Inside Magnetic Field Free Space

Abnormal electromagnetic emission (EME) signal is one type of the most important precursors before earthquake, which has been widely observed and recorded before large earthquake, but the physical mechanism underlying the phenomenon is unclear and under controversy. Monitoring the EME signals during rock-fracturing experiments in laboratory is an effective way to study the phenomena and their underlying mechanism. Electromagnetic noise is everywhere because industrial and civilian electrical equipments have been widely used, which make difficulties to the in-lab experiments and field monitoring. To avoid the interference from electromagnetic noise, electromagnetic experiments must be carried out inside shielded space. Magnetic Field Free Space (MFFS) was constructed by Institute of Geophysics, China Earthquake Administration in 1980s. MFFS is a near-spherical polyhedron 'space' with 26 faces and inside diameter about 2.3 m. It is enclosed by 8-layer permalloy 1J85 for shielding magnetic field and 2-layer purified aluminium for shielding electric field. MFFS mainly shields static magnetic field by a factor of 160-4000 for the magnetic signals with the frequencies ranging from 0.01 Hz to 10 Hz. The intensity of magnetic field inside the space is less than 20 nT and its fluctuation is less than 0.3 nT in 90 hours. MFFS can dramatically shield EME signals in the frequency range of EME antennas utilized in our experiments, (several to ~320) kHz, by at least 90%, based on observation. Rock specimens (granite, marble) were fractured by two ways inside MFFS. 1) Cuboid bulk specimens were drilled, filled with static cracking agent, and then dilated from inside until fracture. 2) Cylindrical rock specimens were stressed until fracture by using a non-magnetic rock testing machine with the maximum testing force 300kN. EME, acoustic emission (AE) and strain signals were collected synchronously by the same data acquisitor, Acoustic Emission Workstation made by Physical Acoustics Corporation in America. EME and AE signals were collected at sampling rate larger than 1 MHz, while strain signals at sampling interval of 0.1 second. Abnormal strong EME signals were usually observed at the same moment with the main crack indicated by strain signals. There are too many AE signals but too little EME signals collected during the rock-cracking experiments. Even AE signals with amplitude larger than 60dB were hardly found accompanied by EME signals. Few data indicate that EME and AE signals occur simultaneously. Abnormal EME signals were always detected after rock specimens were cracked, indicated by the torn strain gauges. The lag of EME signals behind the cracking signals revealed by strain gauges may indicate that EME signals were caused by the fast movement of charged rock fragments. This study was sponsored by National Natural Science Foundation of China (NSFC, grant 41004029) and Institute of Geophysics, CEA (Project No.: DQJB10B14). We would like to thank Shuqing Qian, Zhiwei Guo and Zhengyi Liu to take part in the experiments.