1. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, P. R. China
2. School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, P. R. China
3. Department of Physics, University of Alberta, Edmonton T6G 2R3, Canada
4. chool of Electrical Engineering, Chongqing University, Chongqing 400044, P. R. China
5. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, P. R. China