1. Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, P. R. China
2. College of Physics and Electronics, Hunan Institute of Science and Technology, Yueyang 414000, P. R. China
3. Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN 37996, USA
4. Key Laboratory of Pollution Ecology and Environment Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, P. R. China