A Surface-to-Borehole TEM System Based on Grounded-wire Sources: Synthetic Modeling and Data Inversion
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Pure and Applied Geophysics
A Surface-to-Borehole TEM System Based on Grounded-wire Sources: Synthetic Modeling and Data Inversion WEIYING CHEN,1,2,3 SIXU HAN,4 MUHAMMAD YOUNIS KHAN,5 WEN CHEN,1,2,3 YIMING HE,1,2,3 LINBO ZHANG,1,2,3 DONGYANG HOU,1,2,3 and GUOQIANG XUE1,2,3 Abstract—The surface-to-borehole transient electromagnetic (TEM) method, which uses a rectangular loop as its source, has shown great potential in mineral exploration at greater depth. However, recent studies have demonstrated that a grounded-wire source can resolve the resistive targets and provide detection at greater depths than the loop source. Thus, we propose a surface-toborehole TEM method using a grounded wire as a transmitter at short offsets. We conducted numerical experiments to explain the variation of different parameters and its comparable effects on the TEM transients. Similarly, a systematic comparison of the crosssectional electric field distribution maps, borehole profiles of the time derivatives of the magnetic field, qualitatively reveal that the proposed method induces detectable responses for conductive and resistive targets at greater depths. In the present study, unlike the loop source configuration, the time derivative of an underground magnetic field in horizontal direction showed high sensitivity to resistive bodies. Furthermore, 1-D inversion was performed to provide quantitative interpretation of the borehole data. The 1-D inversion of the surface and borehole data was realized using an Occam-type inversion scheme. The results of both synthetic and field data indicate that inverting the surface or borehole data separately resulted in degraded resolution in both shallow and deep earth. Thus, we also compared the results from the inversions of surface and borehole data in order to explore the merits of joint inversion in resolving shallow and deep structures. Keywords: Transient electromagnetic method, electric source, surface-to-borehole, underground, Occam’s inversion.
1 Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. E-mail: [email protected] 2 Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China. 3 University of Chinese Academy of Sciences, Beijing 100049, China. 4 Geophysical Prospecting Team, Guangdong Geological and Mineral Bureau, Guangzhou 510800, China. 5 National Centre of Excellence in Geology, University of Peshawar, Peshawar 25120, Pakistan.
1. Introduction The transient electromagnetic (TEM) method has been used for mineral, groundwater and hydrocarbon exploration (Fitterman and Stewart 1986; Williams and Fraser 1992; Maher 1993; Mu¨ller et al. 2002; Barfod et al. 2016). Conventionally, both transmitter and receiver are deployed on the earth surface in a ground-based TEM survey. Among the ground-based TEM methods, grounded-wire TEM has been successfully applied on land and in marine environments, and has shown greater detection depth and higher sensitivity to resistive targets than methods us
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