Analysis of multi-component response characteristics of surface-to-borehole transient electromagnetic method with complex-shaped loop source
WANG Zhi-Xin1,2(), DENG Ju-Zhi1,2(), CHEN Hui1,2, QIU Chang-Kai3, YU Hui1,2, YIN Min1,2, FENG Min1,2
1. State Key Laboratory of Nuclear Resources and Environment, Nanchang 330013, China 2. School of Geophysics and Measurement-control Technology, East China University of Technology, Nanchang 330013, China 3. Development and Research Center, China Geological Survey, Beijing 100083, China
In actual exploration, the surface-to-borehole transient electromagnetic (TEM) method is prone to be affected by irregular transmitter loops and inclined boreholes, resulting in increased measurement errors of the three-component induced magnetic fields and decreased interpretation accuracy. By establishing surface-to-borehole TEM method-based three-component measurement models under the excitement of transmitter loops of various shapes, this study derived the calculation formulas for surface-to-borehole TEM responses under inclined boreholes through coordinate transformation. Then, it achieved one-dimensional forward modeling of the full-space TEM field using a numerical filtering algorithm. The calculation results of multiple typical models indicate that the three components of the transient magnetic fields were significantly influenced by the shapes of the transmitter loops, with the impacts on horizontal components x and y far more significant than those on vertical component z. The transmitter loops of regular polygons with even edges as the sources exhibited uniform and symmetric distribution of the TEM fields of the three components. Under the condition of the same perimeter, a greater number of edges of the transmitter loops associated with greater primary field energy excited by the loops. Therefore, rectangular transmitter loops as the sources prove the most cost-effective. The inclination and offset primarily affected the amplitude of the three-component responses. In contrast, the borehole azimuth mainly influenced the sign of the horizontal components, bearing rich information on location. Therefore, in the exploration using the surface-to-borehole TEM method, it is necessary to accurately determine source paths and the geometric morphologies of boreholes to make essential corrections, thus improving the accuracy and reliability of interpretations.
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