High-resolution inversion imaging method for urban shallow subsurface structures using small-loop transient electromagnetic RDI-SA
The small-loop transient electromagnetic detection system has the advantages of short shutdown time and a small transmitting coil area,resulting in a small detection blind zone and high lateral resolution.Urban shallow subsur-face structures,compared to natural resource exploration objects,have characteristics such as small scale,variable shapes,and significant differences in physical and background parameters.In the inversion calculation,solving a large spatial range leads to prominent problems of multi-solution and convergence.This study proposes a resistivity depth ima-ging(RDI)-simulated annealing(SA)combined inversion method.The transient electromagnetic data is used for re-sistivity depth imaging.The imaging results are utilized to constrain the range of the model parameters to be solved.This guides the determination of the initial state of the SA algorithm and forms the upper and lower bounds of the parameter updating process,improving and optimizing the SA inversion process.Two typical urban underground structures,shallow municipal transportation tunnels and civil air defense chambers,were selected.A small-loop transient electromagnetic detection system developed by ourselves was used for practical detection analysis.The results show that the RDI-SA in-version imaging method accurately restores the spatial characteristics of underground structures.
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