Three-dimensional inversion analysis of transient electromagnetic response signals of water-bearing abnormal bodies in tunnels based on numerical characteristic parameters.

The transient electromagnetic inversion of detection signals mainly depends on fast inversion in the half-space state. However, the interpretation results have a certain degree of uncertainty and blindness, so the accuracy and applicability of the three-dimensional full-space inversion need to be investigated. Two different three-dimensional full-space inversions were carried out. First, the numerical characteristic parameters of the response signals were extracted. Then, the correlations between the numerical characteristic parameters and physical parameters of the water-bearing abnormal bodies were judged, and the judgment criterion of the iterative direction was proposed. Finally, the inversion methods of the iterative algorithm and the BP neural network were utilized based on the virtual example samples. The results illustrate that the proposed numerical characteristic parameters can accurately reflect the response curve of the full-space surrounding rock. The difference in the numerical characteristic parameters was used to determine the update direction and correction value. Both inversion methods have their advantages and disadvantages. A single inversion method cannot realize the three-dimensional inversion of the physical parameters of water-bearing abnormal bodies quickly, effectively and intelligently. Therefore, the benefits of different inversion methods need to be considered to comprehensively select a reasonable inversion method. The results can provide essential ideas for the subsequent interpretation of the three-dimensional spatial response signals of water-bearing abnormal bodies.

Characterisation of transient electromagnetic signals during fixed interference sources in tunnel structure.

The detection effect of the transient electromagnetic method is ambiguous in engineering applications due to the existence of interference sources, so explaining the influence of these fixed interference sources on is crucial. In this paper, the response characterisation of transient electromagnetic signals of fixed interference sources are thoroughly investigated. First, the secondary field generated by these interference sources is analyzed, and a typical fixed interference source is calculated. Then, a sensitivity analysis of the transient electromagnetic response curve is carried out. Finally, the mathematical superposition method for multiple field sources is proposed and verified. The results illustrate that the transient electromagnetic response curve of uniform full-space surrounding rock with a single fixed interference source has an apparent lifting phenomenon in the middle stage and presents an approximate horizontal change rule at the late stage. The transient electromagnetic response curves of multiple field sources separately illustrate the response characterisation of different field sources at different time stages. These research results can provide a valuable reference for the on-site interpretation of detection signals.

The research on minimizing the induction between the transmitting and receiving coils in close range transient electromagnetic inspection of groundwater-related defects in the operating tunnels.

The striking dominance of groundwater-related defects in the operational high-speed railway tunnels in China calls for swift and accurate detection and identification. Thus, it is a new attempt to detect the water-bearing defects at 5 to 10 meters via train-borne transient electromagnetic method in operating tunnels. Due to the short detection distance, the interaction between transmitting and receiving coils is more important than those normally used coils. Thus, numerical and experimental methods are combined to investigate the mutual induction. The influence of turns, current and coil size on the mutual induction and the impact of damping coefficient on the receiving system are manifested. To further verify these findings, full-scale model experiments are conducted. During these physical experiments, the detection results of different coil parameters including coil size, number of turns, and emission current are compared and analyzed. Then, a special effort to minimize the induction between transmitting and receiving coils is expended to acquire the suitable coils for close range detection in the tunnel context. Finally, in order to verify the availability of the detection system, different detection distances are conducted. It turns out that different detection distances have slight difference at the detection results, but they are still within the measuring range of the detection instrument. Obviously, these findings can provide theoretical support for the detection of water-bearing anomalies in operating tunnels and it also has reference significance for the detection of anomalies at close distance.