Inversion of TEM Responses in Tunnel with Steel Infrastructure and Its Application

Steel infrastructure in tunnels significantly affects the accuracy of transient electromagnetic (TEM) data inversion. This study proposes and compares two methods to effectively mitigate this interference: the correction factor method and the prior information constraint method. The correction factor method constructs correction factors using interference data obtained in two ways. The first method, numerical simulation, is theoretically precise but relies heavily on accurate model parameters and requires significant computational resources. The second is conducted by measuring the interference data in the working space. While this approach is simple and operationally convenient, its effectiveness in correction is limited by the measured data quality. The a priori information constraint method improves the inversion by introducing a more accurate initial model. The inversion results of synthetic data indicate that the inversion based on the correction factor method and the prior information constraint method can effectively invert the anomaly. However, the delay effect remains a challenge for the correction factor method. Finally, we invert the field data measured in a mine tunnel by combining the interference processing methods.