Research On Ahead Geological Prospecting In Tunneling Of Fixed Loop TEM Based On Coil Inductance And Magnetic Viscosity Effect Suppression

Transient electromagnetic method(TEM)is a widely used electromagnetic detection method in various research fields.Fixed-loop transient electromagnetics,as a common array form,have the advantages of high detection efficiency and large detection depth.The fixedloop TEM used in tunnels is different from the traditional large fixed-loop TEM.Due to the limitation of the tunnel cross-section,the use of multi-turn small coils as the transmitting coil generates strong interference,especially near the wire of the loop,where the effects of mutual inductance between the transmitting and receiving coils are particularly significant.Therefore,when interpreting the data obtained by the fixed-loop TEM,it is necessary to first eliminate the effects of relevant interference,and then interpret the transient data at non-central points.Starting from the calculation of the transient electromagnetic secondary field,this study proposes an intuitive approximate calculation method for the non-central point response of transient electromagnetic fields,which is used to calculate the transient electromagnetic response at any point within a transmission coil.Based on the traditional equivalent conductive plane method,an approximate model is established,and on this basis,correction formulas and algorithms are derived for the transient electromagnetic response at any observation position within the transmission line.One-dimensional half-space and layered geological formations are simulated numerically to verify the correction formula,and the results show that the correction coefficient can quantitatively reflect the variation of the transient electromagnetic response with offset distance under different time channels.During actual small fixed-loop transient electromagnetic detection,it is easy to be interfered by various factors,leading to abnormal non-central point transient responses that differ from the secondary field variations.Based on previous detection experience and relevant theoretical analysis,it is necessary to suppress and remove the influence of coil inductance and hysteresis response by considering the detection conditions and geological information.The respective variations and influencing factors of coil inductance and magnetic viscosity effect response under different offset distances have been studied and discussed.Then,the transient responses with added inductance and magnetic viscosity effects were forward modeled to discuss their interference with transient electromagnetic responses and the variation patterns of the total response.Based on the study of influencing factors of non-central point responses,a fixed-loop transient electromagnetic interpretation method based on the suppression of coil inductance and magnetic viscosity effects is proposed.By performing noise elimination and secondary field correction on the measured data of uniform layered geological formations under different geological conditions,the inverted results of the processed transient responses are consistent with the drilling information of the actual geological formations,and the variation pattern of the secondary field with offset distance is consistent with the theoretical analysis results.Finally,the method proposed in this paper is applied to an actual tunnel ahead geological prospecting project.The range and depth of anomaly was accurately delineated,and the results are verified by advance drilling,successfully predicting the presence of a water-bearing cave in front of the tunnel face.The impact factors and variation patterns of the transient response with offset distance within the fixed-loop were studied and discussed through an equivalent conductive plane model.Based on this,a correction calculation was proposed for fast approximate interpretation of the fixed-loop TEM.Further research was conducted on the factors affecting the transient electromagnetic response within the loop,based on the variation patterns of the secondary field response.A small fixed-loop transient electromagnetic interpretation method based on suppression of coil inductance and magnetic viscosity effects was proposed.This study provides a theoretical basis and foundation for further research and data interpretation of small fixed-loop TEM,and can also serve as a reference for forward and inverse calculations of noncentral point transient electromagnetic responses in large fixed-loop.