The Study Of Forward Modeling For Fixed-Source Tem Detection In Tunnels With Irregular Loop

Transient Electromagnetic Method (Transient Electromagnetic Method, TEM for short) is a time-domain electromagnetic method with artificial sources. For the advantages of deeper probing depth, high lateral resolution, low resistance sensitive response and high efficiency, it has been applied in mineral exploration, hydrogeological investigation and engineering investigation and other fields widely.For the geological forecast in tunnels, TEM uses moving-source device such as central loop, that arranging multi-turn square or circular and other regular transmit frame on tunnel face, and receiveing electromagnetic signals in the transmission line frame by the receiving loop (or probe), then point by point moving transmit frame and receive loop (or probe) at the same time along the survey line on tunnel face. This way of working is simple and easy to explain, but there are corresponding disadvantages, such as:less probing depth, strong mutual inductance, small detection range, less data and not conducive to 3D imaging of entire working face and so on. In order to improve these problems, the author proposed fixed -source transient electromagnetic method for geological forecast in tunnels. The method used fixed-source device and collected every measuring point’s data one by one or at the same, by designing semicircular irregular loop based on the tunnel face contour, and arranging a plurality of parallel survey lines in it. This way of working can take advantage of the space of tunnel face, and lay the foundation for the future development of a fixed-source multi-array observation system.Because fixed-source transient electromagnetic method have made large changes on transmit and receive way, if we applied the forward and inversion method of existing central loop work device directly, the results are bound to occur large error,even make mistakes. Therefore, we need to re-establish the appropriate forward modeling and inversion method. Given the limited stduy time, and forward modeling as the basis of inversion, this paper only researched forward modeling method of fixed-source TEM. Based on the thought of horizontal electric dipole constituting irregular loop source, the study established the theoretical expression of fixed-source transient electromagnetic response at the surface of homogeneous layered medium. We got Hankel integral and cosine integral of electric dipole by means of linear digital filtering algorithm, and then achieved forward modeling of Fixed-source TEM. This method can calculate TEM’s irregular loop, and receiving position can be placed at the non-center position in loop, and applicable to the geological forecast of tunnel face under different circumstances.By comparing the forward modeling results of the fixed-source TEM and traditional TEM with central loop device, it verified the correctness of the forward modeling theory and numerical calculation method for fixed-source transient electromagnetic. To better understand the application performance and feature of forward modeling theory for fixed-source transient electromagnetic method, this paper built three forward models of typical unfavorable geological bodies in geological forecast for tunnels——water-filled faults and fractures (a thin layer of low resistivity), fracture zone filled with water and mud (a thick layer of low resistivity) and dry and no filling faults,fracture zone (the high-resistivity layer), by analyzing the EMF decay curve and its differential curve obtained by fixed-source TEM forward simulation, the results of electromagnetic response were consistent with the structural characteristics of geoelectric models, thereby further validated the forward modeling method of fixed-source TEM. Finally, the research carried out comparative analysis between the corresponding physical simulation and numerical simulation by the tunnel model for geological forecast Similarly, the practical application of fixed-source TEM forward modeling can be better examined.