Boulder detection by multi-source transient electromagnetic method in shield TBM tunnel

At the present time, shield constructions have become essential engineering methods for urban subway systems, with increasing numbers of urban subway projects implementing shield constructions. However, the main prospecting problems encountered in shield tunnel constructions inve determining the locations of boulder groups. Due to their characteristics of strong abrasiveness and unstable mechanical properties, boulder groups present severe challenges to the construction of shield tunnels. Therefore, it is necessary to design an advance prospecting method which has high-resolution, large penetration depth, and the ability to adapt to the narrow prospecting spaces of shield machinery. It has been found that transient electromagnetic multi-source prospecting methods can effectively solve prospecting problems and the difficulties related to the resolution of boulder groups. Furthermore, the forms of emission with galvanic sources, along with the transmitting and receiving by the x component of the electric fields, can effectively solve the interference problems encountered when using shield machinery. In addition, full-field apparent resistivity imaging methods have been used to obtain the prospecting results of boulder groups. However, electromagnetic shield advance prospecting methods continue to encounter difficulties in accurately determining boulders and other targets (high-resistance targets) in the soil (low resistance background). In order to address these issues, it is considered that loading multi-resolution prospecting methods using wave-field transformation and pseudo-seismic migration imaging, along with other solutions, are important directions for future research in this field.